EqualsHashCodeDemo.java

import java.util.*;

/**
 * Demonstrates the proper contract for equals() and hashCode()
 * Key concepts: Contract rules, HashMap behavior, common pitfalls
 */
public class EqualsHashCodeDemo {
    
    public static void main(String[] args) {
        System.out.println("=== Equals and HashCode Contract Demo ===\n");
        
        demonstrateProperImplementation();
        demonstrateBrokenContract();
        demonstrateHashMapBehavior();
    }
    
    private static void demonstrateProperImplementation() {
        System.out.println("1. Proper Implementation");
        
        Person p1 = new Person("Alice", 25);
        Person p2 = new Person("Alice", 25);
        Person p3 = new Person("Bob", 30);
        
        System.out.println("   p1.equals(p2): " + p1.equals(p2) + " (same content)");
        System.out.println("   p1.equals(p3): " + p1.equals(p3) + " (different content)");
        System.out.println("   p1.hashCode(): " + p1.hashCode());
        System.out.println("   p2.hashCode(): " + p2.hashCode() + " (same as p1)");
        System.out.println("   p3.hashCode(): " + p3.hashCode() + " (different)\n");
    }
    
    private static void demonstrateBrokenContract() {
        System.out.println("2. Broken Contract Example");
        
        BrokenPerson bp1 = new BrokenPerson("Charlie", 35);
        BrokenPerson bp2 = new BrokenPerson("Charlie", 35);
        
        System.out.println("   bp1.equals(bp2): " + bp1.equals(bp2) + " (overridden equals)");
        System.out.println("   bp1.hashCode(): " + bp1.hashCode());
        System.out.println("   bp2.hashCode(): " + bp2.hashCode() + " (different! BROKEN)");
        System.out.println("   ⚠️  Contract violated: equal objects must have same hashCode\n");
    }
    
    private static void demonstrateHashMapBehavior() {
        System.out.println("3. Impact on HashMap");
        
        // Proper implementation
        Map<Person, String> properMap = new HashMap<>();
        Person key1 = new Person("David", 40);
        properMap.put(key1, "Engineer");
        
        Person key2 = new Person("David", 40); // Equal to key1
        System.out.println("   Proper implementation:");
        System.out.println("   Map contains key2? " + properMap.containsKey(key2) + " ✓");
        System.out.println("   Retrieved value: " + properMap.get(key2));
        
        // Broken implementation
        Map<BrokenPerson, String> brokenMap = new HashMap<>();
        BrokenPerson bkey1 = new BrokenPerson("Eve", 45);
        brokenMap.put(bkey1, "Manager");
        
        BrokenPerson bkey2 = new BrokenPerson("Eve", 45); // Equal to bkey1
        System.out.println("\n   Broken implementation:");
        System.out.println("   Map contains bkey2? " + brokenMap.containsKey(bkey2) + " ✗");
        System.out.println("   Retrieved value: " + brokenMap.get(bkey2) + " (null!)");
        System.out.println("   ⚠️  HashMap lookup fails due to broken contract\n");
    }
}

// Proper implementation
class Person {
    private final String name;
    private final int age;
    
    public Person(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;
        Person person = (Person) o;
        return age == person.age && Objects.equals(name, person.name);
    }
    
    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
    
    @Override
    public String toString() {
        return "Person{name='" + name + "', age=" + age + "}";
    }
}

// Broken implementation - equals without hashCode
class BrokenPerson {
    private final String name;
    private final int age;
    
    public BrokenPerson(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;
        BrokenPerson that = (BrokenPerson) o;
        return age == that.age && Objects.equals(name, that.name);
    }
    
    // Missing hashCode() - uses Object's default implementation
    // This violates the contract!
}