Computer Science 1301

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Chapter 2

JAVA FUNDAMENTALS


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// A simple Java program that displays the message// Have a good semester! on a line on the computer screen// displayMessage.java Created using: jGrasp

// By L. Thompson

public class displayMessage{

public static void main(String[ ] args){

System.out.println("Have a good semester!");}

}


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// By L. Thompson




}

These are

comments. They

are ignored by the

compiler.


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THE PARTS OF A JAVA PROGRAM

Comments

Comments are notes of explanation used to documentprograms, sections of programs, or program statements for the

humans who must read programs.

The program displayMessage.java begins with the followingcomments:

// A simple Java program that displays the message// Have a good semester! on a line on the computer screen// displayMessage.java Created using: jGrasp// By L. Thompson

Every program should start with a similar comment. Describe

what the program does and supply the name of the file

containing the program and the name of the author of the

program.


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Comments

The compiler, the program that translates Java programs to bytecode, ignores everything from the two forward slashes to theend of the line.

Comments do not end with a semicolon.


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// By L. Thompson




}

The compiler ignores blanklines. It is a good idea to use

blank lines to make your

program easier to read.


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// By L. Thompson




}

This is a

class

definition.

This is the class header for the

class named displayMessage.


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Class Definition

Every Java program must have at least one class definition.

We will use a class as a container that has the statements thatcomprise our application. The class named displayMessage

contains the statements that make up our application in thesample program.




}


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Class Definition




}

The wordpublic is an access specifier that controls where the class canbe accessed from. The keywordpublic in the class header specifiesthat the class is unrestricted (i.e. the class can be accessed from anyother class).

The keyword class marks the beginning of the class. The class name, displayMessage, was chosen by the programmer. It is

a user-defined identifier.

The class header does not end with a semicolon. It is only thebeginning of a class definition; it is not a complete statement.


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Class Definition




}

There cannot be more than one public class in a file.

When a Java file contains a public class, the name of the public

class and the filename (without the .java extension) must be

the same. Therefore, the source file containing the class shownabove must be named:

displayMessage.java


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Class Definition




}

The { is an opening brace.

The } is the closing brace.

All the statements in the class definition are enclosed in a set of

braces {}. We say that the braces delimit the body of the class. It is considered good programming style to indent the

statements inside a set of braces one level.


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// By L. Thompson




}

This is a

method

definition.

This is the method header for the

method named main.


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Method Definition

The method definition shown below is from displayMessage.java:


System.out.println("Have a good semester!");

}

A method is a named block of statements that perform aspecific task when executed.

Every Java application has a method called main. Java

programs begin executing at the main method. The method header is not a complete statement, so it does

not end with a semicolon.

All the statements in a method definition are enclosed in a setof braces {}.

There is no semicolon

here.


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// By L. Thompson




}

This statement tells the

computer to display the

message Have a good

semester! on the computer

screen.


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Theprintln Method

In displayMessage.java we used the following statement:


A sequence of characters between the pair of quotation marksis called a string literal.

The quotation marks delimit the string literal, they will not bedisplayed on the screen.

Theprintln statement ends with a semicolon. It is a complete

Java statement that instructs the computer to display a messageon the computer screen.


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THEprint METHOD,println METHOD, and the

JAVA API

Many of the programs we write will display text in a consolewindow.

The console window is the standard output device.

We will send strings of text to a console window using objectsfrom the Java API (Application Programmer Interface).

The Java API is a standard library of prewritten classesavailable to Java programs for performing common operations.


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JAVA API

The program named displayMessage.java contains the statement:


This statement uses the System class from the Java API. The System class contains objects and methods that perform

system level tasks.

The outobject is a member of the System class that contains themethods namedprintandprintln. These methods perform the

task of writing characters in a console window on the computerscreen.


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JAVA API


The line above is read System dot outdotprintln

The dot is the membership operator in Java. It is used tospecify thatprintln is a member method of the outobject whichis a member of the System class.

*** See Figure 2-3 from the text

Membership operators


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JAVA API


The value that is to be displayed on the computer screen isplaced inside parentheses.

A value passed to a method is called an argument.

The argument passed to theprintln method in this example is thestring literal "Have a good semester!".


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JAVA API

Theprintln method displays a stream of characters and thenadvances the cursor to the beginning of the next line.

Theprintmethod displays a stream of characters, but does not

advance the cursor to the next line.


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JAVA API

// A portion of the Gettysburg Address by Abraham Lincoln// gettysburgA.java Created using: jGRASP// By L. Thompson

public class gettysburgA{


// This version uses the println method

System.out.println("Four score and seven years ago");System.out.println("Our fathers brought forth on this continent,");System.out.println("a new nation, conceived in liberty,");System.out.println("and dedicated to the proposition that all men are created equal.");

}}


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JAVA API

// A portion of the Gettysburg Address by Abraham Lincoln// gettysburgB.java Created using: jGRASP// By L. Thompson

public class gettysburgB{


// This version uses the print method

System.out.print("Four score and seven years ago");System.out.print("Our fathers brought forth on this continent,");System.out.print("a new nation, conceived in liberty,");System.out.print("and dedicated to the proposition that all men are created equal.");

}}

Here we used the print method instead

of the println method.


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JAVA API

The characters sent to console are displayed in a continuousstream.

A subsequent insertion begins where the previous insertion leftoff.

