The Class as a Container

© 2001 by Ashby M. Woolf Revision 3

The Class as a Container

Possibly a Meta-Container


Objective

• Provide an overview to help you read

code in the book

• Not enough depth to author programs

• We will start with the details next week


java Foo

new

static

new Foo()

public class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } static void main(String[] args){ System.out.println(Foo.getK()); Foo f = new Foo(); System.out.println(f.getI()); }}

Foo.class

javac

Foo.java

f

C:>

"Class" Everybody Should Have Some


public class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } static void main(String[] args){ System.out.println(Foo.getK()); Foo f = new Foo(); System.out.println(f.getI()); }}

Foo.java

OK, So I Cheated

public class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } public static void main(String[] args){ System.out.println(Foo.getK()); Foo f = new Foo(); System.out.println(f.getI()); }}

public class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } public static void main(String[] args){ System.out.println(Integer.toString(Foo.getK())); Foo f = new Foo(); System.out.println(f.getI()); }}

public class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } public static void main(String[] args){ System.out.println(Integer.toString(Foo.getK())); Foo f = new Foo(); System.out.println("k = " + f.getI()); }}

public class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } public static void main(String[] args){ System.out.println(Integer.toString(Foo.getK())); Foo f = new Foo(); System.out.println("" + f.getI()); }}


class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } public static void main(String[] args){ System.out.println("" + Foo.getK()); Foo f = new Foo(); System.out.println("" + f.getI()); }}

class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; }}public class Foo { public static void main(String[] args){ System.out.println("" + Foo.getK()); Foo f = new Foo(); System.out.println("" + f.getI()); }}

Breaking Up is Not Hard to DoFoo.java

class Fe { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; }}public class Foo { public static void main(String[] args){ System.out.println("" + Foo.getK()); Foo f = new Foo(); System.out.println("" + f.getI()); }}

class Fe { int i = 4; int getI() { return i; }}class Fi { static int k = 2; static int getK() { return k; }}public class Foo { public static void main(String[] args){ System.out.println("" + Foo.getK()); Foo f = new Foo(); System.out.println("" + f.getI()); }}

class Fe { int i = 4; int getI() { return i; }}

class Fi { static int k = 2; static int getK() { return k; }}

public class Foo { public static void main(String[] args){ System.out.println("" + Foo.getK()); Foo f = new Foo(); System.out.println("" + f.getI()); }}



public class Foo { public static void main(String[] args){ System.out.println("" + Fi.getK()); Fe f = new Fe(); System.out.println("" + f.getI()); }}


Foo.javaBreaking Up is Not Hard to Do




new Fe.class

Fi.class

Foo.class

javac


Fe.javaBreaking Up is Not Hard to Do


newFe.class

Fi.class

Foo.class

javac

class Fe { int i = 4; int getI() { return I; }}


javac

javac

Fi.java

Foo.java


class Foo { int i = 4; int getI() { return i; } static int k = 2; static int getK() { return k; } public static void main(String[] args){ System.out.println("" + Foo.getK()); Foo f = new Foo(); System.out.println("" + f.getI()); }}

Examples in the Thinking in Java

class Foo { // . . .

// Code to illustrate behavior // . . .

public static void main(String[] args){

Foo f = new Foo(); // Demonstrate f's behavior here }}


Coding Style

• Capitalize the first letter of a class name

• The first letter of embedded words are capitalized– MyVeryOwnClass

• The same for most all other names except lowercase the first letter– myVeryOwnOther


Comments in the Code

// following "//" everything is a comment

/* stuff between the slash asterisk on this line

is comment stuff

and the asterisk slash on this line */


Embedded Documentationand javadoc

/**

@tag argument(s)

@tag argument(s)

@tag argument(s)

*/

/**

<ol>

<li> One

<li> Two

</ol>

*/

Preceding a variable, class or method definition.


Embedded Documentation

• @see: referring to other classes• Class tags

– @version– @author– @since

• Method tags– @param– @return– @throws– @deprecated


The beginning ofthe class definition

What a Class Looks Like

class Foo {

// the stuff that really defines a class goes here

}

The class keyword says this isthe beginning of a class definition

The name of the class Foo

The end ofthe class definition



public class Foo2 extends Foo1 implements That {

// the stuff that really defines a class goes here

}

The public keyword may appear, it saysthis class can be accessed by any other class.

The extends keyword may appear, it says that this class is just like the class Foo1 with the stuff of class Foo2 added.

The implements keyword may appear, it saysthis class meets a specification defined ina thing called an interface named That.

Yes there are other keywordsthat may appear along with class.



public class Fe {

// Fe stuff goes here

}

class Fi {

// Fi stuff goes here

}

class Fo {

// Fo stuff goes here

}

class Fum {

// Fum stuff goes here

}

class Fum {


}

class Fo {


}

class Fi {


}

Public class Fe {


}

=

The order doesn't matter!

