Iterating HashMap strange behaviour

Question

I'm encountering something weird to me and I can't get what is going on. I'm creating a HashMap, then adding some key-value pairs over a loop.Inside the loop i'm changing a pre-initialized array on every iteration. So what I expect is always different arrays as values in the map.Here's the code:

int[] valueArray = new int[3];
int key = 0;
Map<Integer, int[]> map = new HashMap<>();
while (key < 5) {
    for (int i = 0; i < 3; i++) {
        valueArray[i] = key;
    }
    System.out.println(Arrays.toString(valueArray));
    map.put(key, valueArray);
    key++;
}
for (int i = 0; i < map.size(); i++) {
    System.out.println("Key: " + i + " Value: " + Arrays.toString(map.get(i)));
}

And here's the output:

[0, 0, 0]
[1, 1, 1]
[2, 2, 2]
[3, 3, 3]
[4, 4, 4]
Key: 0 Value: [4, 4, 4]
Key: 1 Value: [4, 4, 4]
Key: 2 Value: [4, 4, 4]
Key: 3 Value: [4, 4, 4]
Key: 4 Value: [4, 4, 4]

Why do you expect different values? How many `int[]` objects are you operating on here? — Sotirios Delimanolis, May 27 '15 at 05:14

score 0 · Accepted Answer · answered May 27 '15 at 05:15

You are using same array ref every time. to make it work create a new ref on each iteration. like this :

int key = 0;
Map<Integer, int[]> map = new HashMap<>();
while (key < 5) {
int[] valueArray = new int[3];
    for (int i = 0; i < 3; i++) {
        valueArray[i] = key;
    }
    System.out.println(Arrays.toString(valueArray));
    map.put(key, valueArray);
    key++;
}
for (int i = 0; i < map.size(); i++) {
    System.out.println("Key: " + i + " Value: " + Arrays.toString(map.get(i)));
}

Iterating HashMap strange behaviour

1 Answers1