4 th Week Spring 2011 Methods 1. Outline Introduction Why method ? Static methods vs. non-static methods Example: Math class Declare a method Signature.

4th WeekSpring 2011

Methods

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OutlineIntroduction

Why method ? Static methods vs. non-static methods

Example: Math classDeclare a method

Signature Method overloading

Call/invoke/use a methodCalling stackArgument passing

Java API Packages

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Introduction Best way to develop and maintain a large

program is to construct it from small, simple pieces, i.e., divide and conquer Methods facilitate design, implementation,

operation and maintenance of large programs.Dividing a program into meaningful methods

makes the program easier to debug and maintain.

Normally, methods are called on specific objects

static methods can be called without the need for an object of the class to exist

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Program Modules in Java Java programs development involves:

Design and implement new methods and classesStatements in method bodies are written only once, are hidden

from other methods and can be reused from several locations in a program.

Use predefined methods and classes available in the Java Application Programming Interface and other class libraries Java API provides a rich collection of predefined classes

Related classes are typically grouped into packages so that they can be imported into programs and reused import java.util.*;

Software reusability: use existing methods as building blocks to create new programs

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Method (function) CallA method is invoked by a method callWhen the called method completes its task, it either

returns a result or simply returns control to the caller Flow chart representation

Similar to hierarchical form of managementA boss (the caller) asks a worker (the called method) to

perform a task and report back (return) the results after completing the task

The boss method does not know how the worker method performs its designated tasks

The worker may also call other worker methods, unbeknown to the boss

Hiding of implementation details promotes good software engineering

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static Methods, static Fields and Class Math Sometimes a method performs a task that does

not depend on the contents of any object Method applies to the class in which it’s declared Known as a static method or a class method Place the keyword static before the return type in the

declaration You can call any static method by specifying its

class name, followed by a dot (.) and the method name

All Math class methods are staticEach is called by preceding the name of the method with

the class name Math and the dot (.) separator Method arguments may be constants, variables

or expressions

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static Methods, static Fields and Class Math

Class Math declares commonly used mathematical constantsMath.PI (3.141592653589793) : ratio of a circle’s

circumference to its diameter Math.E (2.718281828459045) : base value for

natural logarithms (calculated with static Math method log)

declared in class Math as public, final and static public allows you to use these fields in your own

classes final indicates a constant—value cannot changestatic allows them to be accessed via the class

name Math and a dot (.) separatorstatic fields are also known as class variables

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static Methods, static Fields and Class MathWhy must main be declared static?

When you execute the Java Virtual Machine (JVM) with the java command, the JVM attempts to invoke the main method of the class you specify

Declaring main as static allows the JVM to invoke main without creating an object of the class

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OutlineIntroduction





Java API Packages

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Example of static methodClass MaximumFinder (Fig. 5.3) has two

methods—main (lines 8–25) and maximum (lines 28–41)

The maximum method determines and returns the largest of three double values

Most methods do not get called automaticallyYou must call method maximum explicitly to

tell it to perform its task

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Methods declarationpublic static double maximum(double x, double y, double z)

{

…

}Method header

modifiers: public, private, staticreturn type: the data type of the value returned by the

method, or void if the method does not return a value. method name: the rules for field names apply to

method names as well, but the convention is a little different.

parameter list in parenthesis: a comma-delimited list of input parameters, preceded by their data types

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Methods declarationpublic static double maximum(double x, double y, double z)

{

double maximumValue=x;

if (y > maximumValue)

maximumValue = y;

if (z > maximumValue)

maximumValue = z;

return maximumValue;

}Method body: enclosed between braces—the

method's code, including the declaration of local variables, goes here.

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Methods Signaturepublic static double maximum(double x, double y, double

z)

{

…

}

Definition: Two of the components of a method declaration comprise the method signature—the method's name and the parameter types. The signature of the method declared above

is: maximum(double, double, double)

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Modifierspublic static double maximum(double x, double y, double z)

{

…

}

public method is “available to the public”Can be called from methods of other classes

static method in the same class can call each other directly Any other class that uses a static method must qualify

method name with class name, e.g., Math.abs (-20.0);For now, we begin every method declaration with

public and static You’ll learn about non-public and non-static methods

later.

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Return type and Parameter listMethod name follows return type

By convention, method names begin with a lowercase first letter and subsequent words in the name begin with a capital letter (e.g., nextInt)

Parameter list: Empty parenthesis: the method does not

require additional information to perform its task

Otherwise, a comma-separated parameter-list: specify one or more parameters represent additional information needed by the methodEach parameter must specify a type and an identifier A method’s parameters are considered to be local

variables of that method and can be used only in that method’s body

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Calling Methodsdouble result = maximum (number1, number2, number3);A method call supplies arguments for each

parameterThere must be one argument for each parameterEach argument must be “consistent with” the

corresponding parameter’s type Arguments are evaluated to determine their

values If an argument is a method call, the method call must

be performed to determine its return value A return statement returns a value (or just

control) to the point in the program from which the method was called

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Return control to callerAfter “returning from a method”, control

resumes at the next statement in the caller … Ways to return (from method body):

When executing return statement: return [expression];With argument: expression is evaluate and the result

is returned to the callerwithout argument : in the method body is executed.

