子实例从父类实例化

Question

It is possiable to instantiate child object from parent class without child class name.

For example, I have next classes:

public class A {
   protected int a;

   public A1() {
       a = 0;
   }

   public int getA() {
       return a;
   }

   public static A getObject() {
       // some code which I need to write
   }
}

public class A1 extends A {
    public A1() {
        a = 5;
    }
}

public class A2 extends A {
    public A2() {
        a = 10;
    }
}

Usage example:

A a = A1.getObject();
a.getA(); // return 5

a = A2.getObject();
a.getA(); // return 10

a = A.getObject();
a.getA(); // return 0

A1, A2 it is not all child classes. There are may be unlimited numbers.

How can I write getObject() method to it creates A class childs instanses.

PS:

I just need to initialize child class object, but there are large amounts of child classes and I would not to call "new" for all of them and I would not to write static method to initialize it, also, all childs have same constructors. It is big lack that I can't create child instance from parent.

Answer 1

When you write A a = A1.getObject() , you do use child classname (A1). So a) your question is misleading and b) why can't you just write A a = new A1() ?

If you want to write A1.getObject() , then you can redefine getObject() in class A1:

public class A1 extends A {
   public static A getObject() {
     return new A1();
   }

}

Without redefining, there is no way to declare getObject() in class A so that it return objects different classes because A a = A1.getObject() would compile to A a = A.getObject() .

Answer 2

class A
{
   protected int a;

   public A()
   {
       a = 0;
   }

   public int getA()
   {
       return a;
   }

   public static A getObject(Class c) throws Exception
   {
        A obj = (A)c.newInstance();
    return obj;     
   }
}

class A1 extends A
{
    public A1()
    {
        a = 5;
    }
}

class A2 extends A
{
    public A2()
    {
        a = 10;
    }
}
class Test{
    public static void main(String args[]) throws Exception
    {
        A a = A1.getObject(A1.class);
        System.out.println(a.getA()); // return 5

        a = A2.getObject(A2.class);
        System.out.println(a.getA()); // return 10

        a = A.getObject(A.class);
        System.out.println(a.getA()); // return 0
    }
}