Even if the output is broken into several print statements, oneon each line, the output is displayed as one long continuousstream.

Spaces we want included in the output must be included in theliterals we send to the method.


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JAVA API

Here we try to make the output look a little bit nicer by embedding

some spaces in our strings.

// A portion of the Gettysburg Address by Abraham Lincoln// gettysburgC.java Created using: jGRASP// By L. Thompson

public class gettysburgC{


// This version uses the print method, but in this// version I have embedded spaces in the strings to separate// the strings in the output.

System.out.print("Four score and seven years ago ");System.out.print("Our fathers brought forth on this continent,");System.out.print(" a new nation, conceived in liberty,");System.out.print(" and dedicated to the proposition that all men are created equal.");

}

}

Inserted spaces at

these points


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JAVA API

It is possible to mix the use of theprintandprintln methods to getthe desired output.

// A portion of the Gettysburg Address by Abraham Lincoln// gettysburgD.java Created using: jGRASP// By L. Thompson

public class gettysburgD{


// This version uses a combination of the print and println methods

System.out.println("Four score and seven years ago");System.out.print("Our fathers brought forth on this continent,");System.out.println(" a new nation, conceived in liberty,");System.out.print("and dedicated to the proposition that all men are created equal.");

}}


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JAVA API

Escape Sequences

Another way to tell the computer to move the cursor to the

beginning of a new line is to embed the newline escape

sequence in a string literal at the point you want the new

line to begin.

The newline escape sequence is \n.

In general, an escape sequence is a backslash, \, followed

by one or more characters.

Escape sequences are used to control the way output is

displayed.


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JAVA API

Escape Sequences

Causes a double quotation mark to be printeddouble quote\

Causes a single quotation mark to be printedsingle quote\

Causes a backslash to be printedbackslash\\

Causes the cursor to go to the beginning of the current line, not

the next linecarriage return\r

Causes the cursor to back up, or move left, one positionbackspace\b

Causes the cursor to skip over to the next tab stoptab\t

Advances the cursor to the next line for subsequent printingnewline\n


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JAVA API

Escape Sequences// A portion of the Gettysburg Address by Abraham Lincoln

// gettysburgE.java Created using: jGRASP

// By L. Thompson

public class gettysburgE

{

public static void main(String[ ] args)

{

// This version uses a combination of the print and println methods

// It also uses the newline and horizontal tab escape sequences

System.out.println("Four score and seven years ago");

System.out.print("Our fathers brought forth on this continent,");

System.out.print("\na new nation,conceived in liberty,\n");System.out.print("and dedicated to the proposition\n\nthat all men are reated\tequal.");

}

}


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VARIABLES

A variable is a named location in the computers memory thatcan store a value that may be changed during the execution ofthe program.

Part of the job of programming is determining what values

need to be stored. The values stored in variables are often the inputs of the

program and the results of calculations/processing.


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VARIABLES

Declaring Variables

In Java, variables must be declared before they are used.

A variable declaration is a Java statement that tells the

compiler that storage is needed for a value of a particular data

type.

A variable declaration consists of the data type of the variable(the kind of data it can store) followed by the name of the

variable.

A variable declaration ends with a semicolon.


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VARIABLES

Declaring Variables

The statement below declares a variable namedpayments that can beused to store an integer (whole number) value.

int payments;

The declaration

begins with the data

type. The keyword

intis a data type for

whole numbers.


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VARIABLES

Declaring Variables

The statement below declares a variable named interestRate thatcan be used to store a double (real number) value.

double interestRate;

The keyword double

is a data type for

real numbers.


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VARIABLES

Declaring Variables

You may declare several variables of the same data type in a singlestatement by separating the variable names with commas.

int payments, month, day, year;

The statement above is equivalent to the group of statementsbelow:

int payments;

int month;int day;int year;


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VARIABLES

Rules for Naming Variables and Other Identifiers

There are some rules that must be followed when creating the namesof variables and other identifiers.

1. Do not use any of the Java key words as identifiers.

2. The first character of an identifier must be an a-z, A-Z, anunderscore character (_), or the dollar sign ($).

3. After the first character you can use letters a-z or A-Z, digits 0-9,underscores, or dollar signs.

4. Spaces are not allowed in identifiers.


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VARIABLES

Conventions for Naming Variables

It is good programming style to give your identifiersmeaningful names. This means giving them names that give anindication of their usage.

Write variable names using lowercase letters, capitalizing the

first letter of the second and subsequent words that make up thevariable name. This helps to visually break up the words sincespaces cannot be used.

double accountBalance, interestRate;


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VARIABLES

Assigning a Value to a Variable

Values may be assigned to variables using an assignmentstatement.

We can store the value 36 in the variable namedpayments using

the following assignment statement:

payments = 36;

An assignment copies the value of the expression on the right

of the assignment operator into the location correspondingto the variable that is on the left of the assignment operator.


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// A program to calculate and display the area of two circles// AreaOf2Circles.java Created using: jGRASP// by L. Thompson

public class AreaOf2Circles{public static void main(String[ ] args){

double area, radius;

// Calculate and display 1st area

radius = 2.0;area = 3.14159 * radius * radius;System.out.print("The area of a circle with radius " + radius);System.out.println(" is " + area);

Continued on the next slide


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Continued from the previous slide

// Calculate and display 2nd area


}}

Sample output:

The area of a circle with radius 2.0 is 12.56636

The area of a circle with radius 3.0 is 28.274309999999996


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VARIABLES

When a numeric variable is used in a mathematical expression, thevalue stored in the memory location corresponding to the variableis used in the calculation.