When program flow reaches right brace of the method body.

Syntax error: if void function returns a value, or non-void function does not return a value.

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Scope of Declarations Declarations introduce names that can be

used to refer to classes, methods, variables and parameters

Any block may contain variable declarations Cannot declare a method outside a class

declaration, or inside another method declaration

Scope of a declaration:the portion of the program that can refer to the

declared entity by its name Such an entity is said to be “in scope” for that

portion of the program

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Scope RulesScope of a parameter declaration: the body of

the method in which the declaration appears. Scope of a local-variable declaration: from

the point at which the declaration appears to the end of that block.

Scope of a local-variable declared in initialization section of a for statement’s header: the body of the for statement and the other expressions in the header.

A method or field’s scope: the entire body of the class.

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Scope Rules (cont’d)Shadowing: If a local variable or

parameter in a method has the same name as a field of the class, the field is “hidden” until the block terminates executionAvoid shadowing by use different names !

Declaring a local variable for multiple times leads to compilation error

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OutlineIntroduction





Java API Packages

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Method OverloadingMethod overloading: one can declare multiple

methods of same name in a class, as long as they have different of parameter listUseful for creating several methods that perform

similar tasks on different types or different numbers of arguments

Compiler distinguishes overloaded methods by signaturesA combination of the method’s name and the number,

types and order of its parameters Method calls cannot be distinguished by return type

Internally, compiler uses longer method names: include original method name, types of each parameter, and exact order of parameters, to determine whether methods in a class are unique

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Method OverloadingE.g., Math methods min and max are

overloaded with four versions each:One with two double parametersdouble min (double x, double y)One with two float parametersfloat min (float x, float y); One with two int parametersOne with two long parameters

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Literal integer values are treated as type int

Literal floating-point values are treated as type double

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By default, floating-point values are displayed with six digits of precision if the precision is not specified in the format specifier.

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OutlineIntroduction





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Calling/Invoking MethodsWithin same class, use a method name by

itselfe.g., In main()System.out.printf (“Square of interger 7 is %d\

n”,square(7));To call a method on an object, use the object

variable, followed by a dot (.) and method namex = input.nextInt ();

To call a static method of a class: use class name and a dot (.), followed by method nameMath.abs(-12.0)

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Activation RecordsHow does called method know where to return to ?

i.e., what’s next statement to execute after method return ? For each method invocation, an activation record

(stack frame) is used to keep: return address of the calling methodcontains the memory for the local variables used

in the methodWhen a program calls a method, the activation

record (stack frame) is created and pushed onto program-execution stack

When method returns to its caller, its activation record is popped off stack and those local variables are no longer known to program

If a series of method calls occurs, successive activation records are pushed onto stack in last-in, first-out order

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Stack data structureStack data structure

Analogous to a pile of dishes When a dish is placed on the pile, it’s normally

placed at the top (referred to as pushing onto the stack)

Similarly, when a dish is removed from the pile, it’s always removed from the top (referred to as popping off the stack)

Last-in, first-out (LIFO) data structures—the last item pushed (inserted) on the stack is the first item popped (removed) from the stack

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Argument Promotion and CastingArgument promotion: converting an argument’s

value, if possible, to the type that the method expects to receive

Promotion rules: specify which conversions are allowed—that is, which conversions can be performed without losing data Apply to expressions containing values of two or

more primitive types and to primitive-type values passed as arguments to methods

Each value is promoted to the “highest” type in the expression

These conversions may lead to compilation errors if Java’s promotion rules are not satisfied Use explicit cast in this case

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What if we add x = maximumValue here?Does the actual parameter, number 1, corresponding to x, change its value ?

Java only support pass-by-value: a copy of the parameter is passed to the method. Method works on the copy of the parameter.

OutlineIntroduction





Exercise

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Implement binary search as methodImplement the binary search as a static

methodHow to pass an array as argument ?public static int BinarySearch (int array[], int x)Recall array is a reference object, you can use array.length to retrieve its length

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Sorting Algorithms

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Sorting algorithms are one of the most heavily studied topics in Computer ScienceSorting is critical to improve searching efficiency

There are many well known sorting algorithms in Computer Science, we focus on two:BubbleSort: a very simple but inefficient

sorting algorithmMergeSort: a slightly more complex but

efficient sorting algorithm

BubbleSort Algorithm Overview

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BubbleSort: repeatedly scan the array, in each iteration “bubbles" the largest element in the unsorted part of the list to the end After 1 iteration, largest element in last positionAfter 2 iterations, largest element in last position

and second largest element in second to last position

….requires n-1 iterations

at (n-1)-th iteration, only one item left, must already be in proper position (i.e., the smallest must be in the leftmost position)

BubbleSort Algorithm

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Input: n-element array L Bublesort Algorithm

1 Repeat as i varies from n-1 down to 12 Repeat as j varies from 0 to i – 13 If lj > lj+1 swap lj with lj+1

Outer loop (1-3): i controls which part of the array is scanned for each iteration. (Only unsorted part is checked.)

