How to cast parent into child in Java

Question

I am doing this:

Child child = (Child)parent;

Which gives me an error, I found it isn't possible to do it like this. I don't know exactly why, but I think it should be possible, if Child class inherits from Parent class, it contains the Parent object data.

My questions are:

• Why it doesnt work?

• How can i make this work, without setting every single parent's attribute like this

:

class Parent{
    public int parameter1;//...
    public int parameter1000;
}

class Child extends Parent
{
    public Child(Parent parent)
{
        this.parameter1 = parent.parameter1;//...
        this.parameter1000 = parent.parameter1000;
}
}

Answer 1

Do you really want to cast you parent into the child class? If the given Object (given as a Parent object) was created as a child class, you can access the correct functions without the cast. Here is an example: In the main you see a Vector 'ps' to store Parent objects, but added are Child objects which extend from the Parent. While iterateing through that vector, I dont know which kind of child I get next (thus I cannot cast to the correct class), but due to the keyword Override of the function I call, the correct child/method is used.

import java.util.Vector;
import lib.Parent;
import lib.C1;
import lib.C2;

public class MyClass {
    public static void main(String []args){
        System.out.println("Hello World");
        C1 c1 = new C1();
        C2 c2 = new C2();
        
        ps.add(c1);
        ps.add(c2);
        
        for(int k = 0; k < ps.size(); k++){
            ps.get(k).setInteger(); // no cast needed
            System.out.println("stored entity " + ps.get(k).i);
        }
        // or use a ranged for loop
        for(Parent p: ps){
            p.setInteger();
            System.out.println("stored entity " + p.i);
        }
    }
     
    static Vector<Parent> ps = new Vector<>();
}

The Parent class: The keyword abstract forces every child to implement that method, so if you have a Parent object you can safely call this method. Since the Parent class itself is abstract you wont have an object which is just a Parent object since that is not allowed and wont compile.

package lib;
public abstract class Parent{
    public Integer i;
    
    public abstract void setInteger();
}

And here are two Child classes which have an different implementation of the setInteger() method.

package lib;
import lib.Parent;

public class C1 extends Parent{
    
    @Override
    public void setInteger(){
        i = new Integer(1);
    }
}

Second Child:

package lib;
import lib.Parent;

public class C2 extends Parent{
     
    @Override
    public void setInteger(){
        i = new Integer(2);
    }
}

The Output is

stored entity 1
stored entity 2
stored entity 1  <-- ranged for
stored entity 2  <-- ranged for

Edit: The file structre looks like this

|-MyClass.java
|-lib
  |-Parent.java
  |-C1.java
  |-C2.java

Answer 2

Well you could just do :

Parent p = new Child();
// do whatever
Child c = (Child)p;

Or if you have to start with a pure Parent object you could consider having a constructor in your parent class and calling :

class Child{
    public Child(Parent p){
        super(p);
    }
}
class Parent{
    public Parent(Args...){
        //set params
    }
}

Or the composition model :

class Child {
    Parent p;
    int param1;
    int param2;
}

You can directly set the parent in that case.

You can also use Apache Commons BeanUtils to do this. Using its BeanUtils class you have access to a lot of utility methods for populating JavaBeans properties via reflection.

To copy all the common/inherited properties from a parent object to a child class object you can use its static copyProperties() method as:

BeanUtils.copyProperties(parentObj,childObject);

Note however that this is a heavy operation.

Answer 3

change 'this' to 'super' in your child constructor, and remove the parent parameter, instead replace that with the two parameters int parameter1;//...int parameter1000; :)

class Parent{
public int parameter1;//...
public int parameter1000;
}

class Child extends Parent
{
    public Child(int parameter1, int parameter1000)
    {
        super.parameter1 = parameter1
        super.parameter1000 = parameter1000;
    }
}