将对象向下转换为 java 中的引用

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.

Answer 1

Object obj = new Reference();

This is an upcasting scenario.

---

If you want downcasting, then it will be,

Reference ref = (Reference) obj;

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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
}