Java中String、StringBuilder、StringBuffer的区别

Java中String、StringBuilder、StringBuffer是我们再熟悉不过的类型了，那么它们之间到底有啥区别呢。

首先，String类型是不可变的，StringBuilder、StringBuffer是可变的。

其次，StringBuilder与StringBuffer类似，无非就是在单线程的情况下StringBuilder要比StringBuffer快。如果要确保线程安全，那就用StringBuffer。

就这么两点。下面看一段代码感受一下String类型拼接，编译器到底是怎么处理的。

package strings;

public class Concatenation {
public static void main(String[] args) {
String mango = "mango";
String s = "abc" + mango + "def" + 47;
System.out.println(s);
}
} /* Output:
abcmangodef47

用javap反编译一下上面的class文件

javap -c Concatenation.class

于是得到以下字节码：

Compiled from "Concatenation.java"
public class strings.Concatenation {
public strings.Concatenation();
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 mango
2: astore_1
3: new           #3                  // class java/lang/StringBuilder
6: dup
7: invokespecial #4                  // Method java/lang/StringBuilder."<init>":
()V
10: ldc           #5                  // String abc
12: invokevirtual #6                  // Method java/lang/StringBuilder.append:(L
java/lang/String;)Ljava/lang/StringBuilder;
15: aload_1
16: invokevirtual #6                  // Method java/lang/StringBuilder.append:(L
java/lang/String;)Ljava/lang/StringBuilder;
19: ldc           #7                  // String def
21: invokevirtual #6                  // Method java/lang/StringBuilder.append:(L
java/lang/String;)Ljava/lang/StringBuilder;
24: bipush        47
26: invokevirtual #8                  // Method java/lang/StringBuilder.append:(I
)Ljava/lang/StringBuilder;
29: invokevirtual #9                  // Method java/lang/StringBuilder.toString:
()Ljava/lang/String;
32: astore_2
33: getstatic     #10                 // Field java/lang/System.out:Ljava/io/Prin
tStream;
36: aload_2
37: invokevirtual #11                 // Method java/io/PrintStream.println:(Ljav
a/lang/String;)V
40: return
}

在12到21行编译器将String转换为StringBuilder，这样做就是为了更高效。