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VARIABLES

Displaying the Values of Variables

We can display the value of a variable using aprintorprintlnstatement.

int apples;

apples = 5;System.out.print(apples); // 5 is displayed

Notice that the variable name is not enclosed in quotationmarks.

If quotation marks were placed around apples, it would be astring literal, and the characters between the quotation marks

would be displayed on the computer screen, not the value in the

apples variable.


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THE STRING CONCATENATION

OPERATOR (+)

The + operator can be used in two ways

As the addition operator on numeric values

As the concatenation operator when at least one of itsoperands is a string

If either side of the + operator is a string, string concatenation isperformed and the resulting value is a string.


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OPERATOR (+)

In the statement below, the concatenation operator creates the new

string Hello World which is then passed as the argument to the

rintln method and is displayed on the computer screen.

System.out.println("Hello " + "World");


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OPERATOR (+)

In the statement below, the concatenation operator creates the new

string The value is: 2 which is then passed as the argument to the

rintln method and is displayed on the computer screen. Notice the

value ofnumberis not a string. The + operator converts the integer

2 to a string and then appends this string to the string The valueis:, creating a new string.

int number;

number = 2;

System.out.println("The value is: " + number);


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LITERALS

A literal is aconstant value written in the code of a program.

Literals are frequently used as:

Values assigned to variables/attributes

Operands in a arithmetic operations

Display values


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// A program to calculate and display the area of two circles// AreaOf2Circles.java Created using: jGRASP// by L. Thompson

public class AreaOf2Circles{

public static void main(String[] args){

double area, radius;

// Calculate and display 1st area


// Calculate and display 2nd area


}}

Highlighted are

some of the literalsused in this

program. Can you

find more?


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PRIMITIVE DATA TYPES

Values in Java are classified according to their data type.

The data type of a value determines how the value isrepresented in the computer and the kinds of operations that canbe performed on the value.


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Numeric Data Types

Numeric values can be divided into two classifications:

Integer

Floating-point

Integers are whole numbers: 294

111123

950

Floating-point numbers may have a decimal point:

12.1

169.5

1347.98


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Numeric Data Types

The integer and floating-point categories are separated into a

number of types that differ in the amount of storage allocated and

the range of values that can be stored.


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Numeric Data Types

Table 2-5 textData Type

(Key words) Size Range

byte 1 byte Integers in the range-128 to +127

short 2 bytes Integers in the range of -32,768 to +32,767

int 4 bytes Integers in the range of -2,147,483,648 to +2,147,483,647

long 8 bytes Integers in the range of -9,223,372,036,854,775,808 to +9,223,372,036,854,775,807

float 4 bytes Floating-point numbers in the range of

3.4 10-38 to 3.4 1038, with 7 digits of accuracydouble 8 bytes Floating-point numbers in the range of

1.7 10-308 to 1.7 10308, with 15 digits of accuracy


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Numeric Data Types

Integer Date TypesInteger data types are used for storing whole numbers.

Example:

byte rating;

int offset;long acctNum;short days;

offset = 7;acctNum = 1234567890;


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Numeric Data Types

Integer Data Types - Integer LiteralsWhen you write an integer literal in a Java program, it is

automatically stored as a type int.

The assignment statement in the segment below is assigning a

value of type intto a variable of type short:

byte rating;int offset;long acctNum;short days;

days = 28;


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Numeric Data Types

Floating-Point Data Types Floating-point data types are used for holding numeric values

that may include a decimal point.

Thefloatdata type is considered a single-precision data type.It can hold a floating-point number with 7 digits of accuracy.

The double data type uses twice as much memory as thefloatdata type; it is the double-precision floating-point data type.The double can hold a floating-point value with 15 digits ofaccuracy.


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Numeric Data Types

Floating-Point Data TypesExample:

double range;float amount;

range = 5.5;


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Numeric Data Types

Floating-Point Data TypesFloating-point values are typically stored inside the computer in anotation similar to scientific notation, called E notation.

Decimal Notation Scientific Notation E Notation36.78 3.678 X 101 3.678E1

218976.9 2.189769 X 105 2.189769E5

.00445 4.45 X 10-3 4.45E3

*** See Table 2-6 of the text


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Numeric Data Types

Floating-Point Data Types - Floating-Point Literals Floating-point literals are stored in memory with type double.

Because of this, a problem can arise when you assign afloating-point literal to a variable of typefloat.

Java is a strongly typed language, which means it will only let

you store a value of a compatible data type in a variable. Since a double value can be much larger than a variable of type

floatcan hold and the double has greater precision, the

assignment statement below, which assigns the value to the

variable named amount, will produce a compiler error.


amount = 27.98;range = 1.53e-5;


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Numeric Data Types

Floating-Point Data Types - Floating-Point LiteralsYou can get around this problem by specifying that the floating-

point literal be stored as a type float by using the suffix F after the

literal. The following segment will work.


amount = 27.98F;range = 1.53e-5;


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The boolean Data Type

Expressions that evaluate to a true or false value are calledBoolean expressions.

The boolean data type allows you to create variables that canhave either the value true or the value false.

The contents of a boolean variable may not be copied to avariable of any other data type.


