equals and hashCode Contract

Example

import java.util.Objects;

public class Main {
    public static void main(String[] args) {
        String a = new String("upi"), b = new String("upi");
        System.out.println("a == b:      " + (a == b));
        System.out.println("a.equals(b): " + a.equals(b));

        Money m1 = new Money(100, "INR"), m2 = new Money(100, "INR");
        System.out.println("m1.equals(m2): " + m1.equals(m2));
        System.out.println("same hashCode: " + (m1.hashCode() == m2.hashCode()));
    }
}

final class Money {
    private final long amount;
    private final String currency;
    Money(long amount, String currency) { this.amount = amount; this.currency = currency; }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Money other)) return false;
        return amount == other.amount && currency.equals(other.currency);
    }

    @Override
    public int hashCode() { return Objects.hash(amount, currency); }
}

Output

a == b:      false
a.equals(b): true
m1.equals(m2): true
same hashCode: true