泛型Java超级

Question

下面代码中的listNum.add(数字)有什么问题；(Reference- http://docs.oracle.com/javase/tutorial/java/generics/lowerBounded.html)

它给出了编译错误，因为方法add(捕获#1-of？超长)不适用于参数(数字)

public class GenericSuper {

   List<? super Long> listNum = new LinkedList < Number >();
   List<? super ExportException> listExp= new LinkedList<RemoteException>();

   public List<? super ExportException> addList()
   {
      Number num = 10;
      listNum.add(num);
      RemoteException rme = new RemoteException();
      listExp.add(rme);
      return rme;
   }
}

Answer 1

listNum可能是List<Long>的一个实例，您不能将Number添加到Long列表中，因为它会抛出类类型转换异常。

解决方案：

Long

1. make a listNum a
   1. alistNuma
      1. alistNuma
         1. alistNuma
            1. aLong

Answer 2

package main.java.com.mysystems.generics;

import java.rmi.RemoteException;
import java.rmi.server.ExportException;
import java.rmi.server.SocketSecurityException;
import java.util.LinkedList;
import java.util.List;

//peCS
//Consumer super- you want to add item i.e of super type to the collection.
public class GenericSuper {


    List<? super Long> listNum = new LinkedList < Number >();// listNum is variable that can be assigned list of type super class to Long.
    //And you can put Long and its subclass. And you can only get Object type since it can hold anything from Long to Object.

    List<? super ExportException> listExp= new LinkedList<RemoteException>(); // listExp is variable that can be assigned any list of type super class to ExportException. 
    // And you can  put ExportException and all subclasses of ExportException to the list. And you can only get Object type since it can hold anything from ExportException to Object.

    public List<? super ExportException> addList()
    {
        LinkedList < Number > numList = new LinkedList < Number >();

        Number num = 10.10;
        numList.add(num);
        listNum = numList;
        //listNum.add(num); //Compilation error as only long or its subclass can be added.
        listNum.add(20L);

        for(Object e:listNum){ // Note you can only get Object type and will have to use instance of to do anything meaningful.
            System.out.println(e);
        }
        SocketSecurityException sse = new SocketSecurityException("Hello");
       listExp.add(sse);
       return listExp;
       }
   }

Answer 3

问题不在于泛型的实现，而在于它是一个基本的Java Collection框架错误，在这个错误中你不能将Number类型添加到Long type.Generics中，只是在编译time.Had泛型时不在这个地方解决它，你会得到运行时异常，即ClassCastException。