The difference in concat speed between '+' operator of String and append() of StringBuilder

StringVsSringBuilder.java

public class StringVsSringBuilder {
    public static void main(String[] args) {
        String string = "";
        StringBuilder stringBuilder = new StringBuilder();
        String value = "abcd";

        /* String의 '+' 연산 */
        long start = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
            string += value;
        }
        long end = System.currentTimeMillis();
        System.out.println((end - start) / 1000.0 + "초");

        /* StringBuilder의 append 연산 */
        start = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
            stringBuilder.append(value);
        }
        end = System.currentTimeMillis();
        System.out.println((end - start) / 1000.0 + "초");
    }
}