Is hash code of java.lang.String really cached?

问题

String s1 = "String1";      
System.out.println(s1.hashCode()); // return an integer i1 


Field field = String.class.getDeclaredField("value");  
field.setAccessible(true);  
char[] value = (char[])field.get(s1);  
value[0] = 'J';  
value[1] = 'a';  
value[2] = 'v';  
value[3] = 'a';  
value[4] = '1'; 
System.out.println(s1.hashCode()); // return same value of integer i1 

Here even after I changed the characters with the help of reflection, same hash code value is mainatained.

Is there anything I need to know here?

回答1:

A String is meant to be immutable. As such, there is no point having to recalculate the hashcode. It is cached internally in a field called hash of type int.

String#hashCode() is implemented as (Oracle JDK7)

public int hashCode() {
    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;
    }
    return h;
}

where hash initially has a value of 0. It will only be calculated the first time the method is called.

As stated in the comments, using reflection breaks the immutability of the object.

来源：https://stackoverflow.com/questions/21000611/is-hash-code-of-java-lang-string-really-cached