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Theboolean Data Type

// A program to demo the boolean data type// BooleanDemo.java Creating using: JGRASP// by: L. Thompson

public class BooleanDemo{

public static void main(String[ ] args)

{int number;boolean result;

number = 15;

result = number > 5;System.out.println("The value of result is " + result);

result = number < 10;System.out.println("Now the value of result is " + result);

}}


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Theboolean Data Type

Sample output for BooleanDemo.java:

The value of result is true

Now the value of result is false


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Thechar Data Type

A variable of type char, meaning character, can be used to store asingle character.

Example:

char grade;

grade = A;


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Thechar Data Type

Character constants/literals are enclosed in single quotationmarks as in A. If you leave off the quotation marks, the

compiler will thinkA is a variable you forgot to define whichis a syntax error.

A is not equivalent to A. A is a string literal. String literals cannot be assigned to character variables.

Characters are actually encoded in binary (sequences of onesand zeroes). Java uses Unicode to encode characters. Unicodeuses 16 bits (2 bytes) to encode each character.

*** See Appendix B of the text


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Thechar Data Type


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Thechar Data Type

Example:

char grade;

grade = A; // Assigns the binary equivalent of 65 to grade


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Thechar Data Type

The following statements contain a syntax error:

char grade;

grade = A; // Cannot assign a string to a character variable


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VARIABLE ASSIGNMENT AND

INITIALIZATION

The = symbol is the assignment operator in Java.

The assignment operator is a binary operator, because it takestwo operands.

The operand on the left of the assignment operator must

correspond to a location in memory whose contents may bechanged. The operand on the right of the assignment operatorcan be any expression that has a value.

An assignment statement stores the value of the expression thatis the right operand of the assignment operator in the memory

location corresponding to the left operand of the assignmentoperator.


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VARIABLE ASSIGNMENT AND

INITIALIZATION

You may give a variable an initial value when you declare thevariable. This is called initializing the variable.

Examples:

float average = 96.2F;char letterGrade = 'A';long ID = 123456789;float salary = 564248.56F, bonus, retirement = 175.00f;boolean error = false;

Notice that the variable bonus is uninitialized, it contains anunknown bit pattern. It is an error to print this value or use it in acalculation before it is given a value, because you have no ideawhat it contains.


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READING INPUT FROM THE KEYBOARD

The Scanner Class

The standard input device is normally the computer keyboard.

The Java API has an object System.in which is associated withthe standard input device. We will use this object inconjunction with the Scannerclass to read input data from the

keyboard. The Scannerclass has methods that can be used to read input

and format it as primitive data values or strings.


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The Scanner Class

To use the Scannerclass in our program we must put thefollowing statement near the top of our file, before any classdefinition:

import java.util.Scanner;

This statement tells the compiler where to find the Scannerclass.


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The Scanner Class

You must also create a Scannerobject and connect it to theSystem.in object. You can do this with a statement like thefollowing:

Scanner keyboard = new Scanner(System.in);


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The Scanner Class


The words Scanner keyboarddeclare a variable namedkeyboardwith data type Scanner. This variable will referencean object of the Scannerclass.

You could have chosen any name you wanted for the variable,but keyboardis a good one since you are going to use it toaccess the keyboard.


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The Scanner Class


We use the assignment operator, =, because we want to assign avalue to the keyboardvariable.

G


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The Scanner Class

Scanner keyboard =new Scanner(System.in);

The word, new, is a Java key word. The new key word is usedto create an object in memory.

The type of object is listed after the key word new, this is

Scanner. So we are creating an object of the Scannerclass. Inside the parentheses, we have System.in. Here we are saying

that we want the object we are creating to be connected withthe System.in object, which again is associated with thekeyboard.

We are assigning the address of the object created to ourvariable named keyboard, so keyboardwill reference theobject we have linked with the actual keyboard.



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The Scanner Class

Every object created from the Scannerclass has methods that reada string of characters entered at the keyboard, converts them to aspecified type, and returns this value to be stored in a variable ofcompatible type.



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The Scanner Class

For example the code below could be used to read an integerentered at the keyboard and store it in an integer variable namedage.

int age;

System.out.print(Enter your age: );age = keyboard.nextInt( );



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The Scanner Class

The following statement is a prompt:

System.out.print(Enter your age: );

A prompt is a message that lets the user know what input is

expected. Whenever you want the user to enter a value at the keyboard,

you must display an appropriate prompt on the computerscreen.



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The Scanner Class

int age;

System.out.print(Enter your age: );age = keyboard.nextInt( );

We are calling the nextInt( ) method of the keyboardobject toget a value entered by the user at the keyboard.

The nextInt( ) method formats the characters entered bythe user as an intand returns the integer value.

The integer value is assigned to the variable named age.



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The Scanner Class

If you had named your Scannerclass object something else, saydog, the statements above would need to be changed as follows:

Scanner dog = new Scanner(System.in);

int age;

System.out.print(Enter your age: );age = dog.nextInt( );



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The Scanner Class

*** See Table 2-17 of the text for a listing of other useful

methods of the Scanner class

*** See the Java API online


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// A program to calculate and display the area of a rectangle with side lengths entered by the user.// Source filename: RectangleArea.java Created using: jGrasp// Written by: L. Thompson

import java.util.Scanner;

public class RectangleArea{


double length, width; // The length and width of the rectangledouble area; // The area calculated for the rectangle

// Create a Scanner object to read input


// Get the dimensions of the rectangle from the user

System.out.print("Enter the length of the rectangle: ");

length = keyboard.nextDouble( );System.out.print("Enter the width of the rectangle: ");width = keyboard.nextDouble( );



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Continued from the previous slide

// Calculate the area of the rectangle

area = length * width;

// Display the results of the calculation

System.out.print("\nA rectangle with length " + length + "\nand width ");System.out.println(width + " has an area of " + area);

}}

ARITHMETIC OPERATORS


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Java provides many operators that are useful for manipulatingdata and performing arithmetic operations.

