Question

I am using this code to check that array is present in the HashMap:

public class Test {
    public static void main(String[] arg) {
        HashMap<int[], String> map = new HashMap<int[], String>();
        map.put(new int[]{1, 2}, "sun");
        System.out.println(map.containsKey((new int[]{1, 2})));
    }
}

But this prints False. How can I check that array is present in the HashMap?

Answer 1

The problem is because the two int[] aren’t equal.

System.out.println(
    (new int[] { 1, 2 }).equals(new int[] { 1, 2 })
); // prints "false"

Map and other Java Collections Framework classes defines its interface in terms of equals. From Map API:

Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the containsKey(Object key) method says: “returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k)).”

Note that they don’t have to be the same object; they just have to be equals. Arrays in Java extends from Object, whose default implementation of equals returns true only on object identity; hence why it prints false in above snippet.

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You can solve your problem in one of many ways:

• Define your own wrapper class for arrays whose equals uses java.util.Arrays equals/deepEquals method.
  • And don’t forget that when you @Override equals(Object), you must also @Override hashCode
• Use something like List<Integer> that does define equals in terms of the values they contain
• Or, if you can work with reference equality for equals, you can just stick with what you have. Just as you shouldn’t expect the above snippet to ever print true, you shouldn’t ever expect to be able to find your arrays by its values alone; you must hang-on to and use the original references every time.

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See also:

• Overriding equals and hashCode in Java
• How to ensure hashCode() is consistent with equals()?
• Understanding the workings of equals and hashCode in a HashMap

API

• Object.equals and Object.hashCode
  • It’s essential for a Java programmer to be aware of these contracts and how to make them work with/for the rest of the system