More Features of the Java Language

Overriding Methods

The ability of a subclass to override a method in its superclass allows a class to inherit from a superclass whose behavior is "close enough" and then override methods as needed.

For example, all classes are descendents of the Object class. Object contains the toString method, which returns a String object containing the name of the object's class and its hash code. Most, if not all, classes will want to override this method and print out something meaningful for that class.

Let's resurrect the Stack class example and override the toString method. The output of toString should be a textual representation of the object. For the Stack class, a list of the items in the stack would be appropriate

public class Stack
{
    private Vector items;

    // code for Stack's methods and constructor not shown

    // overrides Object's toString method
    public String toString() {
        StringBuffer result = new StringBuffer();
        result.append("[");
        for (int i = 0; i < n; i++) {
            result.append(items.elementAt[i].toString());
            if (i == n-1) result.append("]");
            else result.append(",");
        }
        return result.toString();
    }
}

The return type, method name, and number and type of the parameters for the overriding method must match those in the overridden method. The overriding method can have a different throws clause as long as it doesn't declare any types not declared by the throws clause in the overridden method. Also, the access specifier for the overriding method can allow more access than the overridden method, but not less. For example, a protected method in the superclass can be made public but not private.

Calling the Overridden Method

Sometimes, you don't want to completely override a method. Rather, you want to add more functionality to it. To do this, simply call the overridden method using the super keyword. For example, Stack's implementation of finalize (which overrides Object's finalize method) should include any finalization done by the Object class:

protected void finalize() throws Throwable {
    items = null;
    super.finalize();
}

Methods a Subclass Cannot Override

A subclass cannot override methods that are declared final in the superclass (by definition, final methods cannot be overridden). If you attempt to override a final method, the compiler displays an error message similar to the following and refuses to compile the program:

FinalTest.java:7: Final methods can't be overridden. Method void iamfinal() is final in class ClassWithFinalMethod.
    void iamfinal() {
         ^
1 error

Also, a subclass cannot override methods that are declared static in the superclass. In other words, a subclass cannot override a class method.

Methods a Subclass Must Override

A subclass must override methods that are declared abstract in the superclass, or the subclass itself must be abstract. The upcoming Writing Abstract Classes and Methods section discusses abstract classes and methods in detail.

More Features of the Java Language