Operators can be classified as unary, binary, or ternarydepending on the number of operands the operator requires.

Unary operators require a single operand. Binary operators require two operands.

Ternary operators require three operands.



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Negation Operator ()

The negation operator is a unary operator that negates the value ofits operand.

The following statements define a variable named angle and assignthe value -50 to that variable.

int angle;

angle = 50;



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Addition Operator (+)

The addition operator is a binary operator that returns the sum of itstwo operands.

The following statement adds the value in amountto the value in

bonus and stores the result in total.

total = amount + bonus;



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Subtraction Operator()

The subtraction operator is a binary operator that returns the resultof subtracting its right operand from its left operand.

The following statement subtracts the value 27 from 70 and storesthe result (43) in amount.

amount = 70 27;



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Multiplication Operator (*)

The multiplication operator is a binary operator that returns theproduct of its two operands.

The following statement multiplies the value in rate by the value intime and stores the result in distance.

distance = rate * time;



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Division Operator (/)

The division operator is a binary operator that returns the quotient ofits left operand divided by its right operand.

The following statement divides the value inplayers by the value inmaxPlayers and stores the result in teams.

teams = players / maxPlayers;



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If both operands of the division operator are integers the

quotient is an integer. The fractional part is discarded.

In the statements below, the expression 161 / 2 has the value 80,because both operands of the division operator are of type int. The.5 is discarded. The value 80.0 is assigned tox.

double x;

x = 161 / 2;



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If at least one of the operands of the division operator is a

floating-point type the quotient is a floating-point type.

In the statements below, the expression 161.0 / 2 has the value 80.5.The value 80.5 is assigned to the variablex.

double x;

x = 161.0 / 2;



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What is the data type of the result ofplayers / maxPlayers in thestatements below?

double teams;

teams = players / maxPlayers;



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Modulus Operator(%)

The modulus operator is a binary operator that returns the remainderof a division operation. We can use this operator to find the numberof items left over after an integer division.

The following statement divides 12by 10 and stores the remainderof the division (2) in nextDigit.

nextDigit = 12 % 10;

OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE

An expression may consist of a literal value, a variable, or be acombination of operators and operands.

The evaluation of some expressions like 4 + 21 isstraightforward.

Others are not so straightforward, for instance 16 8 / 4. Thevalue of this expression depends on whether the subtraction orthe division is performed first.

OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE

In Java, mathematical expressions are processed from left toright.

When two operators share an operand, the operation specifiedby the operator with the highest precedence is performed first.

*** See Table 2-8 and Appendix C of the text

Division has higher precedence than subtraction.

In the expression:

16 8 / 4

the division, 8 / 4, is performed first, producing a quotient of 2.

Then 2 is subtracted from 16 producing a result of 14.

OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE



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OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE

If two operators sharing an operand have the same precedence,the operators work according to their associativity.

Associativity is either left to right or right to left.

For instance, if a multiplication and division operator share an

operand these operations will be performed left to right since

multiplication and division associate from left to right.

OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE

What is the value of the following expression?

2 + 3 * 4

OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE


10 + 3 * 8

OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE


17 7 % 4 * 2

OPERATOR PRECEDENCE


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OPERATOR PRECEDENCE


15 + 20 / 8 + 6 * 2

GROUPING WITH PARENTHESES


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You may enclose an expression in parentheses to ensure that it isevaluated before other expressions that involve its operands.

Example:

y = (a b) / 4;

Without the parentheses, the value ofb / 4 would have been

calculated first and then this value would have been subtracted from

a.



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G OU NG N S S

Example:

x = 10 (8 4);

The value of the expression, 10 (8 4), is 6. Without the

parentheses, the expression is 10 8 4, in this expression 8 wouldhave been subtracted from 10 first, resulting in a value of 2, and

then 4 would have been subtracted from 2, to produce the value 2.

TheMath Class


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The Java API provides a class namedMath which includes manymethods for performing mathematical operations.

TheMath Class


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TheMath.pow( ) Method

Java does not have an exponent operator.

To raise a number to a power we can use theMath.pow( )method. You can think of a method as a routine that performs a

specific operation.

TheMath.pow( ) method takes two double arguments.Arguments are data values sent to a method.

TheMath.pow( ) method raises the value of the first argument tothe power of the second argument and returns the result of this

calculation as a value of type double.

TheMath Class


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Suppose that we want to calculate the volume of a cube. Thevolume of a cube, v, is specified by the algebraic formula:

v = s3

where s is the length of the side of the cube.

In our program, we could specify this calculation as follows:

v = Math.pow(s, 3);

TheMath Class


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v = Math.pow(s, 3);

Math.pow(s, 3) is a call to thepow method which is a member oftheMath class.

This call is an expression, the value of this expression is the

value returned by the method. In our case, this value is assignedto the variable v.

