String，StringBuilder字符串拼接性能问题

这是源程序：

String s = "";
        for (int i = 0; i < 50000; i ++)
        {
            s += i;
        }
        
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 50000; i ++)
        {
            sb.append(i);
        }
        StringBuilder sb2 = new StringBuilder();
        for (int i = 0; i < 50000; i ++)
        {
            sb2.append("s" + i);
        }
        
        String s3 = "";
        for (int i = 0; i < 50000; i ++)
        {
            s3 = s3.concat(String.valueOf(i));
        }
        String s4 = "";
        for (int i = 0; i < 50000; i ++)
        {
            s4 = s4.concat("s" + String.valueOf(i));
        }String s = "";
        for (int i = 0;