hashCode() vs equals() in Java

1. Introduction

In Java, hashCode() and equals() are two fundamental methods from the Object class used in the context of object comparison and hashing. The hashCode() method provides a unique integer representation of an object, primarily used in hash-based collections like HashMap. The equals() method is used to determine the equality of two objects, based on their state.

2. Key Points

1. hashCode() returns an integer that represents the object's memory address by default.

2. equals() method is used to check if two objects are the same in terms of their state.

3. Consistency between equals() and hashCode() is important: if two objects are equal according to the equals(Object) method, then calling hashCode() on each of the two objects must produce the same integer result.

4. hashCode() is used in hashing to decide where to store the object in memory, while equals() determines the actual equality of two objects.

3. Differences

hashCode()	equals()
Computes an integer representation of the object.	Determines if two objects are equal.
Used primarily by hashing-based collections.	Used to compare objects for equality.
The default method returns a memory address-based hash code.	The default method checks for reference equality.

4. Example

class Person {
    private String name;
    private int age;

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

    // Overriding equals() method to compare two Person objects based on their properties
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Person person = (Person) obj;
        return age == person.age && name.equals(person.name);
    }

    // Overriding hashCode() to be consistent with how we defined equals()
    @Override
    public int hashCode() {
        int result = name.hashCode();
        result = 31 * result + age;
        return result;
    }
}

public class Main {
    public static void main(String[] args) {
        Person person1 = new Person("John", 30);
        Person person2 = new Person("John", 30);

        // Same state, so equals() should return true
        System.out.println("person1.equals(person2): " + person1.equals(person2));

        // Consistent hashCode, because person1 equals person2
        System.out.println("person1.hashCode() == person2.hashCode(): " + (person1.hashCode() == person2.hashCode()));
    }
}

Output:

person1.equals(person2): true
person1.hashCode() == person2.hashCode(): true

Explanation:

1. The Person class overrides the equals() method to compare objects by their name and age.

2. It also overrides hashCode() to ensure that the hash code is consistent with the equals() method.

3. When two Person objects have the same name and age, equals() returns true and their hash codes are the same.

5. When to use?

- Override equals() when you need to define a custom equality check between two objects.

- Override hashCode() whenever equals() is overridden to maintain the general contract for the hashCode() method, which states that equal objects must have equal hash codes.