» Override Hashcode() Method in Java with example

1) If two objects are equal by equals() method then there hashcode returned by hashCode() method must be same.

2) Whenever hashCode() mehtod is invoked on the same object more than once within single execution of application, hashCode() must return same integer provided no information or fields used in equals and hashcode is modified. This integer is not required to be same during multiple execution of application though.

3) If two objects are not equals by equals() method it is not require that there hashcode must be different. Though it’s always good practice to return different hashCode for unequal object. Different hashCode for distinct object can improve performance of hashmap or hashtable by reducing collision.

To better understand concept of equals and hashcode and what happens if you don’t override them properly I would recommend understanding of How HashMap works in Java

Overriding hashCode method in Java
Override java hashcode exampleWe will follow step by step approach for overriding hashCode method. This will enable us to understand the concept and process better.

1) Take a prime hash e.g. 5, 7, 17 or 31 (prime number as hash, results in distinct hashcode for distinct object)
2) Take another prime as multiplier different than hash is good.
3) Compute hashcode for each member and add them into final hash. Repeat this for all members which participated in equals.
4) Return hash

  Here is an example of hashCode() method

   @Override
    public int hashCode() {
        int hash = 5;
        hash = 89  hash + (this.name != null ? this.name.hashCode() : 0);
        hash = 89  hash + (int) (this.id ^ (this.id >>> 32));
        hash = 89  hash + this.age;
        return hash;
    }

It’s always good to check null before calling hashCode() method on members or fields to avoid NullPointerException, if member is null than return zero. Different data types has different way to compute hashCode.Integer members are simplest we just add there value into hash, for other numeric data-type are converted into int and then added into hash