居然还有人对字符串拼接操作时StringBuilder/StringBuffer取代“+”有疑问

首先说明：

至于是StringBuilder还是StringBuffer去取代“+”，这就是实际应用场景需要考量的，看你的应用场景需要考虑线程安全性与否！

每个观点的提出都要有理有据：

如下是两段字符串拼接两种实现方式：

public class TestA {
    public static void main(String[] args) {
        String str = "A" ;
        StringBuilder sb = new StringBuilder(str) ;
        sb.append("B");
        sb.append("C"); 
        System.out.println(str);
    }
}

第二段：

public class TestB {
    public static void main(String[] args) {
        String str = "A" ;
        str += "B" ;
        str += "C" ;
        System.out.println(str);
    }
}

然后用javap去分析：

TestA的分析结果：

public static void main(java.lang.String[]);
  Code:
   0:   ldc     #2; //String A
   2:   astore_1
   3:   new     #3; //class java/lang/StringBuilder
   6:   dup
   7:   aload_1
   8:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   11:  astore_2
   12:  aload_2
   13:  ldc     #5; //String B
   15:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/S
tringBuilder;
   18:  pop
   19:  aload_2
   20:  ldc     #7; //String C
   22:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/S
tringBuilder;
   25:  pop
   26:  getstatic       #8; //Field java/lang/System.out:Ljava/io/PrintStream;
   29:  aload_1
   30:  invokevirtual   #9; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   33:  return

}

 

TestB的分析结果：

public static void main(java.lang.String[]);
  Code:
   0:   ldc     #2; //String A
   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/S
tringBuilder;
   14:  ldc     #6; //String B
   16:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/S
tringBuilder;
   19:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   22:  astore_1
   23:  new     #3; //class java/lang/StringBuilder
   26:  dup
   27:  invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V
   30:  aload_1
   31:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/S
tringBuilder;
   34:  ldc     #8; //String C
   36:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/S
tringBuilder;
   39:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   42:  astore_1
   43:  new     #3; //class java/lang/StringBuilder
   46:  dup
   47:  invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V
   50:  aload_1
   51:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/S
tringBuilder;
   54:  ldc     #9; //String D
   56:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/S
tringBuilder;
   59:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   62:  astore_1
   63:  getstatic       #10; //Field java/lang/System.out:Ljava/io/PrintStream;
   66:  aload_1
   67:  invokevirtual   #11; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   70:  return

}

 

TestA只在第3行产生了1个StringBuilder对象，而TestB在第3，23，43产生了3个StringBuilder对象；

不论是从执行步骤复杂程度还是从StringBuilder实例化对象数量来讲，使用StringBuilder的效率明显高于使用“+”拼接字符串 。使用“+”拼接字符串JVM会将其转换为StringBuilder，但是每“+”一次，就是多一个StringBuilder对象，但是使用StringBuilder拼接字符串，只会有一个StringBuilder对象，另外注意，当使用StringBuilder拼接字符串个数大于它的容量capacity（默认是16）时，它会进行扩容，所以，在能够能够确定最大拼接数的情况下，在new StringBuilder()时，指定capacity-- StringBuilder sb = new StringBuilder(str,3) ;，可以进一步减少空间占用，提高性能！