在Java中使用generic arguments时的基本问题

      问题怎么来的？

      在编码时，声明一个HashMap类型的变量时，在generic arguments部分将Integer写为了int，像下面这样：

HashMap<int, String> var;

      未等编译，IDE已经给出了提示，有问题。

      问题原因：Java中，在generic arguments不能使用基本数据类型，比如int等。

      对这个问题的回答，可以看看下面这个web：

      http://stackoverflow.com/questions/1780385/java-hashmapstring-int-not-working

      其中，回答者cletus，还就auto-boxing/unboxing进行说明了。下面是其回答的原文：

         You can't use primitive types as generic arguments in Java. Use instead:

        Map<String, Integer> myMap = new HashMap<String, Integer>();

        With auto-boxing/unboxing there is little difference in the code. Auto-boxing means you can write:

        myMap.put("foo", 3);

        instead of:

        myMap.put("foo", new Integer(3));

        Auto-boxing means the first version is implicitly converted to the second. Auto-unboxing means you can write:

        int i = myMap.get("foo");

        instead of:

        int i = myMap.get("foo").intValue();

        The implicit call to intValue() means if the key isn't found it will generate a NullPointerException, for example:

        int i = myMap.get("bar"); // NullPointerException

        The reason is type erasure. Unlike, say, in C# generic types aren't retained at runtime. They are just "syntactic sugar" for explicit casting to save you doing this:

        Integer i = (Integer)myMap.get("foo");

        To give you an example, this code is perfectly legal:

        Map<String, Integer> myMap = new HashMap<String, Integer>();
        Map<Integer, String> map2 = (Map<Integer, String>)myMap;
        map2.put(3, "foo");

      另外，附上一篇关于Java generics的文章，它的内容以及附录的参考资料，有帮助。

      多角度看 Java 中的泛型