hashCode() method using HashMap

Question

Do I have to override hashCode() method, if I'm going to customize HashMap?

UPD: For example:

import java.util.HashMap;
import java.util.Objects;

/**
 *
 * @author dolgopolov.a
 */
public class SaltEntry {

    private long time;
    private String salt;

    /**
     * @return the time
     */
    public long getTime() {
        return time;
    }

    /**
     * @param time the time to set
     */
    public void setTime(long time) {
        this.time = time;
    }

    /**
     * @return the salt
     */
    public String getSalt() {
        return salt;
    }

    /**
     * @param salt the salt to set
     */
    public void setSalt(String salt) {
        this.salt = salt;
    }

    public boolean equals(SaltEntry o) {
        if (salt.equals(o.getSalt()) && time == o.getTime()) {
            return true;
        } else {
            return false;
        }
    }

    public int hashCode() {
        return Objects.hash(String.valueOf(salt + time));
    }

    public static void main(String[] args) {
        SaltEntry se1 = new SaltEntry(), se2 = new SaltEntry();
        se1.setSalt("1");
        se2.setSalt("1");
        se1.setTime(1);
        se2.setTime(1);
        HashMap<String, SaltEntry> hm = new HashMap<String, SaltEntry>();
        hm.put("1", se1);
        hm.put("2", se2);
        System.out.println(hm.get("1").getSalt());

    }

}

Short answer is yes. It is always advisable to override hashCode() — Saif Asif
– Saif Asif, Commented May 21, 2014 at 10:19
hashCode and equals methods should be overridden in the class used as key for the Map. — Luiggi Mendoza
– Luiggi Mendoza, Commented May 21, 2014 at 10:19
@LuiggiMendoza: Only if you need a form of equality other than reference equality... — Jon Skeet
– Jon Skeet, Commented May 21, 2014 at 10:22

NiziL · Accepted Answer · 2014-06-02 11:44:42Z

4

You have to override both hashCode() and equals() of the class A if you're going to use A as key in HashMap and need another form of equality than reference equality.

A little trick to implements hashcode() since Java 7: use Objects#hash(Object...).
For example:

public class A {
  Object attr1, attr2, /* ... , */ attrn ;

  @Override
  public int hashcode() {
    return Objects.hash(attr1, attr2, /* ... , */ attrn);
  }
}

edited Jun 2, 2014 at 11:44

answered May 21, 2014 at 10:20

NiziL

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3 Comments

Luiggi Mendoza Over a year ago

Check which class is used for the key.

Luiggi Mendoza Over a year ago

Also, note that Objects is available since Java 7. If you're working on a prior version, you have to find another way to implement hashCode.

NiziL Over a year ago

@LuiggiMendoza Right ! I'm going to edit my answer, thank you :)

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