看一個小例子

public class Base {
    public void showMessage()
    {
        System.out.println("this is Base");
    }
}

public class A extends Base {

    public void showMessage()
    {
        System.out.println("this is A");
    }
}

public class B extends Base {

    public void showMessage()
    {
        System.out.println("this is B");
    }
}
 

可以看到每一字繼承Base類的子類如果都需要showMessage這個函式，但是卻每個都重寫了一遍這個函式，無疑就是程式碼冗餘

可以這樣改：

public class Base {
    public static void showMessage(Object obj)
    {
        if(obj instanceof A)
        {
            System.out.println("this is A");
        }

        if(obj instanceof B)
        {
            System.out.println("this is B");
        }
    }
}

public class A extends Base {

}

public class B extends Base {

    public static void main(String[] args)
    {
        B b=new B();
        A a=new A();

        Base.showMessage(a);
        Base.showMessage(b);
    }
}
 

執行結果：