forum

home / developersection / forums / what issues should be considered when overriding equals and hashcode in java?

Ask Question

What issues should be considered when overriding equals and hashCode in Java?

Anonymous User 2858 13-May-2015
What issues / pitfalls must be considered when overriding equals and hashCode?
java java 
Updated on 14-May-2015
Anonymous User

Other

I am a content writter !

Message
Can you answer this question?
Answer

1 Answers

Anonymous User

Anonymous User

14-May-2015
The theory (for the language lawyers and the mathematically inclined):

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().
Use the excellent helper classes EqualsBuilder and HashCodeBuilder from the Apache Commons Lang library. An example

public class Person {
    private String name;
    private int age;
    // ...
    @Override
    public int hashCode() {
        return new HashCodeBuilder(17, 31). // two randomly chosen prime numbers
            // if deriving: appendSuper(super.hashCode()).
            append(name).
            append(age).
            toHashCode();
    }
    @Override
    public boolean equals(Object obj) {
       if (!(obj instanceof Person))
            return false;
        if (obj == this)
            return true;
        Person rhs = (Person) 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.


advertising
advertising

Liked By

We use cookies to ensure you have the best browsing experience on our website. By using our site, you acknowledge that you have read and understood our Cookie Policy & Privacy Policy