字符串连接:使用StringBuffer#append来替代”+”真的会带来性能提升吗？

We know:

1.StringBuffer sb = new StringBuffer(); //默认的缓冲区大小是16

2.用 + 来连接字符串，编译的时候，会自动转换成StringBuffer

for example：【参考于大神之作：http://www.ticmy.com/?p=69】

String s = "today is " + "a good day";

编译直接形成：

 Code:
0:   ldc     #5; //String today is a good day

 String tmp = " on the desk" ;
String s2 = "there are " + count + " books " + tmp;
编译直接形成：

  Code:
   0 :   iconst_2
   1 :   istore_0
   2 :   ldc     # 8 ; //String  on the desk
   4 :   astore_1
   5 :   new     # 9 ; //class java/lang/StringBuilder
   8 :   dup
   9 :   invokespecial   # 10 ; //Method java/lang/StringBuilder."":()V
   12 :  ldc     # 11 ; //String there are
   14 :  invokevirtual   # 12 ; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   17 :  iload_0
   18 :  invokevirtual   # 13 ; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
   21 :  ldc     # 14 ; //String  books
   23 :  invokevirtual   # 12 ; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   26 :  aload_1
   27 :  invokevirtual   # 12 ; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   30 :  invokevirtual   # 15 ; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   33 :  astore_2

但是对于循环内部（每次创建会引起性能损耗不少）：

 String s3 = "" ;
        for ( int i= 0 ; i< 100 ; i++) {
            s3 = s3 + i;
        }
  Code:
   0 :   ldc     # 16 ; //String
   2 :   astore_0
   3 :   iconst_0
   4 :   istore_1
   5 :   iload_1
   6 :   bipush  100
   8 :   if_icmpge       36
   11 :  new     # 9 ; //class java/lang/StringBuilder
   14 :  dup
   15 :  invokespecial   # 10 ; //Method java/lang/StringBuilder."":()V
   18 :  aload_0
   19 :  invokevirtual   # 12 ; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   22 :  iload_1
   23 :  invokevirtual   # 13 ; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
   26 :  invokevirtual   # 15 ; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;

We get:

1.有的时候默认的缓冲区大小不够用，需要扩大，这就会影响到性能。如果我们在初始化的时候，设置一个合适的大小，new StringBuffer(xxx);这样就能在一定程度上提高性能。

2.在循环中使用 + 时，编译器的优化并不如我们期望的那么好，所以就要自己来吧new StringBuffer(xxx)拿出来，写外边了

String s3 = "" ;
StringBuilder tmp = new StringBuilder();
tmp.append(s3);
for ( int i= 0 ; i< 100 ; i++) {
    tmp.append(i);
}
s3 = tmp.toString();