String hashCodes

Question

Small question.

String s1 = "test";
String s2 = "test";

s1,s2 both have same hashCode value

String sn1 = new String("java");
String sn2 = new String("java");

all of them said sn1 and sn2 have different hashCode values and those are different objects

When I am printing hashCode values it gives same value that means sn1 and sn2 points to the same object or not ?

In this article [link](http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java) — CHAITHANYA PADALA, Mar 20 '14 at 08:10
Nobody responding to the question that you linked to said that s1 and s2 have different hash codes. — Dawood ibn Kareem, Mar 20 '14 at 08:13

score 3 · Answer 1 · answered Mar 20 '14 at 08:06

3

Hash codes should be equal if objects are equal (the reverse is not true, however). Since

  sn1.equals(sn2) // true

We can conclude that

  sn1.hashCode() == sn2.hashCode() // true

answered Mar 20 '14 at 08:06

Dmitry Bychenko

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score 1 · Accepted Answer · answered Mar 20 '14 at 08:08

It is often useful to look at the java code to see what is happening.

For example the String#hashCode method

public int hashCode() {
    int h = hash;
    if (h == 0) {
        int off = offset;
        char val[] = value;
        int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }

As you can see the result is based upon the value of the String, not its memory location.

bobbel · Answer 3 · 2014-03-20T08:55:36.613

If you are comparing Strings like:

s1 == s2

then you're comparing the pointer (the reference) of the String. This reference is not what you're getting from the String#hashCode method.

If you are comparing Strings like:

s1.equals(s2)

then the method String#equals uses the String#hashCode methods of both Strings and compares the results.

Try it out by yourself:

public class StringEquality {
    public static void main(String[] args) {
        String s1 = new String("java");
        String s2 = new String("java");

        // hashCodes
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());

        // equality of hashCodes
        System.out.println(s1.equals(s2));

        // references
        System.out.println(System.identityHashCode(s1));
        System.out.println(System.identityHashCode(s2));

        // equality of references
        System.out.println(s1 == s2);
    }
}

Will print something like:

score 0 · Answer 4 · answered Mar 20 '14 at 08:02

0

All of who? sn1 and sn2 are different objects, but they are identical, so they have the same hash code.

answered Mar 20 '14 at 08:02

Dawood ibn Kareem

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sn1 and sn2 are not identical, they are equal. :-) – Seelenvirtuose Mar 20 '14 at 08:07
In this article [link](http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java) – CHAITHANYA PADALA Mar 20 '14 at 08:11
@Seelenvirtuose That's what identical means. – Dawood ibn Kareem Mar 20 '14 at 08:11
As the "==" operation is often referred to as _identity_ comparison, I though it's better not to say, they are identical. Btw: I know, what you meant. And you also clearly stated that sn1 and sn2 are different objects. So everything is ok with your answer. :-) – Seelenvirtuose Mar 20 '14 at 08:16
1

The sock on my left foot is identical to the sock on my right foot. But I don't have both feet shoved in the same sock. – Dawood ibn Kareem Mar 20 '14 at 08:17
All men are created equal - not identical – Scary Wombat Mar 20 '14 at 08:20

String hashCodes

4 Answers4