例：

class Foo {
    String myFoo;
    public Foo(){}
    public Foo(Foo old) {
        this.myFoo = old.myFoo;
    }
}

class Bar extends Foo {
    String myBar;
    public Bar(){}
    public Bar(Bar old) {
        super(old); // copies myFoo
        this.myBar = old.myBar;
    }
}

class Copier {
    Foo foo;

    public Foo makeCopy(Foo oldFoo) {

        // this doesn't work if oldFoo is actually an
        // instance of Bar,because myBar is not copied
        Foo newFoo = new Foo(oldFoo);
        return newFoo;

        // unfortunately,I can't predict what oldFoo's the actual class
        // is,so I can't go:
        // if (oldFoo instanceof Bar) { // copy Bar here }
    }
}