Java HashMap get works but containsKey does not

Question

I am trying to locate a key in a HashMap. I can print the selected key by using \'get\' but when I use \'containsKey\' in an if statement, it is not found.

I KNOW

Answer 1

Take a peak at the source code for the HashMap implementation. Both get and containsKey use the hasCode() and equals() methods of your key object.

The only real difference, and as was pointed out, it is a trivial null check, is in the comparisons:

get:

((k = e.key) == key || key.equals(k))

containsKey:

((k = e.key) == key || (key != null && key.equals(k)))

where e is of type Entry for a HashMap.

So, if you do not have a strong implementations of hashCode() and/or equals() you will have a problem. Additionally, if your keys were mutated (I see that you did not declare the class fields final) you could have an issue.

Take the following example:

public class HashMapTest {
    static class KeyCheck {
        int value;
        public KeyCheck(int value) { this.value = value; }
        public void setValue(int value) { this.value = value; }
        @Override public int hashCode() { return value; }
        @Override public boolean equals(Object o) {
            return ((KeyCheck)o).value == this.value;
        }
    }

    public static void main(String args[]) {
        HashMap map = new HashMap();
        KeyCheck k1 = new KeyCheck(5);
        KeyCheck k2 = new KeyCheck(5);

        map.put(k1, "Success");

        System.out.println("Key: " + k1 + " Get: " + map.get(k1) +
                           " Contains: " + map.containsKey(k1));
        System.out.println("Key: " + k2 + " Get: " + map.get(k2) +
                           " Contains: " + map.containsKey(k2));

        k1.setValue(10);

        System.out.println("Key: " + k1 + " Get: " + map.get(k1) +
                           " Contains: " + map.containsKey(k1));
        System.out.println("Key: " + k2 + " Get: " + map.get(k2) +
                           " Contains: " + map.containsKey(k2));
    }
}

This will print out:

Key: HashMapTest$KeyCheck@5 Get: Success Contains: true
Key: HashMapTest$KeyCheck@5 Get: Success Contains: true
Key: HashMapTest$KeyCheck@a Get: null Contains: false
Key: HashMapTest$KeyCheck@5 Get: null Contains: false

As you can see, in this case the mutability caused the hashCode() to change, which ruined everything.