How does string class does caching of Hashcode in java

Question

Here what is written in String API for Hashcode------
public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
                     


        

Answer 1

But how this happens for two string created with new like :

String a = new String("abc"); String b = new String("abc"); 

It doesn't happen. Caching only occurs for a single String object.

 int h = hash;
 if (h == 0 && value.length > 0) {
        char val[] = value;

        for (int i = 0; i < value.length; i++) {
            h = 31 * h + val[i];
        }
        hash = h;
 }

For a single String, hash is where the hash code is stored. If it's 0, then it recomputes it -- storing it in the local variable h -- and once the hash code is newly computed, it's stored back in hash so it can be used in the future.

If Strings were mutable, then the stored hash code would be wrong after the string got changed, so any way of changing the string would have to reset hash to 0 to indicate it had to be recomputed anew. That gets messy, though, on many many different levels, which is why String is not mutable.