Help me with shuffle in list.

[Erdene Yadam]

I have a problem with my code.
Every men and women have same preference list on output.
They all should have different preference list. why collections.shuffle is not shuffling it? it shuffles it only once. i dont understand why

int list = 5;
        String[] menList = new String[list];
        String[] womenList = new String[list];
        List<String> men = Arrays.asList(menList);
        List<String> women = Arrays.asList(womenList);
        for(int i = 0; i < list; i++) {
            menList[i] = "M" + (i+1);
            womenList[i] = "W" + (i+1);
        }
        System.out.println("List: " + men);
        System.out.println("List: " + women);
        List<String> menTempList = new ArrayList<>(men);
        List<String> womenTempList = new ArrayList<>(women);
        HashMap<String, List<String>> menPref = new HashMap<>();
        HashMap<String, List<String>> womenPref = new HashMap<>();
        
        for (int i = 0; i < list; i++) {
            Collections.shuffle(womenTempList);
            menPref.put(men.get(i), womenTempList);
        }
        System.out.println("Preference list: " + menPref);

        for (int i = 0; i < list; i++) {
            Collections.shuffle(menTempList);
            womenPref.put(women.get(i), menTempList);
        }
        System.out.println("Preference list: " + womenPref);
        }

they all have same preference list. men and women
I want every preference list to be shuffled
Output:

Preference list: {M1=[W4, W3, W5, W1, W2], M2=[W4, W3, W5, W1, W2], M3=[W4, W3, W5, W1, W2], M4=[W4, W3, W5, W1, W2], M5=[W4, W3, W5, W1, W2]}
Preference list: {W1=[M4, M5, M2, M1, M3], W2=[M4, M5, M2, M1, M3], W3=[M4, M5, M2, M1, M3], W4=[M4, M5, M2, M1, M3], W5=[M4, M5, M2, M1, M3]}

[Tushar Goel]

because you are storing same object in each index of map. So, when you shuffled the same object again then values in all index in map also updated.

And you will get last shuffled value in all the indices.

[Henry Wong]

This is a duplicate of this topic of...

https://coderanch.com/t/670802/java/java/HashMap

And the OP got the answer on that one too.


To the OP ... Please stop creating duplicate topics. This causes duplicate effort.

Henry