如何安全返回通用对象(How to safely return generic objects)

 我正在做一些实验，试图了解返回泛型的问题。 以下程序运行无误，并在结尾打印“FOO gets BAR”。 有人可以解释为什么吗 GetMap（）成员将HashMap <Integer，Integer>不安全地转换为Map <K，V>，在测试用例中是Map <String，String>。 我所有的阅读建议我应该得到一个ClassCastException，但我没有得到一个。 
 
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class GenericTrial
{
 public static <K,V> Map<K,V> GetMap()
 { 
  return (Map<K,V>)new HashMap<Integer,Integer>();
 }

    public static void main(String[] args)
    {
     try
     {
      Map<String,String> m = GetMap();
      m.put("FOO", "BAR");
      System.out.println("FOO gets " + m.get("FOO"));
     }
     catch (Exception e)
     {
      System.out.println("Got exception");
     }
    }
}

I'm doing some experiments, trying to understand the issues surrounding returning generics. The following program runs without error and prints "FOO gets BAR" at the end. Can anyone explain why? The GetMap() member has unsafely cast a HashMap<Integer,Integer> to a Map<K,V>, which in the test case is a Map<String,String>. All my reading suggests I should get a ClassCastException, but I'm not getting one.
 
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class GenericTrial
{
 public static <K,V> Map<K,V> GetMap()
 { 
  return (Map<K,V>)new HashMap<Integer,Integer>();
 }

    public static void main(String[] args)
    {
     try
     {
      Map<String,String> m = GetMap();
      m.put("FOO", "BAR");
      System.out.println("FOO gets " + m.get("FOO"));
     }
     catch (Exception e)
     {
      System.out.println("Got exception");
     }
    }
}

原文：https://stackoverflow.com/questions/4168208