When do you use Java's @Override annotation and why?

Answer 1

• Used only on method declarations.
• Indicates that the annotated method declaration overrides a declaration in supertype.

If used consistently, it protects you from a large class of nefarious bugs.

Use @Override annotation to avoid these bugs: (Spot the bug in the following code:)

public class Bigram {
    private final char first;
    private final char second;
    public Bigram(char first, char second) {
        this.first  = first;
        this.second = second;
    }
    public boolean equals(Bigram b) {
        return b.first == first && b.second == second;
    }
    public int hashCode() {
        return 31 * first + second;
    }

    public static void main(String[] args) {
        Set s = new HashSet();
        for (int i = 0; i < 10; i++)
            for (char ch = 'a'; ch <= 'z'; ch++)
                s.add(new Bigram(ch, ch));
        System.out.println(s.size());
    }
}

source: Effective Java