Cannot Instantiate Type in generics

Question

I have this class

public class Tree<T> {

    //List of branches for this tree
    private List<Tree<? super T>> branch = new ArrayList<Tree<? super T>>();
    public Tree(T t){ this.t = t; }
    public void addBranch(Tree< ? super T> src){ branch.add(src); }
    public Tree<? extends T> getBranch(int branchNum){
        return (Tree<? extends T>) branch.get(branchNum);
    }


    private T t;

}

And I am trying to create a variable out of this class using this

public static void main(String[] args){ 
        Tree<? super Number> num2 = new Tree<? super Number>(2);
    }

and it is giving me this error

Cannot instantiate the type Tree<? super Number>

possible duplicate of [Creating new generic object with wildcard](http://stackoverflow.com/questions/9147129/creating-new-generic-object-with-wildcard) — Paul Bellora, Aug 30 '12 at 15:19
Well, that's not an exact duplicate, because that is a Java 7 question. — Erick Robertson, Aug 30 '12 at 15:21
The OP on that post is asking about the compile error - the misconception is the same. — Paul Bellora, Aug 30 '12 at 15:23

score 4 · Accepted Answer · answered Aug 30 '12 at 15:15

4

While instantiating generics should be replaced with corresponding objects.

Ex:

 Tree<Integer> num2 = new Tree<Integer>(2);

answered Aug 30 '12 at 15:15

kosa

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what if I wanted to add a sub type of Trees of Number? like maybe a Tree ? – KyelJmD Aug 30 '12 at 15:16
Exactly, you need to explicitly tell what type you want to add to Tree while Instantiating. Tree class is defined as Generic so that at instantiate time you can tell concrete type. – kosa Aug 30 '12 at 15:18
what I was aiming to do that is the Tree will contain various subclasses of Number. like Tree,Tree and so on. – KyelJmD Aug 30 '12 at 15:20
Then instantiate it using `Tree`. – Erick Robertson Aug 30 '12 at 15:22
@ErickRobertson: Yes exactly, instaintiate with Number and se.e – kosa Aug 30 '12 at 15:24

score 4 · Answer 2 · answered Aug 30 '12 at 15:32

Wildcards ? cannot be used when creating new instances. You should change your code to something like that

import java.util.ArrayList;
import java.util.List;

public class Test1 {
  public static void main(String[] args){
    Tree<? super Number> num2 = new Tree<Number>(2);
    num2.addBranch(new Tree<Number>(1));
    Tree<? super Number> num3 = (Tree<? super Number>) num2.getBranch(0);
    System.out.println(num3);
  }
}

class Tree<T> {

  //List of branches for this tree
  private List<Tree<? super T>> branch = new ArrayList<Tree<? super T>>();
  public Tree(T t){ this.t = t; }
  public void addBranch(Tree<Number> src){ branch.add((Tree<? super T>) src); }
  public Tree<? extends T> getBranch(int branchNum){
    return (Tree<? extends T>) branch.get(branchNum);
  }

  public String toString(){
    return String.valueOf(t);
  }

  private T t;

}

Cannot Instantiate Type in generics

2 Answers2

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