Downcasting Objects to references in java

Question

I am working on an assignment and don't understand downcasting. For example, class Reference.java and I'm in a method that passes an object in. If that object is of type (not sure if that is the right word) then create a new Reference object. This is what I have right now but it doesn't work.

public method(Object obj){
     if( obj instanceof Reference){
          Object obj = new Reference();
     }
}

Now the constructor for Reference.java needs two parameters and I also don't understand how in the world this would work when downcasting.

`Object obj = new Reference();` should probably be `Reference ref = (Reference) obj;` — Lino, Apr 01 '21 at 20:53
You can't declare 2 variables (including parameters) with the same identifier in the same scope. You already have something called `obj` so use another name. — Michael, Apr 01 '21 at 20:55

score 2 · Accepted Answer · answered Apr 02 '21 at 01:27

Object obj = new Reference();

This is an upcasting scenario.

If you want downcasting, then it will be,

Reference ref = (Reference) obj;

When it works/when it is possible,

Suppose, you create an Object class instance using Reference class (child class reference)

Object obj = new Reference();

This is an upcasting. And you can do that,

In this case, you can downcast without any ClassCastException

Reference ref = (Reference) obj;

But when it is not clear to you, is it Cast safe or not? This time instanceof helps you. So the full working code should be-

class Reference extends Object{ //not necessary extends Object class, but for 
                                //clearification
  ...

  public static void method(Object obj){
    if(obj instanceof Reference){
      Reference ref = (Reference) obj;
    }
  }
}

In the main method,

public static void main (String [] args) {  
    Object _obj = new Reference();  
    Reference.method(_obj);  
    //downcast successfull
}

Thank you for explaining this. – Anthony Apr 04 '21 at 19:35 — Anthony, Apr 04 '21 at 19:35

Downcasting Objects to references in java

1 Answers1