String 拼接字符串每次都会创建新的对象，速度最慢
StringBuilder 线程不安全，适合在单线程中拼接字符串
StringBuffer 线程安全，适合多线程中使用

我们在代码中测试下使用效率：

/**
     * 用String拼接字符串
     */
    public static void testStringJoint(){

        String strFinal = "abcdefghijklmnopqrstovwxyz";
        String a = "";
        Random rand = new Random();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i <10000; i++){
            int randInt = rand.nextInt(10);
            a +=  strFinal.substring(randInt, randInt + 14);
        }
        long endTime = System.currentTimeMillis();
        System.out.println(endTime-startTime);
    }

    /**
     * 用String拼接字符串
     */
    public static void testStringBuilder(){

        String strFinal = "abcdefghijklmnopqrstovwxyz";
        StringBuilder sb = new StringBuilder();
        Random rand = new Random();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i <10000; i++){
            int randInt = rand.nextInt(13);
            sb.append(strFinal.substring(randInt, randInt + 14));
        }
        long endTime = System.currentTimeMillis();
        System.out.println(endTime-startTime);
    }

返回结果如下：

image.png

循环次数为10000次的时候，用String拼接的时间是923ms，而StringBuilder的时间是1ms，这完全不在一个数量级了