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How to Define Methods with Behavior: Java

Objects have behavior that is implemented by its methods. Other objects can ask an object to do something by invoking its methods. This section tells you everything you need to know about writing methods for your Java classes.

In Java, you define a class’s methods in the body of the class for which the method implements some behavior. Typically, you declare a class’s methods after its variables in the class body although this is not required.

Implementing Methods

Similar to a class implementation, a method implementation consists of two parts: the method declaration and the method body

methodDeclaration {
        methodBody
}

The Method Declaration

At minimum, a method declaration has a name and a return type indicating the data type of the value returned by the method:

returnType methodName( ) {
. . .
}

This method declaration is very basic. Methods have many other attributes such as arguments, access control, and so on.

Objects as Instances of Class

A class defines only a blueprint and its concrete version comes into effect only through objects that implement the functionality as defined by class. Recall our class example of City class. The objects created from this class will have two variables: name and population; and they will be able to represent cities. The object of City class will also have a method namely display ( ). An object of a class is typically named by a variable of the class type. For example, the program CityTrial in Example 4.7 declares the two variables metl1 and metro2 to be of type City, as follows;

City metro1, metro2;

This gives us variables of the class City, but so far there are no objects of the class. Objects are class value that are named by the variables. To obtain an object you must use the new operator to create a “new” object. For example, the following creates an object of the class City and names it with the variable metro1:

Metro = new city ( );

For now you need not go into details, simply note that the statement like:

Class-variable = new class-Name ( );

Creates a new object of the specified class and associates it with the class type variables. Since the class variable now names an object of the class, we will often refer to the class variable as an object of the class. (This is really the same usage as when we refer to an int variable n as “the integer n”, even though the integer is strictly speaking not n but the value of n.)

Unlike what we did in previous lines, the declaration of a class type variable and the creation of the object are more typically combined into one statement as follows:
City metro1 = new City( );
To instantiate an object, Java uses the keyword new.

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