Java equals() and hashCode() Contract in 2 min

Photo by Christopher Gower on Unsplash

Equals

• equals() method is used to compare logical equality of objects
• Default implementation (from Object class) compares references
• To compare object data, we must override equals()

Example

import java.util.Objects;

public class Employee {
    private int id;
    private String name;
    private int age;
    private String dept;
    public Employee(int id, String name, int age, String dept) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.dept = dept;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Employee other = (Employee) obj;
        return this.id == other.id
                && this.age == other.age
                && Objects.equals(this.name, other.name)
                && Objects.equals(this.dept, other.dept);
    }
}

Runner Class

public class Main {
    public static void main(String[] args) {
    
    Employee e1 = new Employee(1, "sagar", 26, "A");
    Employee e2 = new Employee(1, "sagar", 26, "A");
    Employee e3 = new Employee(2, "sagar", 26, "B");
    
        System.out.println(e1 == e2);        // false (reference check)
        System.out.println(e1.equals(e2));   // true  (value check)
        System.out.println(e1.equals(e3));   // false  (value check)
    }
}

hashCode

• Whenever we override equals(), we must override hashCode()
• Hash-based collections (HashSet, HashMap) rely on hashCode()
• If not overridden, duplicate logical objects may get stored

Get DebugBytez’s stories in your inbox

Join Medium for free to get updates from this writer.

Remember me for faster sign in

Issue Example (without hashCode())

HashSet<Employee> set = new HashSet<>();
set.add(e1);
set.add(e2);

System.out.println(set.size()); // 2 ❌

Solution (Override hashCode())

// Overriding in Employee Class

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

After Fix

HashSet<Employee> set = new HashSet<>();
set.add(e1);
set.add(e2);

System.out.println(set.size()); // 1 ✅

Important Contract Rule

Fields used in equals() and hashCode() must be the same

❌ Wrong

equals()  → id, name, dept
hashCode() → id, name, age, dept

✅ Correct

equals()  → id, name, age, dept
hashCode() → id, name, age, dept

Interview One-Liner :

If two objects are equal according to equals(), they must have the same hashCode(). Otherwise, hash-based collections may behave incorrectly.