Although hash values are different, still why are my objects stored in the same location?

Question

I have a Movie class and I override only hashCode() method. Please find below the java class

public class Movie {

private String actor;
private String name;
private String releaseYr;

public String getActor() {
    return actor;
}

public void setActor(String actor) {
    this.actor = actor;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getReleaseYr() {
    return releaseYr;
}

public void setReleaseYr(String releaseYr) {
    this.releaseYr = releaseYr;
}
@Override
    public int hashCode() {
        return actor.hashCode() + name.hashCode() + releaseYr.hashCode();
    }


}

I have created two Movie objects and both the object's all the property values are same and put them in a HashMap. Below is the code

import java.util.HashMap;

public class Test {

public static void main(String[] args) {

    Movie m1 = new Movie();
    m1.setActor("Akshay");
    m1.setName("Taskvir");
    m1.setReleaseYr("2010");

    Movie m2 = new Movie();
    m2.setActor("Akshay");
    m2.setName("Taskvir");
    m2.setReleaseYr("2010");


    HashMap<Movie, String> map = new HashMap<Movie, String>();

    map.put(m1, "Value of m1");
    map.put(m2, "Value of m2");

}

}

I am getting the expected result. As I override only hashCode() method and both the object's hash values are same, so they are stored in the same index location of HashMap table array. Below is the expected result in debugging mode.

But if I do not override the hashCode() method but override the equals() method they are stored in the same index location of HashMap table array. Although I can see the hash values are different. Below is my equals method

@Override
public boolean equals(Object obj) {

    Movie m1 = (Movie) obj;
    boolean result = false;

    if (m1.getActor().equals(this.actor) && m1.getName().equals(this.name)
            && m1.getReleaseYr().equals(this.releaseYr)) {
        result = true;
    }

    return result;
}

The output in debugging mode

If I do not override the equals and hashCode methods then also I am getting same unexpected result.

As per my understanding, if I do not override equals and hashCode methods or only override the equals method then m1 and m2 objects should stored in the different location as hash values are different for m1 and m2 objects. But in this case it is not happening.

Can some one please explain me why with different hash values, my objects stored in the same location?

I have used Java 8.

Mureinik Mureinik · Accepted Answer · 2018-12-23T17:43:08

Hash codes have a huge range, from Integer.MIN_VALUE to Integer.MAX_VALUE, while a HashMap usually has much fewer buckets (by default, 16 for a newly instantiated HashMap, at least with OpenJDK 11). Thus, it's entirely possible, even expected, that hash codes will collide, and multiple objects will be added to the same bucket. However, note that if you're not overriding hashCode() this behavior is completely incidental, and can't be relied upon.

Although hash values are different, still why are my objects stored in the same location?

2 Answers

Override nothing

Only override hashCode()

Override both hashCode() and equals()

Only override equals()