Suppose s has the value 3.3, the value returned by the method

call and assigned to v is 35.937.

TheMath Class


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The value returned by theMath.pow( ) method can be usedanywhere a double value can be used.

For example the algebraic expression j = y (x + 2)6could

be encoded in Java as:

= y - Math.pow(x + 2, 6);

TheMath Class


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TheMath.sqrt( ) Method

Java does not have a square root operator.

We can use theMath.sqrt( ) method to find the square root of anumber.

TheMath.sqrt( ) method takes a double argument and returnsthe square root of the argument as a value of type double.

TheMath Class


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TheMath.sqrt( ) Method

For example the algebraic expression:

could be encoded in Java as:

z = 4 * Math.sqrt(a + b);

z a b 4

COMBINED ASSIGNMENT OPERATORS


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In programs, it is quite common to get the current value of avariable, modify it, and then reassign the new value back to the

original variable.

Example:

x = x + 10;

This statement specifies that the value 10 is added to the current

contents of the variablex and the resulting value becomes the new

contents ofx. Effectively, this statement increases the value of thevariablex by 10.



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+=

Java provides a shorthand way to write this statement using thecombined assignment += operator.

Instead of writing

x = x + 10;

we can write:

x += 10;

The (+=) operator adds the value of the right operand to the current

value of its left operand and stores the result in the left operand.



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=

The (-=) operator subtracts the value of the right operand from thecurrent value of its left operand and stores the result in the left

operand.

For example:

balance = balance withdrawals;

could be written:

balance - = withdrawals;



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*=

The (*=) operator multiplies the value of the right operand by thecurrent value of its left operand and stores the result in the left

operand.

For example:

j = j * a;

could be written:

*= a;



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/=

The (/=) operator divides the current value of its left operand by thevalue of the right operand and stores the result in the left operand.



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%=

The (%=) operator divides the current value of its left operand bythe value of the right operand and stores the remainder of the

division in the left operand.



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The left operand of a combined assignment operator must be avariable.




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The combined assignment operators have lower precedencethan the arithmetic operators +, -, *, /, and %.

*** See Appendix C of the text

For example:

w /= b + 1;

is equivalent to

w = w / (b + 1);

CONVERSION BETWEEN PRIMITIVE DATA


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TYPES

The Java compiler checks that the values assigned to variables areof a type compatible with the variables data type.



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TYPES

For example, the compiler permits the following statements:

int x = 5;float y;

y = x; // Assigns an int to a float



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TYPES

However, the compiler rejects the assignment statement in thesegment below, saying possible loss of precision.

double g = 13.9;int z;

z = g; // Assigns a double to an int

The compiler will not allow this assignment, because the data type

double can hold a fractional value and also it can hold larger and

smaller values than an intcan hold.



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TYPES

In Java the primitive data types are ranked. One data type outranksanother if it can hold a larger value.

*** See Figure 2-6 of the text

Numeric data type

doublefloatlongintshortbyte

Highest Ranking

Lowest Ranking



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TYPES

If you write an assignment statement that assigns a value of a lower-ranking data type to a variable of a higher-ranking data type, Java

automatically converts the value of the lower-ranking data type to

the higher-ranking data type before the assignment. This type of

conversion is called a widening conversion.

The following is an example of a widening conversion:

int x = 5;float y;

y = x; // Assigns an int to a float


TYPES


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TYPES

In general, Java does not automatically perform narrowingconversions. A narrowing conversion is a conversion from a

higher-ranking data type to a lower-ranking data type.

The following is an example of a narrowing conversion:


z = g; // Assigns a double to an int


TYPES


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TYPES

The Cast Operator The cast operator is a unary operator that allows you to convert

a value to a different data type.

You can specify either a narrowing or a widening conversionusing the cast operator.

The cast operator is a parenthesized data type.

It specifies that the following operand is to be converted to thedata type in parentheses.


TYPES


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TYPES

The Cast Operator


z = (int) g; // Makes a copy of the value in g that is type int & then assigns this value to z

When you convert a floating-point type to an integer type, anyfractional portion is truncated (discarded).

It is not rounded. In the statement above, the value 13 isassigned toz.

The operand of the cast operator is not changed by theoperation. A copy of the value is made that is of the specifieddata type. The value in the variable g is not changed in thesegment above, it is still 13.9.

See the programDemoTypeConv.java on webct. Compile this


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p g yp j p

program, find and correct all syntax errors using the cast operator.

Are there still logical errors? How could these be fixed? Make sure

you understand the automatic data type conversions and theimplications of these?


TYPES


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TYPESAutomatic Type Conversion in Arithmetic Expressions

When an arithmetic operator has operands of different data types,Java will temporarily convert the operand of the lower-ranking datatype to the higher-ranking data type.

The result of the operation will be the same data type as the higher-

ranking operand. When values of type byte and shortare used in arithmetic

expressions, they are temporarily converted to type int.


TYPES


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Given the following declarations:

int x = 16, y = 7;float f = 3.0F;

What is the value and data type of the following expression?

x / 5


TYPES


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int x = 16, y = 7;float f = 3.0F;


x / f


TYPES


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int x = 16, y = 7;float f = 3.0F;


f * 5 / 6


TYPES


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int x = 16, y = 7;float f = 3.0F;


17 / (y - 5) * f


TYPES


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int x = 16, y = 7;float f = 3.0F;


4.4 + x % 9

What happens if you try to assign the value of the expression above

to the integer variabley?

