Create subclass object from another subclass object

Question

There are three classes. One of them is parent class, and others are subclasses. For example:

Parent Class: Parent

Subclasses: SubClass1, SubClass2

My question is that how can I convert SubClass1 object into SubClass2 object without typecasting exception ?

Answer 1

You can't, unless you create an is-a relationship between them. Sibling type instances cannot be cast to eachother.

You will have to do something like:

SubClass2 extends Parent
SubClass1 extends Subclass2

Now both inherit from Parent and you can write

Subclass2 obj = new Subclass1();

Or do the reverse.

Answer 2

Actually it's possible!! if you don't care about the lose of data in the new object .

class Parent{
    String A;
    String B;
    public String getA();
    public String setA();
    public String getB();
    public String setB();
}
class SubClass1 extend Parent{
    String C;
    public String getC();
    public String setC();
}

class SubClass1 extend Parent{
    String D;
    public String getD();
    public String setD();
}

public SubClass1 convertTOSubClass2(SubClass2 obj2){
    SubClass1 obj1 = new SubClass1();
    obj1.setA(obj2.getA());
    obj1.setB(obj2.getB());
    ... 
    return obj1;
}

In this case, we simply ignore the C and D . I think it's OK for a few Fields (A,B) to set, but no good for many Fields(AZ) or even Parent has Parent with many fields...

I am also looking for better/elegant solution , something like " copy all fields of parent class object ", because sometimes we do want to reuse the data of the existing object.

== UPDATE ==

Apache BeanUtils : copyProperties(Object,Object) could be helpful.

Answer 3

您不能这样做，因为SubClass1可能具有SubClass2没有的成员， SubClass2 。

Answer 4

Say that the parent is Fruit and the subclasses are Banana and Apple . You are asking how to make a Banana into an Apple ? You can't. A banana isn't an apple. The exception is telling you exactly that.

Answer 5

您可以伪造编译器，但可以确保运行时获得ClassCastException

Answer 6

You can't, and it would not really make sense to. Imagine you parent class being "Animal" and subclass1 an Elephant while subclass2 is a Mouse, what you are asking is for a way to look at the Elephant as if it was a Mouse.