Java – what is the purpose of using local variables to hold global variables?

I looked at string Hashcode () method source code This is the implementation of 6-b14 and has changed

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;
}

My question is about this line:

int len = count;

Where count is a global variable representing the number of string characters

Why is the local variable len used here for loop conditions rather than the global variable itself? Because there are no manipulated variables, you can only read If you use global fields to read or write local variables, is it a good habit to use local variables? If the answer is yes, why read?

Solution

Looking in the string class, I found something about string Comments in the trim () method to avoid strange assignment of local variables of getfield opcode

public String trim() {
    int len = value.length;
    int st = 0;
    char[] val = value;    /* avoid getfield opcode */

    while ((st < len) && (val[st] <= ' ')) {
        st++;
    }
    while ((st < len) && (val[len - 1] <= ' ')) {
        len--;
    }
    return ((st > 0) || (len < value.length)) ? substring(st,len) : this;
}

So the whole thing seems to be about performance, as Frank olschewski pointed out

In Java bytecode, instance variables are actually referenced by objects and names (using the getfield instruction) Without optimization, the VM must do more to access variables

Therefore, the potential performance loss of the code is that it uses the relatively expensive getfield instruction every time it passes through the loop The local assignment in this method does not require getfield for each loop

The JIT optimizer may or may not optimize the loop, so developers may take a safe path to manually enforce it

Avoiding getfield opcode has a separate question with details