Does storing the str.length() value in a variable before using it in a for loop have any performance improvements in Java?

Question

In short, does the JVM internally optimize the following code

public void test(String str)
{
    int a = 0;

    for( int i = 0; i < 10; i++)
    {
        a = a + str.length();
    }
}

to behave as efficiently as the one below:

public void test(String str)
{
    int len = str.length();
    int a = 0;

    for( int i = 0; i < 10; i++)
    {
        a = a + len;
    }
}

If it does optimize, does it do so by caching the str.length() value internally?

Answer 1

nice answer Elliot F.

I did a much simpler test and ran the two methods with a very large number of repetitions and timed each.

The first method (where the length is only calculated once) was consistently faster than the second method.

Here's the whole test class I created;

package _testing;

import java.util.Date;

public class Speed {

    long count = 5000000;
    public static void main(String[] args) {

        long start, finish; 
        Speed sp = new Speed(); 

        start = new Date().getTime();
        sp.test("test");
        finish = new Date().getTime();
        System.out.println("test 1:"+(finish - start));

        start = new Date().getTime();
        sp.test2("test");
        finish = new Date().getTime();
        System.out.println("test 2:"+(finish - start));

    }


    public void test(String str) {
        int len = str.length();
        int a = 0;
        for (int i = 0; i < count; i++) {
            a = a + len;
        }
    }

    public void test2(String str) {
        int a = 0;
        for (int i = 0; i < count; i++) {
            a = a + str.length();
        }
    }   
}

The output looks like this;

test 1:7
test 2:22