Java的Generics的几点限制

参见

http://docs.oracle.com/javase/tutorial/java/generics/restrictions.html

To use Java generics effectively, you must consider the following restrictions:

• Cannot Instantiate Generic Types with Primitive Types
• Cannot Create Instances of Type Parameters
• Cannot Declare Static Fields Whose Types are Type Parameters
• Cannot Use Casts or instanceof With Parameterized Types
• Cannot Create Arrays of Parameterized Types
• Cannot Create, Catch, or Throw Objects of Parameterized Types
• Cannot Overload a Method Where the Formal Parameter Types of Each Overload Erase to the Same Raw Type

Cannot Instantiate Generic Types with Primitive Types

这个是说你不能用primitive type来做类型参数，就是说下面这个使用泛型的例子是不对的：

class Pair<K, V> {

private K key;
private V value;

public Pair(K key, V value) {
this.key = key;
this.value = value;
}

// ...
}

Pair<int, char> p =
new Pair<>(8, 
'a'); // compile-time error

class Pair<K, V> {

    private K key;
    private V value;

    public Pair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    // ...
}

Pair<int, char> p = new Pair<>(8, 'a');  // compile-time error

Cannot Create Instances of Type Parameters
这个是不能直接new T()来创建类型参数的实例。你必须用反射的技术来做到：

public static <E> 
void append(List<E> list, Class<E> cls) throws Exception {
E elem = cls.newInstance(); // OK
list.add(elem);
}

public static <E> void append(List<E> list, Class<E> cls) throws Exception {
    E elem = cls.newInstance();   // OK
    list.add(elem);
}

这里反射的技术就是传一个类作为参数。


Cannot Declare Static Fields Whose Types are Type Parameters

这个容易引起混乱的，看下面这个例子：
如果类的静态变量允许是类型参数：

public class MobileDevice<T> {
private static T os;

// ...
}
MobileDevice<Smartphone> phone = new MobileDevice<>();
MobileDevice<Pager> pager = new MobileDevice<>();
MobileDevice<TabletPC> pc = new MobileDevice<>();

public class MobileDevice<T> {
    private static T os;

    // ...
}
MobileDevice<Smartphone> phone = new MobileDevice<>();
MobileDevice<Pager> pager = new MobileDevice<>();
MobileDevice<TabletPC> pc = new MobileDevice<>();

那么os的实际类型是什么呢？乱套了肯定。所以Java禁止这样做。

Cannot Use Casts or instanceof with Parameterized Types
因为java在编译是做了type erasure，所以无法区分ArrayList<Integer>和ArrayList<String>，所以Java不允许如下code编译通过：

public static <E> 
void rtti(List<E> list) {
if (list instanceof ArrayList<Integer>) {
// compile-time error
// ...
}
}

public static <E> void rtti(List<E> list) {
    if (list instanceof ArrayList<Integer>) {  // compile-time error
        // ...
    }
}