Why Should I Override the equals() and hashCode() Methods in Java?

What is the importance of overriding the equals() and hashCode() methods in Java?

public class Person {
    private String name;
    private int age;

    @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 && Objects.equals(name, person.name);
    }

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

Overriding the equals() and hashCode() methods in Java is critical because these methods determine how objects are compared for equality and how objects are stored in hash-based collections like HashMap and HashSet. Failing to override these methods correctly can lead to unexpected behavior in your applications, especially when objects are used as keys in maps or stored in sets.

@Override
public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null || getClass() != obj.getClass()) return false;
    // Comparison Logic
}

@Override
public int hashCode() {
    // Return combined hash value
}

Causes

• Using default implementation of equals() which compares object references instead of their actual data.
• Not overriding hashCode() when equals() is overridden, which can lead to issues in hash-based collections.

Solutions

• Always override equals() in conjunction with hashCode() to maintain object equality and hash functionality.
• Ensure that equals() checks for type and nullity, and compares all relevant fields, while hashCode() combines those fields into a single integer.

Mistake: Only checking one field in the equals() method.

Solution: Make sure to compare all fields that define equality.

Mistake: Failing to provide a hashCode() that reflects changes to fields involved in equals().

Solution: Whenever fields that affect equality are modified, ensure your hashCode() still returns consistent results.