Posted in java, Software Engineering

How to override equals and hashCode and What is the effective way to do it?

Before we begin, let’s read something from java Documentation

equals() (javadoc) must define an equality relation (it must be reflexive, symmetric, and transitive). In addition, it must be consistent (if the objects are not modified, then it must keep returning the same value). Furthermore, o.equals(null) must always return false.

hashCode() (javadoc) must also be consistent (if the object is not modified in terms of equals(), it must keep returning the same value).

The relation between the two methods is:

Whenever a.equals(b), then a.hashCode() must be same as b.hashCode().
In practice:

If you override one, then you should override the other.

Use the same set of fields that you use to compute equals() to compute hashCode().

Luckily there are the excellent helper classes EqualsBuilder and HashCodeBuilder from the Apache Commons Lang library. An example:

public class Employee {

private String name;

private int age;

// …

public int hashCode() {

return new HashCodeBuilder(17, 31). // two randomly chosen prime numbers

// if deriving: appendSuper(super.hashCode()).

append(name).

append(age).

toHashCode();

}

public boolean equals(Object obj) {

if (!(obj instanceof Employee))

return false;

if (obj == this)

return true;

Employee rhs = (Employee) obj;

return new EqualsBuilder().

// if deriving: appendSuper(super.equals(obj)).

append(name, rhs.name).

append(age, rhs.age).

isEquals();

}

}

Also remember:

When using a hash-based Collection or Map such as HashSet, LinkedHashSet, HashMap, Hashtable, or WeakHashMap, make sure that the hashCode() of the key objects that you put into the collection never changes while the object is in the collection. The bulletproof way to ensure this is to make your keys immutable, which has also other benefits.

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