Duplicate HashSet entries with original class

TL;DR

You should implement both of hashCode and equals for using in HashSet.

Env

openjdk version "1.8.0_265"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_265-b01)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.265-b01, mixed mode)

Try

Below MyModel class is my sample class with some properties and provide same hashcode with same property values.
However equals returns false.

public class MyModel {
    private String name;
    private int age;

    public MyModel(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        return false;
    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        return result;
    }
}

import org.junit.Test;

import java.util.HashSet;

import static org.junit.Assert.*;

public class MyModelTest {

    @Test
    public void testHashSet() {
        HashSet<MyModel> set = new HashSet<>();

        // Add models with same property values.
        set.add(new MyModel("taro", 10));
        set.add(new MyModel("taro", 10));

        // We want the set has only 1 entry.
        assertEquals(1, set.size());
    }

}

But, you will see the result as below.

java.lang.AssertionError: 
Expected :1
Actual   :2

For pass the assertion, you should implement equals also.

public class MyModel {
    private String name;
    private int age;

    public MyModel(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        MyModel myModel = (MyModel) o;

        if (age != myModel.age) return false;
        return name != null ? name.equals(myModel.name) : myModel.name == null;
    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        return result;
    }
}

Why

In OpenJDK, HashSet depends on HashMap implementation.

• HashSet::add

https://github.com/openjdk/jdk/blob/jdk8-b120/jdk/src/share/classes/java/util/HashSet.java#L219

• HashMap::putVal

https://github.com/openjdk/jdk/blob/jdk8-b120/jdk/src/share/classes/java/util/HashMap.java#L633-L634