Cannot Instantiate Type in generics

Question

I have this class

public class Tree {

    //List of branches for this tree
    private List> branch = new ArrayList<         


        

Answer 1

While instantiating generics should be replaced with corresponding objects.

Ex:

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

Answer 2

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;

}