Java FAQ：abstract クラスとは？

Java FAQ：S015 Q-04

abstract class C2008052500a {
    abstract void foo();
    void bar() {
        System.out.println("bar");
    }
}

class C2008052501 extends C2008052500a {
    void foo() {
        System.out.println("foo");
    }
}

public class C2008052500 {
    public static void main(String[] args) {
        new C2008052501().foo();
        new C2008052501().bar();
    }
}

で、

foo
bar