File Fe.java File Fe.java


What can you expect to find inside a class?


• More Classes• Comments and Embedded Documentation• Fields Containing (Variables)

– Primitive Values– References to Objects

• Methods, Arguments and Return Values (Subroutines)• Initializers and Constructors (Chap. 4)



public class Fe {


class Fi {


class Fo {


class Fum {


}

}

}

}

Sometimes classes can be inside other classesbut we will discuss that later.

File Fe.java



import java.util.*;

public class Fe {


}

class Fi {


}

class Fo {


}

class Fum {


}

Import Statements Precede any Classes

What is animport statement and

why do I care?

File Fe.java



import java.util.*;

public class Fe {

boolean theEarthIsFlat = false;

int i = 4;

int j, k;

float f = 8.7f;

String s = new String("Hello");

Date d = new Date();

}

Fields (attributes or variables)

File Fe.java



import java.util.*;

public class Fe { boolean theEarthIsFlat; float f; int i ,j, k; String s; Date d; Fe() { theEarthIsFlat = false; i = 4; f = 8.7f; s = new String("Hello"); d = new Date(); }}

Fields (attributes or variables)

File Fe.java


All the Primitives

boolean b; // true, false

char c; // 16 bit Unicode 0 to +2+16-1

byte by; // 8 bit integer -2-7 to +2+7-1

short s; // 16 bit integer -2-15 to +2+15-1

int i; // 32 bit integer -2-31 to +2+31-1

long l; // 64 bit integer -2-63 to +2+63-1

float f; // 32 bit floating point IEEE754 Standard

double d;// 64 bit floating point IEEE754 Standard



Wrapper Classes for the Primitives

Wrapper Class PrimitiveBoolean b = new Boolean(true); // boolean

Character c = new Character(‘c’); // char

Byte by = new Byte(20); // byte

Short s = new Short(500); // short

Integer i = new Integer(50000); // int

Long l = new Long(1000000000); // long

Float f = new Float(23.45f); // float

Double d = new Double(234.567f); // double


All the Classes

String s;// String a string of Unicode characters

// s is a reference to a String object

Date d; // Date a date/time object

// d is a reference to a Date object

• About 1500 other classes included in the JDK from Sun,• Plus all the classes you and your friends define, and• All the classes available from third party providers.



Creating and Referencing Objects

String s; // s is now a String reference

// set to null

s = new String(“Hello”); // A String object is created

// and s is set to reference

// the new object

OR

String s = new String(“Hello”); // All in one line

OR

String s = “Hello”; // A special shorthand

// allowed for String only.

System.out.println(s); // Using the reference to

// print the object.

Using String as an Example


Java has Arrays

k[0] = 10

k[1] = 20

k[2] = 30

k[3] = 40

int[] k; // Makes k a reference to an array of ints

int k[]; // Equivalent to int[] k;

outputs

A Simple Example Now (a whole chapter later)

int k[] = {10, 20, 30, 40};

System.out.println("k[0] = " + k[0]);





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

}}

int k[] = {10, 20, 30, 40};





Java has ArraysLets Make a Complete Program Out Of It

outputs

File: SimpleArrayThing.java

k[0] = 10

k[1] = 20

k[2] = 30

k[3] = 40


public class SimpleArrayThing {

public static void main(String[] args) {

}

}

String[] k = {"ten", "twenty", "thirty", "forty"};





Java has Arrays

k[0] = ten

k[1] = twenty

k[2] = thirty

k[3] = forty

How About Objects

outputs

File: SimpleArrayThing.java


// import here if needed

public class MyClassName {

public static void main(String[] args) {

}

}

//

// This is where you try simple declarations

// and executable statements.

//

// Then print the results.

System.out.println("variable = " + variable);

A Static Place for Simple Testing

File: MyClassName.java


The Staff of Hotel Java Cleans up abandoned Objects

String s;

s = "Hello";

s = "Goodbye";


Scoping References and Object Lifetimes

/* Stuff between these things are comments */

{ int i = 42;

/* only i available */

{ int j = 100;

String s = new String("Hi There");

/* i, j, and s are available */

} /* only i available */

/* j and s out of scope */

/* What happens to "Hi There"? */

}


Summary

• Java Programming Centers Around Classes

• Classes contain data and methods

• Static members have a single identity

• The "new" keyword produces a new Object

of the Class


Exercises

• Enter the first Foo class and compile and run it• Modify Foo to have separate Fe and Fi classes

compile and run.• Make Foo, Fe and Fi into separate files, compile and

run.• Do exercise 9 page 131. (see also page 128)


End of Content

The Class as a Container

Documents