Why is it important to maintain consistency between the equals and hashCode methods when overriding them?

In Java, the methods equals and hashCode are closely related: if two objects are considered equal according to equals, then their hashCode should return the same value. This is important for the correct functioning of hash-based collections, such as HashMap or HashSet. If this rule is violated, objects may be stored incorrectly or not found in such collections, leading to errors and hard-to-detect bugs.

Example:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    MyClass myClass = (MyClass) o;
    return id == myClass.id;
}

@Override
public int hashCode() {
    return Objects.hash(id);
}

Here, equals compares based on the id field, and hashCode is also based on this field, ensuring consistency.