Creating Named Constants Using the Key Word

final


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final

A named constant is a location in memory, given a name, whichcontains a data value that cannot be changed during the execution of

the program.

The declaration of a named constant looks like the declaration of a

variable, except the key wordfinal precedes the data type in thedeclaration of a constant.

Example:

final double SALES_TAX_RATE = .0825;


final


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final

Examples:

final double RATE_PER_MILE = .50;final double BONUS_LEVEL = 1000000;final double INTEREST_RATE = .05667;final int INCHES_PER_FOOT = 12;

final double LBS_PER_KG = 2.20;

A named constant is a way of representing a literal value. Insteadof using the literal value in expressions, we use the named

constant. Named constants are used to make programs more

readable and to facilitate the modification of programs. By convention, programmers use all uppercase letters and

separate words using the underscore character when naming

constants. This helps distinguish named constants from variable

names.


final


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final

Examples:

final double RATE_PER_MILE = .50;final double BONUS_LEVEL = 1000000;final double INTEREST_RATE = .05667;final int INCHES_PER_FOOT = 12;

final double LBS_PER_KG = 2.20;

A named constant must be given a value when it is declared.

It is a syntax error to write a statement that attempts tochange the value in a named constant after its declaration.

TheMath.PINamed Constant


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TheMath class, from the Java API, provides a named constant,Math.PI,which we can use in our programs. This constant has the value

3.141592653589793, which is an approximation of the mathematical value

pi.

In our programAreaOf2Circles.java, we could have used the followingstatement to calculate the area of a circle:

area = Math.PI * Math.pow(radius, 2.0);

SCOPE


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The scope of a variable is the part of the program where thevariable can be accessed by name.

Variables declared inside a method are called local variables.

The scope of a local variable begins at its declaration and ends atthe closing brace, }, of the block in which it is defined.

The compiler reads a program from top to bottom. The compilermust encounter the declaration of a variable before it finds the

variable used in any other statement, or a syntax error occurs.

You cannot declare two variables with the same name in the samescope.

THE String CLASS


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Java does not have a primitive data type for holding a string ofcharacters.

The Java API includes a class named String. Objects of theString class can be used to store and process strings.

THE String CLASS

Objects and Classes


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Objects and Classes

Objects are software entities that have attributes and methods. An objects attributes are data values that are stored in the

object.

An objects methods are procedures for performing operationson the objects attributes.

A class is a pattern from which objects are created/instantiated.

THE String CLASS


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The first step in using an object of the String class is to declare avariable that can be used to reference an object of this class. This can

be accomplished with a statement like the following:

String street;

This statement declares a variable named streetthat will reference an

object of the String class.

THE String CLASS

Variables of Primitive Data Types vs Class Type


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Variables of Primitive Data Types vs. Class Type

Variables

A variable of a primitive data type, like int,float, double, char, long,

etc. holds an actual data value.

The statement below declares a variable of the primitive data type int.

The variable named numbercontains the data value 25.int number = 25;

number 25

THE String CLASS

Variables of Primitive Data Types vs Class Type


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Variables of Primitive Data Types vs. Class Type

VariablesA class type variable holds the memory address of an object. We saythat a class variable references the object or is a reference variable.

The statement below declares a variable that can hold the address ofa String object.

String street;

street address

A String object

THE String CLASS


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We can create/instantiate an object of the String class using the new

operator.

String street;

street = new String("1600 Pennsylvania Avenue");

This statement creates an object of the String class that holds the

string 1600 Pennsylvania Avenue and assigns the address of this

object to the reference variable named street.

THE String CLASS


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Java provides a shorthand way of creating a String object. The

following creates a String object in memory and stores the address

of this object in the variable named street.

street = "1600 Pennsylvania Avenue";

Objects of other classes cannot be created in a similar way.

THE String CLASS


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Objects of the String class have numerous methods for working with

strings.


THE String CLASS


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Objects of the String class have a method named length( ) which

returns the number of characters in the string, including spaces.

The value 24 would be assigned to the integer variable named

numChars in the statements below:

String street = new String("1600 Pennsylvania Avenue");

int numChars= street.length( );

THE String CLASS


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Objects of the String class have a method named charAt( ) which

returns the character at the specified index in the string. The first

character in a string is at index 0; the second character is at index 1,

etc.

The character 6 would be assigned to the character variable namedletterin the statements below:


char letter = street.charAt(1);

THE String CLASS


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Objects of the String class have a method named toLowerCase( )

which returns a new string with all the characters converted to

lowercase.

The variable named newString will reference a String object

containing the string 1600 pennsylvania avenue after

the statements below are executed:


String newString = street.toLowerCase( );

THE String CLASS


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Objects of the String class have a method named toUpperCase( )

which returns a new string with all the characters converted to

uppercase.

THE String CLASS


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The methods discussed above do not change the original string.

ThetoLowerCase( ) andtoUpperCase( ) methods create new

String objects.

*** See the Java API for additional methods for working with

objects of the String class

See the program StringDemo.java. See if you can figure out

what is displayed by tracing the code (i e Try not to use the


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what is displayed by tracing the code. (i.e. Try not to use the

computer, you will not have access to a computer during the

exam).

