System.currentTimeMillis();
//当前时间与1970年1月1日午夜UTC之间的毫秒差。

public class HelloWorld {
    public static void main(String[] args) {

        String s1 = "";
        StringBuilder s2 = new StringBuilder("");
        long time = System.currentTimeMillis();
        long spendTime;
        int n = 100000;
        for (int i = 0; i < n; i++) {
            s1 += "abc";
        }
        spendTime = System.currentTimeMillis()-time;
        System.out.println(spendTime/1000.0 + "s");//String操作花费的时间

        time = System.currentTimeMillis();

        for (int i = 0; i < n; i++) {
            s2.append("abc");

        }
         spendTime = System.currentTimeMillis()-time;
        System.out.println(spendTime/1000.0 + "s");//StringBuilder操作花费的时间

    }
}

StringBuilder明显快于String。