(七)『Javaプログラミング思想』——多態の欠陥

1.プライベートメソッドを上書きできません

package chapter8;



/**

 *  " " 

 */

public class PrivateOverride {

    private void f() {

        System.out.println("private f()");

    }



    public static void main(String[] args) {

        PrivateOverride po=new Derived();

        po.f();

        Derived d=new Derived();

        d.f();

    }

}



class Derived extends PrivateOverride {

    public void f() {

        System.out.println("public f()");

    }

}

【運転結果】:private f()public f()
2.ドメインがマルチステートでない(通常の方法のみがマルチステートである)

/**

 * 

 */

package chapter8;



/**

 *  ： 

 */

class Super {

    public int field = 0;



    public int getField() {

        return field;

    }

}



class Sub extends Super {

    public int field = 1;



    public int getField() {

        return field;

    }



    public int getSuperField() {

        return super.field;

    }

}



public class FieldAccess {

    public static void main(String[] args) {

        Super sup = new Sub();

        System.out.println("sup.field = " + sup.field + " sup.getField() = "

                + sup.getField());

        Sub sub = new Sub();

        System.out.println("sub.field = " + sub.field + " sub.getField() = "

                + sub.getField() + " sub.getSuperField = "

                + sub.getSuperField());

    }

}

【運転結果】:
sup.field = 0 sup.getField() = 1sub.field = 1 sub.getField() = 1 sub.getSuperField = 0
3静的方法は多態性を呈しない

package chapter8;



/**

 *  

 */

class StaticSuper {

    public static String StaticGet() {

        return "Base staticGet()";

    }



    public String dynamicGet() {

        return "Base dynamicGet()";

    }

}



class StaticSub extends StaticSuper {

    public static String StaticGet() {

        return "Derived staticGet()";

    }



    public String dynamicGet() {

        return "Derived dynamicGet()";

    }

}



public class StaticPolymorphism {

    public static void main(String[] args) {

        StaticSuper sup = new StaticSub();

        System.out.println(sup.StaticGet());

        System.out.println(sup.dynamicGet());

    }

}

【運転結果】:
Base staticGet()Derived dynamicGet()