7 copyright © 2005, oracle. all rights reserved. creating classes and objects

7Copyright © 2005, Oracle. All rights reserved.

Creating Classes and Objects

7-2 Copyright © 2005, Oracle. All rights reserved.

Objectives

After completing this lesson, you should be able to do the following:

• Define instance variables and methods

• Define the no-arg (default) constructor method

• Instantiate classes and call instance methods

• Perform encapsulation by using packages to group related classes

• Control access with public and private access modifiers

• Use class variables and methods


Using Java Classes

Packages

Methods Objects

Objectreferences

Attributes

Contained in a class


title: “Gone with…”rating: “PG”

title: “Last Action…”rating: “PG-13”

Comparing Classes and Objects

• An object is an instance of a class.

• Objects have their own memory.

• Class definitions must be loaded to create instances.

public void displayDetails()

private String title;

private String rating;

public void setRating()

Movie

mov1 mov2


Creating Objects

• Objects are typically created by using the new operator:

• For example, to create two Movie objects:

Movie mov1 = new Movie("Gone ...");

Movie mov2 = new Movie("Last ...");

ClassName objectRef = new ClassName();


title: “Last Action…”rating: “PG-13”


Using the new Operator

The new operator performs the following actions:

• Allocates and initializes memory for the new object

• Calls a special initialization method in the class, called a constructor

• Returns a reference to the new object

Movie mov1 = new Movie("Gone with…");

mov1(When instantiated)



Comparing Primitives and Objects

Primitive variables hold a value.

int i;

int j = 3; Movie mov1 = new Movie();

Object variableshold references.

title: nullrating: null

mov1

Movie mov1;

mov10

3

i

j

null


Using the null Reference

• A special null value may be assigned to an object reference, but not to a primitive.

• You can compare object references to null.

• You can remove the association to an object by setting the object reference to null.

Movie mov1; //Declare object reference

…

if (mov1 == null) //Ref not initialized?

mov1 = new Movie(); //Create a Movie object

…

mov1 = null; //Forget the Movie object


Assigning References

Assigning one reference to another results in two references to the same object:

Movie mov1 = new Movie("Gone...");

mov1

Movie mov2 = mov1;

mov2





Declaring Instance Variables

Instance variables are declared within the class, but outside the methods or instance or static intializers.

public class Movie {

public String title;

public String rating;

public float getPrice(){

return price;

}

}

Movie mov1 = new Movie();


mov2

mov1

Create movies:


Accessing public Instance Variables

public instance variables can be accessed by usingthe dot operator:


public String title;

public String rating;

…

}Movie mov1 = new Movie();

mov1.title = "Gone ...";

…

if (mov1.title.equals("Gone ... ") )

mov1.rating = "PG";


Defining Methods

A method in Java is equivalent to a function or subroutine in other languages.

modifier returnType methodName (argumentList) {

// method body

…

};


Calling a Method

Objects communicate by using messages:

• All methods are defined within a class and are not defined globally as in traditional languages.

• When you call a method, it is always in the context of a particular object.– myPen.write( ): Object-oriented programming– Write (myPen): Traditional structured

programming


Specifying Method Arguments: Examples

• Specify the number and type of arguments in the method definition:

• If the method takes no arguments, then leave the parentheses empty:

public void displayDetails() {

System.out.println("Title is " + title);

System.out.println("Rating is " + rating);

}

public void setRating(String newRating) {

rating = newRating;

}


Returning a Value from a Method

• Use a return statement to exit a method and to return a value from a method:

• If the return type is void, then no return is needed.

• You can use a return without a value to terminate a method with a void return type.

public class Movie { private String rating; … public String getRating () { return rating }}


Calling Instance Methods


private String title, rating;

public String getRating(){

return rating;

}

public void setRating(String newRating){

rating = newRating;

}

}Movie mov1 = new Movie();

String r = mov1.getRating();

if (r.equals("G")) … Use the dot operator:


Applying Encapsulation in Java

• Instance variables must be declared as private.

• Only instance methods can access private instance variables.

• private decouples the interface of the class from its internal operation.


String rating = mov1.getRating();

String r = mov1.rating; // error: private

...

if (rating.equals("G"))

var

aMethod

aMethod()


Passing Primitives into Methods

When a primitive or object reference value is passed into a method, a copy of the value is generated:

public void aMethod(int arg) {

if (arg < 0 || arg > 100)

arg = 0;

System.out.println("arg: " + arg);

}

int num = 150;

anObj.aMethod(num);

System.out.println("num: " + num);

arg150

num150



Passing Object References into Methods

When an object reference is passed into a method, the object is not copied but the pointer to the object is copied:

public void aMethod(Movie ref2) {

ref2.setRating("R");

}

mov1

ref2

Movie mov1 =

new Movie("Gone…");

mov1.setRating("PG");

anObj.aMethod(mov1);


What Are Class Variables?

Class variables:• Belong to a class and are common to all instances

of that class• Are declared as static in class definitions


private static double minPrice; // class var

private String title, rating; // inst vars

Movie class variable Movie objects

titlerating

titlerating

titlerating

minPrice


Initializing Class Variables

• Class variables can be initialized at declaration.

• Initialization takes place when the class is loaded.

• Use a static initializer block for complex initialization.

• All class variables are initialized implicitly to default values depending on data type.


private static double minPrice = 1.29;

private String title, rating;

private int length = 0;


What Are Class Methods?

Class methods are:

• Shared by all instances

• Useful for manipulating class variables

• Declared as static

A class method is called by using the name of the class or an object reference.

public static void increaseMinPrice(double inc) {

minPrice += inc;

}

Movie.increaseMinPrice(.50);

mov1.increaseMinPrice(.50);


Guided Practice: Class Methodsor Instance Methods


private static float price = 3.50f; private String rating; … public static void setPrice(float newPrice) { price = newPrice; } public String getRating() { return String; }}

Movie.setPrice(3.98f); Movie mov1 = new Movie(…);mov1.setPrice(3.98f);String a = Movie.getRating();String b = mov1.getRating();Legal or not?


Examples in Java

Examples of static methods and variables:• main()• Math.sqrt()• System.out.println()

public class MyClass {

public static void main(String[] args) { double num, root; … root = Math.sqrt(num); System.out.println("Root is " + root); } …


Creating Classes Using the Class Editor


What Are Java Packages?

oe

Customer Order Util

OrderEntry OrderItem


Grouping Classes in a Package

• Include the package keyword followed by the package name at the top of the Java source file. Use the dot notation to show the package path.

• If you omit the package keyword, then the compiler places the class in a default “unnamed” package.

• Use the –d flag with the javac compiler to create the package tree structure relative to the specified directory.

• Running a main() method in a packaged class requires:– That the CLASSPATH contains the directory having

the root name of the package tree– That the class name must be qualified by its

package name


Setting the CLASSPATH with Packages

The CLASSPATH includes the directory containing the top level of the package tree:

Package name .class location

C:\>set CLASSPATH=E:\Curriculum\courses\java\les06CLASSPATH


Access Modifiers

private

protected

acmevideo acmetools

public public


Summary

In this lesson, you should have learned the following:

• A class definition specifies a template for building objects with identical features, such as instance variables and methods.

• An object is an instance of a particular class.– Create an object by using new.– Manipulate an object by using its public instance

methods.


Practice 7: Overview

This practice covers:

• Defining new classes

• Specifying the classes’ instance variables and instance methods

• Creating Customer objects in main()• Manipulating Customer objects by using public

instance methods

7 copyright © 2005, oracle. all rights reserved. creating classes and objects

Documents

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