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:(Ljava/lang/String;)Ljava/lang/StringBuilder;      15: aload_1      16: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;      19: ldc           #7                  // String def      21: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/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/PrintStream;      36: aload_2      37: invokevirtual #11                 // Method java/io/PrintStream.println:(Ljava/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。