Why do I need to override the equals and hashcode method in Java ?

Let's analyze what would happen if we override one but not the other and we attempt to use a Map.

public class MyClass {

    private final String importantField;

    private final String anotherField;

    public MyClass(final String equalField, final String anotherField) {

        this.importantField = equalField;

        this.anotherField = anotherField;

    }

    public String getEqualField() {

        return importantField;

    }

    public String getAnotherField() {

        return anotherField;

    }

    @Override

    public int hashCode() {

        final int prime = 31;

        int result = 1;

        result = prime * result

                + ((importantField == null) ? 0 : importantField.hashCode());

        return result;

    }

    @Override

    public boolean equals(final Object obj) {

        if (this == obj)

            return true;

        if (obj == null)

            return false;

        if (getClass() != obj.getClass())

            return false;

        final MyClass other = (MyClass) obj;

        if (importantField == null) {

            if (other.importantField != null)

                return false;

        } else if (!importantField.equals(other.importantField))

            return false;

        return true;

    }

}

Override only hashCode

Imagine you have this :

MyClass first = new MyClass("a","first");

MyClass second = new MyClass("a","second");

If you only override hashCode then when you call myMap.put(first,someValue) it takes first, calculates its hashCode and stores it in a given bucket.

Then when you call myMap.put(first,someOtherValue) it should replace first with second as per the Map Documentation because they are equal (according to our definition).

But the problem is that equals was not redefined, so when the map hashes second and iterates through the bucket looking if there is an object k such that second.equals(k) is true it won't find any as second.equals(first) will be false.

Override only equals

If only equals is overridden, then when you call myMap.put(first,someValue) first will hash to some bucket and when you call myMap.put(first,someOtherValue) it will hash to some other bucket.

So, although they are equal, as they don't hash to the same bucket (different hashCode) the map can't realize it and both of them stay in the map.