Question

why does this run:

    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; 
    }

and this not:

  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

The method toArray() returns an Object[] instance, which cannot be cast to String[], just like an Object instance cannot be cast to String:

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

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

However, because you can assign String to Object, you can also put String into Object[] (which is what probably confuses you):

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

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

The inverse wouldn’t work, of course

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

When you do this (from your code):

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

You’re not actually casting an Object instance to String, you’re casting a String instance declared as an Object type to String.

The solution to your problem would be any of these:

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

Refer to the Javadoc for more info about Collection.toArray(T[] a)