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Returning a Value from a Method

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 and cannot contain a return statement. Any method that is not declared void must contain a return statement.

Let's look at the isEmpty method in the Stack class:

public boolean isEmpty() {
    if (items.size() == 0) {
        return true;
    } else {
        return false;
    }
}

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 declared return type for the isEmpty method is boolean, and the implementation of the method returns the boolean value true or false, depending on the outcome of a test.

The isEmpty method returns a primitive type. A method can return a reference type. For example, Stack declares the pop method that returns the Object reference type:

public Object pop() {
    if (top == 0) {
        throw new EmptyStackException();
    }
    Object obj = items[--top];
    items[top]=null;
    return obj;
}

When a method uses a class name as its return type, such as pop 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.

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 Object. ImaginaryNumber is a Number because it's a subclass of Number. However, an Object is not necessarily a Number — it could be a String or another type.

You also can use interface names as return types. In this case, the object returned must implement the specified interface.

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