Do we need hashCode Implementation for TreeMap?

Question

I was testing the behaviour of TreeMap and understood the process of sorting. However I am still in the confusing thought that, for retrieval does the custom key class needs to override the hashCode method. Having said that, I have searched enough of google but could not find any justifiable answer.

Below is the example which I worked with.

class Dog implements Comparable<Dog> {
    String color;
    int size;

Dog(String c, int s) {
    color = c;
    size = s;
 }

int hc;

@Override
public int hashCode() {
    hc = super.hashCode();
    return hc;
}

@Override
public int compareTo(Dog o) {
    return this.color.compareTo(o.color);
}
}

Test Code for testing the TreeMap..

public class TestHashMap {
    public static void main(String[] args) {
        Dog d1 = new Dog("a", 1);
        Dog d2 = new Dog("b", 2);
        Dog d3 = new Dog("c", 3);
        Dog d4 = new Dog("d", 4);
        Dog d5 = new Dog("e", 5);

        Dog d = new Dog("c", 3);

        TreeMap<Dog, Integer> hm = new TreeMap<>();
        hm.put(d1, 10);
        hm.put(d2, 15);
        hm.put(d3, 5);
        hm.put(d4, 20);
        hm.put(d5, 25);

        System.out.println("value is :" + hm.get(d));
    }
}

Irrespective of I implement hashCode method or not the value is retrieved correctly, However when debugging , always hashcode method is called , so I am confused if really hashCode implementation is mandatory.

Could anybody please help in understanding the correct behaviour of retrieval from TreeMap.

Please do not copy paste java doc from TreeMap.

Answer 1

The hashCode() is called as a part of default toString() implementation. Since you haven't implemented your own toString(), I'd suspect you printed a Dog somewhere in your code which caused the method invocation.