Question

There are two kinds equals methods?

public boolean equals(Bigram b) {
    return b.first == first && b.second == second;
    }

@Override public boolean equals(Object o) {
    if (!(o instanceof Bigram))
        return false;
    Bigram b = (Bigram) o;
    return b.first == first && b.second == second;
}

compare with the 2 methods,when we want to override the equal method,why we need to define an equals method whose parameter is of type Object!

Answer 1

There is actually a good reason for this:

• You need the equals(Object) method to override the superclass equals method in java.lang.Object
• You often also want an overloaded equals(Bigram) method that handles the case where the compiler can prove that the type is Bigram at compile time. This improves performance by avoiding type checking/casting and gives you better type checking in your code.
• Finally you may want to implement equals in a special way for testing with equality with objects that aren’t themselves an instance of Bigram. This should be used with care (do you really want something that is not a Bigram instance to be considered equal to a Bigram?), but it does have some valid applications (e.g. comparing the contents of different types of collection objects).

Normally however it is best to implement them so that one method calls the other, e.g.:

public boolean equals(Bigram b) {
    return b.first == first && b.second == second;
}

@Override public boolean equals(Object o) {
    if (!(o instanceof Bigram)) return false;
    return equals((Bigram)o);
}

This way is more concise and means that you only need to implement the equality testing logic once (Don’t Repeat Yourself!).