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

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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

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.