When is it recommended to implement the equals() method in a class?

The equals() method in a class is recommended to be implemented when objects of this class need to be compared not by reference (the default behavior), but by content — that is, by the value of fields.

This is important if:

• The class represents an entity with a logical sense of equality (for example, a user, a product).
• Objects will be used in collections where correct equality checking is important, such as in HashSet, HashMap.

Example implementation of equals() and hashCode() for the Person class:

public class Person {
    private String name;
    private int 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);
    }
}

If equals() is not overridden, object comparison will be by reference, which often does not match business logic.