String与StringBuilder的区别
String与StringBuilder最主要的区别体现在做字符串连接操作的效率上。
String使用重载运算符+或+=做字符串连接，有代码如下：
public class Concatenation {
public static void main(String[] args) {
String str = "My";
String introduction = str + " name " + " is " + " cgw ";
System.out.println(introduction);
}
}
通过javap进行反编译，我们可以得到JVM生成的字节码：
public 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 My 2: astore_1 3: new #3; //class java/lang/StringBuilder 6: dup 7: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V 10: aload_1 11: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 14: ldc #6; //String name 16: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 19: ldc #7; //String is 21: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 24: ldc #8; //String cgw 26: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)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 }
我们可以看到，在main方法内，JVM是new了一个StringBuilder对象，并通过调用append方法进行字符串连接操作的。
既然这样，那么我们可以不考虑使用StringBuilder对象，而直接使用String，让JVM来为你自动进行优化呢？
请看下面代码：
import java.lang.StringBuilder;
public class WhitherStringBuilder {
public String implicit(String[] fields) {
String result = "";
for(int i = 0 ; i < fields.length ; i++) {
result += fields[i];
}
return result;
}
public String explicit(String[] fields) {
StringBuilder result = new StringBuilder();
for(int i = 0 ; i < fields.length ; i++) {
result.append(fields[i]);
}
return result.toString();
}
}
同样进行反编译得到：
public class WhitherStringBuilder extends java.lang.Object{
public WhitherStringBuilder();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public java.lang.String implicit(java.lang.String[]);
Code:
0: ldc #2; //String
2: astore_2
3: iconst_0
4: istore_3
5: iload_3
6: aload_1
7: arraylength
8: if_icmpge 38
11: new #3; //class java/lang/StringBuilder
14: dup
15: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
18: aload_2
19: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: aload_1
23: iload_3
24: aaload
25: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
28: invokevirtual #6; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
31: astore_2
32: iinc 3, 1
35: goto 5
38: aload_2
39: areturn
public java.lang.String explicit(java.lang.String[]);
Code:
0: new #3; //class java/lang/StringBuilder
3: dup
4: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
7: astore_2
8: iconst_0
9: istore_3
10: iload_3
11: aload_1
12: arraylength
13: if_icmpge 30
16: aload_2
17: aload_1
18: iload_3
19: aaload
20: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
23: pop
24: iinc 3, 1
27: goto 10
30: aload_2
31: invokevirtual #6; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
34: areturn
}
注意：
implicit方法为使用String的+操作进行字符串连接，观察字节码第8-35行，这是一个循环，你会发现，new StringBuilder对象的操作是在循环体内进行的，这说明每次循环都会创建一个新的StringBuilder对象。
explicit方法为使用StringBuilder对象的append方法进行字符串连接，JVM在第0行创建了StringBuilder对象，而循环是在第13至27行，这说明，使用StringBuilder进行字符串连接只会创建一个StringBuilder对象。
由以上得知StringBuilder的字符串连接操作效率要远远高于String。
另StringBuilder的效率要稍高于StringBuffer，因为StringBuffer的线程安全特性会造成额外的开销。