In 1st iteration, check everything, l0, l1, … ln-2 In 2nd iteration, check everything except last

element, l0, l1, …, ln-3 …

Inner loop (2-3): bubble up largest element in unsorted part of list to the end

BubbleSort Example

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Use BubbleSort to sort list of number (9 2 8 4 1 3) into increasing order.

How many comparisons did you do each iteration? Can you find a pattern?

Implement BubbleSort

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Implement the bubble sort algorithm as a static method Start with document, the method head => the

contract Then worry about implementation detail =>

others do not care them Recall: reference type vs. primitive type

variableReference type: stores address of objects

Scanner input;int scores[ ] = new int[20];

Primitive type: stores the value itselfint a=10;

Recursive algorithmsExperiment with Fibonanci function

F(n)=F(n-1)+F(n-2)F(1)=0F(2)=1

Instrument the code so that it displays each invocation of Fib() method

Lab3: Implement merge sort, a recursive sorting

algorithm.Compare bubble sort with merge sort

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OutlineIntroduction





ExerciseRandom number generator

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Random-Number GenerationThe element of chance can be introduced in a

program via an object of class Random (package java.util) or via the static method random of class Math

Objects of class Random can produce random boolean, byte, float, double, int, long and Gaussian values

Math method random can produce only double values in the range 0.0 £ x < 1.0, where x is the value returned by method random

Random class documentationhttp://java.sun.com/javase/6/docs/api/java/util/Random.

html

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Random-Number Generation (cont.)If truly random, then every value in the range

should have an equal chance (or probability) of being chosen each time nextInt is called

The numbers are actually pseudorandom numbers—a sequence of values produced by a complex mathematical calculation Uses current time of day to seed random-number

generator such that each execution of a program yields a different sequence of random values

Random’s overloaded method nextInt( ) int nextInt(): generates a random int value in the

range –2,147,483,648 to +2,147,483,647, inclusive int nextInt (int max): returns a value from 0 up to,

but not including, the argument’s value

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Generalized Scaling and Shifting of Random Numbersscaling factor—represents the number of

unique values that nextInt should produce shift the range of numbers produced by adding

a shifting valuenumber = shiftingValue +

randomNumbers.nextInt( scalingFactor );

shiftingValue: specifies the first number in the desired range of consecutive integers

scalingFactor: specifies how many numbers are in the range

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Generalized Scaling and Shifting of Random NumbersChoose integers at random from sets of values

other than ranges of consecutive integers, generalized as

number = shiftingValue + differenceBetweenValues * randomNumbers.nextInt( scalingFactor );

where shiftingValue specifies the first number in the desired range of values, differenceBetweenValues represents the constant difference between consecutive numbers in the sequence and scalingFactor specifies how many numbers are in the range

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Random-Number Repeatability for Testing and DebuggingThe calculation that produces random numbers

uses the time of day as a seed value to change the sequence’s starting point

Each new Random object seeds itself with a value based on the computer system’s clock at the time the object is created

Sometimes useful to repeat the exact same sequence of pseudorandom numbers during each executionEnables you to prove that your application is working for a

specific sequence of random numbers before you test the program with different sequences of random numbers

When repeatability is important, you can create a Random object with a seed value as an argument to the constructor

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Case Study: A Game of Chance; Introducing EnumerationsRules for the dice game Craps:

You roll two dice. Each die has six faces, which contain one, two, three, four, five and six spots, respectively. After the dice have come to rest, the sum of the spots on the two upward faces is calculated. If the sum is 7 or 11 on the first throw, you win. If the sum is 2, 3 or 12 on the first throw (called “craps”), you lose (i.e., the “house” wins). If the sum is 4, 5, 6, 8, 9 or 10 on the first throw, that sum becomes your “point.” To win, you must continue rolling the dice until you “make your point” (i.e., roll that same point value). You lose by rolling a 7 before making your point.

Simulates the game of craps, using methods to implement the game’s logic

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Introducing Enumerationsprivate enum Status {CONTINUE, WON, LOST};Type Status is a private member of class Craps

it will be used only in that class Status is an enumeration, Special kind of class

that is introduced by keyword enum and a type name

Declares a set of constants represented by identifiers Braces encloses an enum declaration’s body: a comma-

separated list of enumeration constants, each representing a unique value

The identifiers in an enum must be unique Convention: use only upper case letter

Variables of an enum type can be assigned only the constants declared in enumeration

Use Status.CONTINUE, … rather than 0, 1, ..!

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Case Study: A Game of Chance; Introducing Enumerations (cont.)Why we declare some constants as public final static int rather than as enum constants: Java does not allow an int to be compared to an

enumeration constant Unfortunately, Java does not provide an easy way to

convert an int value to a particular enum constant

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Summary

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4 th Week Spring 2011 Methods 1. Outline Introduction Why method ? Static methods vs. non-static methods Example: Math class Declare a method Signature.

Documents

class math class math

static math method log

boss method

object method

method function calla

method bodies

method callwhen

outlineintroductionwhy