java subclass have method to return its class

Question

Ok, maybe this is a stupid question. But i'm just wondering if this can be done in java.

abstract public class ParentClass<T> { 
  abstract public T getTest();
}

in the subclass

public class SubClass extends ParentClass<MyObject> {
  public MyObject getTest() {
    // I can return the object with class MyObject
    return null;
  }
}

My question is can I return the class type in the child method? I mean, is it can be done by adding some code in the ParentClass, so I can do this below?

For example

public class Sub1Class extends parentClass<Object1> {
  public Object1 getTest() { }
  // I want to have a method that return it's class in the superclass
  public Sub1Class getItClassObject() { }
}

other example

public class Sub2Class extends parentClass<Object2> {
  public Object2 getTest() { }
  // I want to have a method that return it's class in the superclass
  public Sub2Class getItClassObject() { }
}

one example again

public class Sub3Class extends parentClass<Object3> {
  public Object3 getTest() { }
  // I want to have a method that return it's class in the superclass
  public Sub3Class getItClassObject() { }
}

if you see, method getItClassObject in Sub1Class, Sub2Class and Sub3Class will follow it's class. But I don't want to add same method for every subclass, just want to add some code (if feasible) in the ParentClasss, so in the subclass, I just can call getItClassObject directly without write all the code in every subclass.

Usually I add method in ParentClass like this.

abstract public class ParentClass<T> {
  abstract public T getTest();
  public Object getItClassObject() { }
}

so in the subclass I just instance the class, but I have to cast again :(

Sub1Class sub1Class = new Sub1Class();
Sub1Class after1Cast = (Sub1Class) sub1Class.getItClassObject();

Sub2Class sub2Class = new Sub2Class();
Sub2Class after2Cast = (Sub2Class) sub2Class.getItClassObject();

I think it cannot be done in java. But I don't know if there is a clue to solve this. Thanks

Answer 1

This is what you want I think. The following compiles:

abstract class A {
    public abstract A getA();
}

class B extends A {
    // Declared to return a B, but it still properly overrides A's method
    @Override
    public B getA() {
        return new B();
    }
}

class C extends A {
    // Declared to return a B, but it still properly overrides A's method
    @Override
    public C getA() {
        return new C();
    }
}

As you can see, A declares that the getA() method returns an A . But, you can restrict the return type in subclasses as shown.

Answer 2

I'm not sure if I understand your intent correctly, but I think the built-in Object.getClass() method will do what you want. Given classes defined as:

public abstract class ParentClass<T> {
  public abstract T getTest();
}

class SubClassString extends ParentClass<String> {
   public String getTest() {
      return "";
   }
}

class SubClassInteger extends ParentClass<Integer> {
   public Integer getTest() {
      return Integer.valueOf(0);
   }
}

getClass() will return the correct run-time class

  public static void main(String[] args) {
      SubClassString subString = new SubClassString();
      // displays "class SubClassString"
      System.out.println(subString.getClass()); 

      SubClassInteger subInteger = new SubClassInteger();
      // displays "class SubClassInteger"
      System.out.println(subInteger.getClass());

      ParentClass<?> parentInstance = new SubClassInteger();
      // displays "class SubClassInteger"
      System.out.println(parentInstance.getClass());  
  }

Answer 3

The only way I can think of is by telling the parent class what the subclass is when you extend it (just like you did with 'T'). Eg:

public abstract class ParentClass<T,U> { 
    abstract public T getTest();
    abstract public U getItClassObject();
}

They you define your subclass like so:

public class Sub1Class extends ParentClass<Object1,Sub1Class> {
    public Object1 getTest() { }
    public Sub1Class getItClassObject() { }
}

Then you can do what you want without the typecast:

Sub1Class sub1Class = new Sub1Class();
Sub1Class after1Cast = sub1Class.getItClassObject();

Answer 4

If your objects have no-arg constructors (or some consistent form of constructor across all of them), you can use reflection to do it. Some pseudocode would be

public class MyClass {

    public MyClass instantiateType() {
       Class<?> actualClass = getClass();
       return actualClass.newInstance();
    }

}

This is using the runtime type of the class, so subclasses will return their type. This works only for a no-arg constructor though.