5.24.Calling an Object's Methods
You also use an object reference to invoke an object’s method. You append the method’s simple name to the object reference, with an intervening dot operator (.). Also, you provide, within enclosing parentheses, any arguments to the method. If the method does not require any arguments, use empty parentheses.
The Rectangle class has two methods: getArea() to compute the rectangle’s area and move() to change the rectangle’s origin. Here’s the CreateObjectDemo code that invokes these two methods:
System.out.println(“Area of rectOne: ” + rectOne.getArea());…rectTwo.move(40, 72);
The first statement invokes rectOne’s getArea() method and displays the results. The second line moves rectTwo because the move() method assigns new values to the object’s origin.x and origin.y.
As with instance fields, objectReference must be a reference to an object. You can use a variable name, but you also can use any expression that returns an object reference. The new operator returns an object reference, so you can use the value returned from new to invoke a new object’s methods:
new Rectangle(100, 50).getArea()
The expression new Rectangle(100, 50) returns an object reference that refers to a Rectangle object. As shown, you can use the dot notation to invoke the new Rectangle’s getArea() method to compute the area of the new rectangle.
Some methods, such as getArea(), return a value. For methods that return a value, you can use the method invocation in expressions. You can assign the return value to a variable, use it to make decisions, or control a loop. This code assigns the value returned by getArea() to the variable areaOfRectangle:
int areaOfRectangle = new Rectangle(100, 50).getArea();
Remember, invoking a method on a particular object is the same as sending a message to that object. In this case, the object that getArea() is invoked on is the rectangle returned by the constructor.