java 泛型的问题

Question

我想写一个方法list->array 想利用泛型，代码如下

public static <T> T[] list2Array(List<T> list){
    T[] array = (T[])list.toArray(new T[list.size()]);
    return array;
}

也就是传过来的String就返回String数组 Integer就返回Integer数组。但是这个new T[list.size]这里有问题，请问怎么解决？谢谢。

Answer 1

这个Effective Java第二版里有说到, Item25.

Because of these fundamental differences, arrays and generics do not
mix well. For example, it is illegal to create an array of a generic
type, a parameterized type, or a type parameter. None of these array
creation expressions are legal: new List[], new List[], new
E[]. All will result in generic array creation errors at compile time.

Generally speaking, arrays and generics don’t mix well. If you find
yourself mixing them and getting compile-time errors or warnings, your
first impulse should be to replace the arrays with lists.

1. 因为数组和泛型不对付, 所以在不做强制类型转换的情况下, 我们是没有办法得到一个泛型数组的实例的, 编译器会限制此类情况发生(new E[], list.toArray());
2. 为什么数组和泛型不对付, Effective Java里有例子. 根本原因在于:

Arrays are covariant. This scary-sounding word means simply that if
Sub is a subtype of Super, then the array type Sub[] is a subtype of
Super[].

给出例子说明我的理解, 类似的代码, 在泛型集合下, 会在静态编译阶段报错; 而泛型数组在运行阶段给出ArrayStoreException:

private <T> void doSomethingToGenArr(T[] tArr){
    Object[] oArr = tArr;
    oArr[0] = new String("aaa");
}

private <T> void doSomethingToGenList(List<T> list){
    List<Object> l1 =  list; /* compile error */
    l1.set(0, new String("aaa"));
}

@Test
public void test111(){
    doSomethingToGenArr(new Integer[2]);
}

@Test
public void test112(){
    doSomethingToGenList(new ArrayList<Integer>());
}

Answer 2

没有泛型数组，这一说

Answer 3

toArray()方法有两种,带参和不带参.
带参的情况下,参数就是一个一个泛型数组,如T[] a,相当于你手动构造一个数组,传入方法中.
toArray()内部是一个copy的实现.

举两个例子.
1:String[][] str_a = (String [][]) arr.toArray(new String[0][0]);

2 :String[][] a = new String[<size>][size];
String [][] str_a = (String [][]) arr.toArray(a);

当然 要保证转型成功,不然会引发ClassCastException.

--------------------------分割线-------------------------

以下是源码中是实现,实际上就是一个copy操作.

 /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list in
     *         proper sequence
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }