無法實例化泛型中的類型

Question

我有這門課

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;

}

我正在嘗試使用此類創建一個變量

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

它給了我這個錯誤

Cannot instantiate the type Tree<? super Number>

Answer 1

雖然實例化泛型應該用相應的對象替換。

例如：

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

Answer 2

通配符? 創建新實例時無法使用。 您應該將代碼更改為類似的代碼

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;

}