Java Reflection: Creating class instance dynamically and assigning it to Parent object

Question

String child = "C";
Parent p = null;
try {
    Class c1 = new Class.forName(child);
    Constructor co = c1.getConstructor();
    // p=co.newInstance(null); //This gives compilatoin error cannot
    // conver object to Parent
    // p=(c1.getClass())co.newInstance(null);//also gives cast errror
    p = (Parent) co.newInstance(null);// this works but it typecasts the
    // child to Parent
} catch (Exception e) {
}

What am i trying to do.

I have multiple Child classes inherited from Parent. I am getting child class name as string input.

I want to instantiate the object of Child class and assign it to Parent. I do not want to type cast Child to Parent. As later in the code i need to compare two Child classes. If I typecast it to Parent. I cannot differentiate between Child1 and Child2.

How do you compare your child1 and child2?

Lynch
– Lynch

2011-07-08 04:23:16 +00:00
Commented Jul 8, 2011 at 4:23 — Lynch
– Lynch, Commented Jul 8, 2011 at 4:23
You might want to check out Polymorphism.

Gyan
– Gyan

2011-07-08 04:25:24 +00:00
Commented Jul 8, 2011 at 4:25 — Gyan
– Gyan, Commented Jul 8, 2011 at 4:25

Dave Jarvis · Accepted Answer · 2020-12-19 04:20:17Z

4

Typecasting has absolutely no effect on the object itself. Using p = (Parent) t simply does a runtime check on t to make sure that the type of t is assignable to Parent (i.e. either t is-a Parent or it is-a subclass of Parent) . Afterward, t will still be a Child1 or whatever its actual type always has been.

Use the explicit cast.

edited Dec 19, 2020 at 4:20

Dave Jarvis

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answered Jul 8, 2011 at 4:21

Mark Peters

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2 Comments

zookastos Over a year ago

What if we want to use methods of child class in this scenario? How to get reference variable of child class?

Mark Peters Over a year ago

@zookastos: If an instance of the child class is what you need, that's what you should cast to. If you only care that it's some child of a certain parent, cast it to the parent.

aroth · Accepted Answer · 2011-07-08 04:29:09Z

1

You might try something like:

Object parent = null;
String child = String.class.getName();  //not necessary, can just use String.class directly
Class childClass = Class.forName(child);
Class parentClass = Object.class;

if (parentClass.isAssignableFrom(childClass)) {
    parent = childClass.newInstance();
}

System.out.println("Parent is:  " + parent);

answered Jul 8, 2011 at 4:29

aroth

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Comments

Charlee Chitsuk · Accepted Answer · 2011-07-08 04:35:24Z

0

Even the Parent is cast form the 2 different children as

Parent parent1 = (Parent)child1;
Parent parent2 = (Parent)child2;

The parent1 and parent2 are totally different based on each child.

You may see the difference by printing them as

System.out.println(parent1.getClass().getName());
System.out.println(parent2.getClass().getName());

Then you may compare it by using the getName() as well.

I hope this may help to achieve the requirement.

Regards,

Charlee Ch.

answered Jul 8, 2011 at 4:35

Charlee Chitsuk

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