obj.hashCode() vs System.identityHashCode( - java help on CodeGym

What is the difference? I thoght the reference of the object will returned by both of them , but it seems not. public class Solution {

public static void main(String[] args) {

        String s = "abcd";
        String s1 = new String("abcd");
        String s2 = new String("abcd");
        String s3 = "abcd";

        System.out.println(Integer.toHexString(System.identityHashCode(s)));
        System.out.println(Integer.toHexString(System.identityHashCode(s1)));
        System.out.println(Integer.toHexString(System.identityHashCode(s2)));
        System.out.println(Integer.toHexString(System.identityHashCode(s3)));
        System.out.println();
        System.out.println(Integer.toHexString(s.hashCode()));
        System.out.println(Integer.toHexString(s1.hashCode()));
        System.out.println(Integer.toHexString(s2.hashCode()));
        System.out.println(Integer.toHexString(s3.hashCode()));
    }
}

Output: 7960847b // i think this is the reference of the object, memory-address 6a6824be // new object = new address. OK. 5c8da962 // new object = new address. OK. 7960847b // String pool. Same adress as the first, it's understandable. OK. 2d9442 // but why are these hashCodes the same? What kind of hashCodes are these??... 2d9442 2d9442 2d9442

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Nouser

Level 36 , Germany

25 February 2021, 08:22

in the String class hashCode is overridden. So it doesn't use the method of the object class anymore and outputs something more meaningful for the programmer or better collections based on hashes. Means equal objects should get the same hashcode (but the same hashcode must not mean equality). That's later an own CG topic, equals and hashcode. just look at the code of the hascode method for UTF-16 strings

public static int hashCode(byte[] value) {
    int h = 0;
    int length = value.length >> 1;
    for (int i = 0; i < length; i++) {
        h = 31 * h + getChar(value, i);
    }
    return h;
}

That's calculated from the data and not the address. So two strings with the same data at different memory locations get the same hashcode. identityHashCode on the other hand is the default implementation of the Object class. And cause the object class can not know when instances of certain classes are equal or should have the same hashCode you just got that defaul returning an integer representation of the memory address. Read more about the contract for hashcode in the api

Gellert Varga

Level 23 , Szekesfehervar, Hungary

25 February 2021, 19:07

Thanx! And what kind of operator is this?

length >> 1;

25 February 2021, 19:19

bitwise right shift (1 position) eg. 0100 shifted 1 bit to the right => 0010 if unsigned, shifting one position means dividing by two, two positions dividing by four

25 February 2021, 19:31

Yeah, really, i already remember... :-) Thank you!

obj.hashCode() vs System.identityHashCode(obj))