Problem That Can Occur When a call to the

nextLine( ) Method is Preceded by a Call to One


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( ) y

the Other Scanner Class Methods

Calling the Scannerclasses nextLine( ) method following a call to

one the Scannerclasses methods for reading a value of a primitive

data type can cause a problem.




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( ) y


When the user types keystrokes at the keyboard, the keystrokesare stored in area of memory called the keyboard buffer.

When the user presses the enter key, a newline character iswritten to the keyboard buffer.

The Scannerclasses methods that read values of primitive datatypes, skip leading newline characters and stop reading at atrailing newline character, leaving this character in the keyboard

buffer.




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( ) y


The nextLine( ) method does not skip leading newline characters.

The nextLine( ) method reads characters from the keyboardbuffer until it reads its first newline character. The nextLine( )method does not put the newline in the String object, but does

advance the read position past it. You might say the nextLine( )method consumes the newline character, but the other methodswe have learned do not.

When a call to the nextLine( ) method reads a newline characterthat was left in the buffer by a preceding read using one of the

other methods of the String class, it appears that the nextLine( )call was skipped, because it does not read any other characters.

We can fix this problem by inserting another call to thenextLine( ) method to consume the problematic newlinecharacter.

*** See the program BufferError java Notice that as it is


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*** See the program BufferError.java. Notice that as it iswritten, the user is not allowed to enter the first name.

Placing a call to thenextLine( ) method after line 21, but

before line 24 will read past the problematic newlinecharacter.

When developing a program echo inputs and


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When developing a program, echo inputs and

intermediate results on the computer screen to verify that

values are getting stored as you expected.

You can take these debug statements out before your code

goes into production.

COMMENTS


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Previously, we learned that comments are notes of explanation usedto document programs, sections of programs, or program statementsfor the humans who must read programs.

COMMENTS


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In Java there are three ways to write comments.

Single-Line comments begin with two forward slashes. Thecompiler ignores everything from the two forward slashes to theend of the line.

Multi-Line comments start with a forward slash followed by anasterisk, /*, and are terminated by an asterisk followed by aforward slash, */. The compiler ignores everything between the

/* and the */. This type of comment can span multiple lines.

Documentation comments begin with a forward slash followedby two asterisks, /**, and end with an asterisk followed by aforward slash, */. The compiler also ignores these comments.

COMMENTS

Documentation Comments


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The Sun JDK includes a program namedjavadoc that reads asource code file and creates an HTML file that includes theinformation provided in the documentation comments.

You should include a documentation comment before each classheader, providing a description of the class.

You should also include a documentation comment before eachmethod header, which gives a description of the method.

The HTML file created byjavadoc is formatted in the same wayas the documentation provided for the Java API, so you canproduce documentation for your classes that look just likestandard Java documentation.

DIALOG BOXES


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A dialog box is a small graphical window that displays a message orrequests input from the user.

We can create dialog boxes using theJOptionPane class.

Two types of boxes we can create using theJOptionPane class are:

Message Dialog A dialog box that displays a message and anOK button.

Input Dialog A dialog box that prompts the user for input,provides a text field where input is to be typed,and displays OK and Cancel buttons.

DIALOG BOXES


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In order to use theJOptionPane class we must put the statementbelow at the top of our file, before any class definition:

import javax.swing.JOptionPane;

DIALOG BOXES

JOptionPane. showMessageDialog( )


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TheJOptionPane class has a static method calledshowMessageDialog( ), which we will use to display a messagedialog.

The statement:

JOptionPane.showMessageDialog(null, "Go Stars!");

displays the message dialog shown below:

DIALOG BOXES

JOptionPane. showMessageDialog( )


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JOptionPane.showMessageDialog(null, "Go Stars!");

The first argument specifies which frame the dialog is to bedisplayed in. In this class we will always specify the default frame

by passing the key word null. The second argument is the messageyou want displayed in the dialog.

The dialog box will close when the user clicks the OK button.

DIALOG BOXES

JOptionPane. showInputDialog( )


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TheJOptionPane class has a static method called showInputDialog( ),which we will use to display an input dialog.

The statement:

String enteredWeight = JOptionPane.showInputDialog("Enter your weight in pounds.");

displays the input dialog shown below:

DIALOG BOXES



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The argument is the message you want displayed in the input dialog.This should be your prompt.

If the user clicks the OK button, the String variable will reference thestring entered by the user in the text field. If the user clicks the Cancelbutton, the String variable will reference a special value null.

DIALOG BOXES



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Notice that the showInputDialog( ) method always returns the usersinput as a String. This is a problem is you wanted a numeric value thatyou could use in a mathematical expression. (You cannot do math on

strings.)

DIALOG BOXES



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To convert a string to numeric value, we can use a method from a Javawrapper class.

There is a class calledDouble that has a static member method calledarseDouble( ), which converts a string to a double, and returns the

double.

We could write the following:


double weightInPounds = Double.parseDouble(enteredWeight);

DIALOG BOXES

Wrapper Classes - Double, Float, Long, Integer,


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Short, and Byte

*** See Table 2-18 of the text for information about other

wrapper classes and their member methods for converting to

other numeric types

DIALOG BOXES

JOptionPane


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When you use theJOptionPane class in your program you must usethe System.exit( ) method to end the program or a thread will continueto execute.

System.exit(0);

The argument, zero, is used to indicate that the program endednormally.


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PROGRAMMING STYLE


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*** Refer to the document,Elements of Good Programming Style,

which is available on webct.

Computer Science 1301

Documents