求教一个把list转换成数组的问题！！！！

[tikyo772]

public class ListToArrayTest {
public static void main(String args[]) {
ArrayList<String> list = new ArrayList<String>();
String group[];
list.add("aaa");
list.add("bbb");
list.add("ccc");
list.add("ddd");

// 1
// Object[] temp;
// temp = list.toArray();
// System.out.println(temp.length);
// group=new String[temp.length];
// for(int i=0;i<temp.length;i++){
// group[i]=(String) temp[i];
// }
//
// 2
group =new String[list.size()];
group= (String[]) list.toArray();

//Object to String


System.out.println("group size is "+group.length);
}
}


报错的是：
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
at ListToArrayTest.main(ListToArrayTest.java:23)


oboject的强转不是应该可以的吗？谢谢！

[shiyiwan]

import java.util.ArrayList;

public class ListToArrayTest {
	public static void main(String args[]) {
		ArrayList<String> list = new ArrayList<String>();
		String group[];
		list.add("aaa");
		list.add("bbb");
		list.add("ccc");
		list.add("ddd");

		// 1
		// Object[] temp;
		// temp = list.toArray();
		// System.out.println(temp.length);
		// group=new String[temp.length];
		// for(int i=0;i<temp.length;i++){
		// group[i]=(String) temp[i];
		// }
		//
		// 2
		group = new String[list.size()];
		list.toArray(group);

		// Object to String

		System.out.println("group size is " + group.length);
		for(String s: list)
			System.out.println(s);
	}
}

[台城柳月]

	public static void main(String args[]) {
	    ArrayList<String> list = new ArrayList<String>();
	    String group[];
	    list.add("aaa");
	    list.add("bbb");
	    list.add("ccc");
	    list.add("ddd");
	    String[] arr = new String[list.size()];
	    list.toArray(arr);
	   

	    for (String s : arr)
	    {
	        System.out.println(s);
	    }
	  }

[huntor]

使用 public <T extends java/lang/Object> T[] toArray(T[]);

[brightyq]

List list = new ArrayList();
list.add("aaa");
list.add("bbb");
list.add("ccc");
list.add("ddd");

int size = list.size();
String[] str = (String[])list.toArray(new String[size]);

[softroad]

list.toArray(group);

[shiyiwan]

下面是这两种方法的区别，查源码和注释就明白了

 /**
     * 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);
    }

    /**
     * Returns an array containing all of the elements in this list in proper
     * sequence (from first to last element); the runtime type of the returned
     * array is that of the specified array.  If the list fits in the
     * specified array, it is returned therein.  Otherwise, a new array is
     * allocated with the runtime type of the specified array and the size of
     * this list.
     *
     * <p>If the list fits in the specified array with room to spare
     * (i.e., the array has more elements than the list), the element in
     * the array immediately following the end of the collection is set to
     * <tt>null</tt>.  (This is useful in determining the length of the
     * list <i>only</i> if the caller knows that the list does not contain
     * any null elements.)
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
	System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

[风尘中国]

参照这样写才对

public class ListToArrayTest {
    public static void main(String args[]) {
        ArrayList<String> list = new ArrayList<String>();
        String group[];
        list.add("aaa");
        list.add("bbb");
        list.add("ccc");
        list.add("ddd");


        group =new String[list.size()];
        group= list.toArray(group);

        

        System.out.println("group size is "+group.length);
        for(String s:group){
             System.out.println(s);
        }
    }
}

[xiaojinhua1988]

学习哈