Java – How to override equals and hashCode

By mkyong | Updated: March 24, 2016

Viewed: 10,040 (+29 pv/w)

Tags: hashcode java

Some Java examples to show you how to override equals and hashCode.

1. POJO

To compare two Java objects, we need to override both equals and hashCode (Good practice).

public class User {
    private String name;
    private int age;
    private String passport;

	//getters and setters, constructor
}

	User user1 = new User("mkyong", 35, "111222333");
    User user2 = new User("mkyong", 35, "111222333");

    System.out.println(user1.equals(user2)); // false

2. Classic Way

The 17 and 31 hash code idea is from the classic Java book – effective Java : item 9

public class User {
    private String name;
    private int age;
    private String passport;

	//getters and setters, constructor

    @Override
    public boolean equals(Object o) {

        if (o == this) return true;
        if (!(o instanceof User)) {
            return false;
        }

        User user = (User) o;

        return user.name.equals(name) &&
                user.age == age &&
                user.passport.equals(passport);
    }

    //Idea from effective Java : Item 9
    @Override
    public int hashCode() {
        int result = 17;
        result = 31 * result + name.hashCode();
        result = 31 * result + age;
        result = 31 * result + passport.hashCode();
        return result;
    }

}

2. JDK 7

For JDK 7 and above, you can use the new Objects class to generate the equals and hash code values.

import java.util.Objects;

public class User {
    private String name;
    private int age;
    private String passport;

	//getters and setters, constructor

    @Override
    public boolean equals(Object o) {

        if (o == this) return true;
        if (!(o instanceof User)) {
            return false;
        }
        User user = (User) o;
        return age == user.age &&
                Objects.equals(name, user.name) &&
                Objects.equals(passport, user.passport);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age, passport);
    }

}

3. Apache Commons Lang

Alternatively, you can use the Apache Commons Lang EqualsBuilder and HashCodeBuilder function.

import org.apache.commons.lang3.builder;

public class User {
    private String name;
    private int age;
    private String passport;

	//getters and setters, constructor

     @Override
    public boolean equals(Object o) {

        if (o == this) return true;
        if (!(o instanceof User)) {
            return false;
        }

        User user = (User) o;

        return new EqualsBuilder()
                .append(age, user.age)
                .append(name, user.name)
                .append(passport, user.passport)
                .isEquals();
    }

    @Override
    public int hashCode() {
        return new HashCodeBuilder(17, 37)
                .append(name)
                .append(age)
                .append(passport)
                .toHashCode();
    }

}

4. Test again

After overrides both equals and hashCode.

	User user1 = new User("mkyong", 35, "111222333");
    User user2 = new User("mkyong", 35, "111222333");

    System.out.println(user1.equals(user2)); // true

References

1. Wikipedia : Java hashCode()
2. The 3 things you should know about hashCode()
3. JDK 7 Objects JavaDoc
4. Apache Commons Lang
5. HashCodeBuilder JavaDoc
6. Stackoverflow : Overriding equals and hashCode in Java?