为什么我们不能从Object []转换为String [],而我们可以从数组中的值转换？

Question

为什么这样运行:

    static TreeMap<String, int[]> configs = new TreeMap<String, int[]>();

    int[] upperarms_body = {2,3,4,6};
    int[] left_arm = {1,2};
    int[] right_arm = {6,7};
    int[] right_side = {5,6,7};
    int[] head_sternum = {3,4};


    configs.put("upperarms_body", upperarms_body);
    configs.put("left_arm", left_arm);
    configs.put("right_arm", right_arm);
    configs.put("right_side", right_side);
    configs.put("head_sternum", head_sternum);



    // create a config counter
    String[] combi = new String[configs.keySet().size()];

    Set<String> s = configs.keySet();
    int g = 0;
    for(Object str : s){
        combi[g] = (String) str; 
    }

而这不是:

  static TreeMap<String, int[]> configs = new TreeMap<String, int[]>();

    int[] upperarms_body = {2,3,4,6};
    int[] left_arm = {1,2};
    int[] right_arm = {6,7};
    int[] right_side = {5,6,7};
    int[] head_sternum = {3,4};

    configs.put("upperarms_body", upperarms_body);
    configs.put("left_arm", left_arm);
    configs.put("right_arm", right_arm);
    configs.put("right_side", right_side);
    configs.put("head_sternum", head_sternum);



    //get an array of thekeys which are strings
    String[] combi = (String[]) configs.keySet().toArray();

Answer 1

该方法toArray()返回一个无法转换为Object[] 实例的实例,String[]就像Object 实例无法转换为String:

// Doesn't work:
String[] strings = (String[]) new Object[0];

// Doesn't work either:
String string = (String) new Object();

但是,因为你可以分配String到Object,你也可以把String进入Object[](这是可能混淆你):

// This works:
Object[] array = new Object[1];
array[0] = "abc";

// ... just like this works, too:
Object o = "abc";

当然,倒数不起作用

String[] array = new String[1];
// Doesn't work:
array[0] = new Object();

当你这样做时(从你的代码):

Set<String> s = configs.keySet();
int g = 0;
for(Object str : s) {
    combi[g] = (String) str; 
}

你实际上并没有将Object 实例转换为String,你正在将一个String声明为Object类型的实例转换为String.

您的问题的解决方案将是以下任何一个:

String[] combi = configs.keySet().toArray(new String[0]);
String[] combi = configs.keySet().toArray(new String[configs.size()]);

有关更多信息,请参阅Javadoc Collection.toArray(T[] a)