java string concatenation decrypted symbol +

I believe many people like me, at the beginning of writing java program, as long as the local connection string appears directly to a "+", that is simple, easy, easy.

Later, as more and more code, more widely read books, get to know, in some circumstances StringBuilder (jdk1.5 before a StringBuffer, StringBuffer is thread-safe, but in normal circumstances it will lead to a sharp decline in the performance of) the efficiency ratio, I believe we can know access to information, java compiler after a compilation, we use the + automatically translate into StringBuilder;

As shown, according to the byte code tool we can clearly know our approach is converted to use StringBuilder, since the compiler has helped us to do, and why do we need to use StringBuilder own it;

private String getStr1() {
    String a = "1";
    String b = "2";
    String c = "3";
    String d = "4";
    String f = "";
    f =  a + b + c + d;
    return f;
}

private String getStr2() {
    String a = "1";
    String b = "2";
    String c = "3";
    String d = "4";
    String f = "";
    b = a + b;
    c = b + c;
    f = c + d;
    return f;
}

As we look at the code, as the effect of these two methods it? The answer is definitely consistent, and that it is consistent both methods performance?

We also take a look at the bytecode of these two methods:

private getStr1()Ljava/lang/String;
   L0
    LINENUMBER 20 L0
    LDC "1"
    ASTORE 1
   L1
    LINENUMBER 21 L1
    LDC "2"
    ASTORE 2
   L2
    LINENUMBER 22 L2
    LDC "3"
    ASTORE 3
   L3
    LINENUMBER 23 L3
    LDC "4"
    ASTORE 4
   L4
    LINENUMBER 24 L4
    LDC ""
    ASTORE 5
   L5
    LINENUMBER 25 L5
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 2
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 3
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 4
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 5
   L6
    LINENUMBER 26 L6
    ALOAD 5
    ARETURN
   L7
    LOCALVARIABLE this Lcom/hik/yuntest/StringTest; L0 L7 0
    LOCALVARIABLE a Ljava/lang/String; L1 L7 1
    LOCALVARIABLE b Ljava/lang/String; L2 L7 2
    LOCALVARIABLE c Ljava/lang/String; L3 L7 3
    LOCALVARIABLE d Ljava/lang/String; L4 L7 4
    LOCALVARIABLE f Ljava/lang/String; L5 L7 5
    MAXSTACK = 2
    MAXLOCALS = 6


  // access flags 0x2
  private getStr2()Ljava/lang/String;
   L0
    LINENUMBER 30 L0
    LDC "1"
    ASTORE 1
   L1
    LINENUMBER 31 L1
    LDC "2"
    ASTORE 2
   L2
    LINENUMBER 32 L2
    LDC "3"
    ASTORE 3
   L3
    LINENUMBER 33 L3
    LDC "4"
    ASTORE 4
   L4
    LINENUMBER 34 L4
    LDC ""
    ASTORE 5
   L5
    LINENUMBER 35 L5
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 2
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 2
   L6
    LINENUMBER 36 L6
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    ALOAD 2
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 3
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 3
   L7
    LINENUMBER 37 L7
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    ALOAD 3
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 4
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 5
   L8
    LINENUMBER 38 L8
    ALOAD 5
    ARETURN
   L9
    LOCALVARIABLE this Lcom/hik/yuntest/StringTest; L0 L9 0
    LOCALVARIABLE a Ljava/lang/String; L1 L9 1
    LOCALVARIABLE b Ljava/lang/String; L2 L9 2
    LOCALVARIABLE c Ljava/lang/String; L3 L9 3
    LOCALVARIABLE d Ljava/lang/String; L4 L9 4
    LOCALVARIABLE f Ljava/lang/String; L5 L9 5
    MAXSTACK = 2

    MAXLOCALS = 6

看到上面的字节码是不是晕了，放心我也看不懂，不过看看我标红的那几句话，同样的效果，同样使用+连接符，方法getStr1只new了一个StringBuilder，而getStr2却new了三个，说明getStr1与我们直接使用StringBuilder的方式是一模一样的，在同一个语句行使用+,它就会直接使用append将字符串追加进去，不产生新的对象，但是分行写，每段的“+”操作，都会产生一个新的对象，从而性能会损失掉，所以如果我们在循环里面拼接字符串，就不要使用“+”。实在是想使用“+”，尽量将语句放在一行解决，但是这样书法又不太美观，总之，一句话，在合适的地方使用“+”，如果你在乎你的性能