Value Based Identity

While reference based identity can be useful, its often not what you want for keys in a HashMap. Ideally if you are looking up "Tow Mater" you shouldn't have to be careful to ensure its the same instance of String, all you care about is that it contains the right characters.

We call this notion of identity "value based." Two things are the same if they contain the same data - i.e. if they represent the same value.

class Main {
    void main() {
        // Strings A and B are distinct instances
        var stringA = new String(new char[] { 'a', 'b', 'c' });
        var stringB = new String(new char[] { 'a', 'b', 'c' });

        // but they will give the same hashCode
        System.out.println(stringA.hashCode());
        System.out.println(stringB.hashCode());

        // and will be equal to eachother
        System.out.println(stringA.equals(stringB));
        System.out.println(stringB.equals(stringA));
    }
}

Strings, all the numeric types like Integer and Double, as well as Booleans are defined in this way.¹ So will any records and enums you make².

void main() {
    new Main().main();    
}

record Pos(int x, int y) {}

class Main {
    void main() {
        // Positions A and B are distinct instances but hold the same values
        var posA = new Pos(5, 5);
        var posB = new Pos(5, 5);

        // therefore they will give the same hashCode
        System.out.println(posA.hashCode());
        System.out.println(posB.hashCode());

        // and will be equal to eachother
        System.out.println(posA.equals(posB));
        System.out.println(posB.equals(posA));
    }
}