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Saturday, July 06, 2013

Returning a Value from a Method

A method returns to the code that invoked it when it
  • completes all the statements in the method,
  • reaches a return statement, or
  • throws an exception (covered later),
whichever occurs first.
You declare a method's return type in its method declaration. Within the body of the method, you use the return statement to return the value.
Any method declared void doesn't return a value. It does not need to contain a return statement, but it may do so. In such a case, a return statement can be used to branch out of a control flow block and exit the method and is simply used like this:
If you try to return a value from a method that is declared void, you will get a compiler error.
Any method that is not declared void must contain a return statement with a corresponding return value, like this:
return returnValue;
The data type of the return value must match the method's declared return type; you can't return an integer value from a method declared to return a boolean.
The getArea() method in the Rectangle Rectangle class that was discussed in the sections on objects returns an integer:
    // a method for computing the area of the rectangle
    public int getArea() {
        return width * height;
This method returns the integer that the expression width*height evaluates to.
The getArea method returns a primitive type. A method can also return a reference type. For example, in a program to manipulate Bicycle objects, we might have a method like this:
public Bicycle seeWhosFastest(Bicycle myBike, Bicycle yourBike,
                              Environment env) {
    Bicycle fastest;
    // code to calculate which bike is 
    // faster, given each bike's gear 
    // and cadence and given the 
    // environment (terrain and wind)
    return fastest;

Returning a Class or Interface

If this section confuses you, skip it and return to it after you have finished the lesson on interfaces and inheritance.
When a method uses a class name as its return type, such as whosFastest does, the class of the type of the returned object must be either a subclass of, or the exact class of, the return type. Suppose that you have a class hierarchy in which ImaginaryNumber is a subclass of java.lang.Number, which is in turn a subclass of Object, as illustrated in the following figure.
The class hierarchy for ImaginaryNumber
The class hierarchy for ImaginaryNumber
Now suppose that you have a method declared to return a Number:
public Number returnANumber() {
The returnANumber method can return an ImaginaryNumber but not an ObjectImaginaryNumber is a Number because it's a subclass of Number. However, an Objectis not necessarily a Number — it could be a String or another type.
You can override a method and define it to return a subclass of the original method, like this:
public ImaginaryNumber returnANumber() {
This technique, called covariant return type, means that the return type is allowed to vary in the same direction as the subclass.