【转】String字符串相加的问题

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String字符串相加的问题

前几天同事跟我说我之前写的代码中在操作字符串时候，使用字符串相加的方式而不是使用StringBuffer或者StringBuilder导致内存开销很大。这个问题一直在困扰我，因为在《Think in java》一书中，作者说使用“+”拼接字符串并不比StringBuffer或者StringBuilder效率低下，因为“+”是java唯一一个系统级的针对字符串的重载过的操作符。

大家都知道String是一个final修饰的类。那么两个字符串使用“+”相加到底会不会导致字符串操作效率受到影响呢？

下面是书中和牛人的理解的结合：

用bytecode来说明问题：

1.使用String + 的方式：
public class Test2 {
public static void main(String[] args) {
String a = "a";
String b = "b";
String c = "c";
String s = a + b + c;
}
}

【同时揭示了为何变量相加时与常量池中的“abc”不相等因为相加变量本质上是一个StringBuilder对象】

对应的bytecode为：

public class Test2 extends java.lang.Object{
public Test2();
Code:
0:   aload_0
1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
4:   return

public static void main(java.lang.String[]);
Code:
0:   ldc     #2; //String a
2:   astore_1
3:   ldc     #3; //String b
5:   astore_2
6:   ldc     #4; //String c
8:   astore_3
9:   new     #5; //class StringBuffer
12:  dup
13:  invokespecial   #6; //Method java/lang/StringBuffer."<init>":()V
16:  aload_1
17:  invokevirtual   #7; //Method java/lang/StringBuffer.append:(Ljava/lang/S
tring;)Ljava/lang/StringBuffer;
20:  aload_2
21:  invokevirtual   #7; //Method java/lang/StringBuffer.append:(Ljava/lang/S
tring;)Ljava/lang/StringBuffer;
24:  aload_3
25:  invokevirtual   #7; //Method java/lang/StringBuffer.append:(Ljava/lang/S
tring;)Ljava/lang/StringBuffer;
28:  invokevirtual   #8; //Method java/lang/StringBuffer.toString:()Ljava/lan
g/String;
31:  astore  4
33:  return

}

2.使用StringBuffer.append()的方式：
public class Test3 {
public static void main(String[] args) {
String a = "a";
String b = "b";
String c = "c";
StringBuffer sb = new StringBuffer();
sb.append(a).append(b).append(c);
String s = sb.toString();
}
}

对应的bytecode为：

public class Test3 extends java.lang.Object{
public Test3();
Code:
0:   aload_0
1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
4:   return

public static void main(java.lang.String[]);
Code:
0:   ldc     #2; //String a
2:   astore_1
3:   ldc     #3; //String b
5:   astore_2
6:   ldc     #4; //String c
8:   astore_3
9:   new     #5; //class StringBuffer
12:  dup
13:  invokespecial   #6; //Method java/lang/StringBuffer."<init>":()V
16:  astore  4
18:  aload   4
20:  aload_1
21:  invokevirtual   #7; //Method java/lang/StringBuffer.append:(Ljava/lang/S
tring;)Ljava/lang/StringBuffer;
24:  aload_2
25:  invokevirtual   #7; //Method java/lang/StringBuffer.append:(Ljava/lang/S
tring;)Ljava/lang/StringBuffer;
28:  aload_3
29:  invokevirtual   #7; //Method java/lang/StringBuffer.append:(Ljava/lang/S
tring;)Ljava/lang/StringBuffer;
32:  pop
33:  aload   4
35:  invokevirtual   #8; //Method java/lang/StringBuffer.toString:()Ljava/lan
g/String;
38:  astore  5
40:  return

}

看到了么？后者不光不比前者效率高，反而多了一些临时变量的存取

至于推荐用StringBuffer来代替String+拼装字符串，说的是循环方式下，如：

public class Test4 {
public static void main(String[] args) {
String s = "s";
for (int i = 0; i < 20; i++) {
s += i;
}
}
}

对应的bytecode为：

public class Test4 extends java.lang.Object{
public Test4();
Code:
0:   aload_0
1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
4:   return

public static void main(java.lang.String[]);
Code:
0:   ldc     #2; //String s
2:   astore_1
3:   iconst_0
4:   istore_2
5:   iload_2
6:   bipush  20
8:   if_icmpge       36
11:  new     #3; //class StringBuffer
14:  dup
15:  invokespecial   #4; //Method java/lang/StringBuffer."<init>":()V
18:  aload_1
19:  invokevirtual   #5; //Method java/lang/StringBuffer.append:(Ljava/lang/S
tring;)Ljava/lang/StringBuffer;
22:  iload_2
23:  invokevirtual   #6; //Method java/lang/StringBuffer.append:(I)Ljava/lang
/StringBuffer;
26:  invokevirtual   #7; //Method java/lang/StringBuffer.toString:()Ljava/lan
g/String;
29:  astore_1
30:  iinc    2, 1
33:  goto    5
36:  return

}

如果用String+的方式，每循环一次，就会重新new一个StringBuffer对象，这样的内存消耗完全是不必要的（在数据量大的情况下，还会导致内存不足的错误），所以要这样做：

public class Test5 {
public static void main(String[] args) {
StringBuffer sb = new StringBuffer("s");
for (int i = 0; i < 20; i++) {
sb.append(i);
}
}
}
这样无论循环多少次，都只会生成一个StringBuffer对象