Services
Technology Web App Development Mobile App Development Windows App Development Database Development UI/UX Development Cloud Development SEO Digital Marketing Guest Posting Content Writing Artificial Intelligence Data Science ERP Development CRM Development CMS Development LMS Development Business Listing Blockchain Technology Startup Promotion AI Tools Integration
Products
Data Converter Cleaner TUC - Unit Converter Import Export Survey Manager Tweet Controller
Developer
Article Blog Forum Interview Beginner News Startup Quiz
Users
Career
Apply Job Internship
Pricing
Login Sign Up Get a Quote

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 2958 13 May 2015
What issues / pitfalls must be considered when overriding equals and hashCode?
java java
Updated 14 May 2015
Anonymous User
Anonymous User

Other

I am a content writter !

Message

1 Answers

Anonymous User
Anonymous User 14 May 2015
Report Abuse
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