java - 当我们在java中使用连接两个字符串对象时编译器如何工作

Question

 class Simple{  
     public static void main(String args[]){  
         String s="Sachin"+" Tendulkar";  
         System.out.println(s);//Sachin Tendulkar  
     }  
 }  

编译器将其转换为:

    String s = (new StringBuilder()).append("Sachin").append(" Tendulkar").toString();  

最后一行是什么意思？

Answer 1

Java 编译器只会在编译时连接这两个字符串，并生成以下指令

代码

public class StringConcatInspection {
    public static void main(String[] args) {
        String s = "Sachin" + " Tendulkar";
    }
}

它将编译为

  Last modified Jul 9, 2014; size 473 bytes
  MD5 checksum 5238429068ccae4ec199f4af970ad46e
  Compiled from "StringConcatInspection.java"
public class StringConcatInspection
  SourceFile: "StringConcatInspection.java"
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #4.#20         //  java/lang/Object."<init>":()V
   #2 = String             #21            //  Sachin Tendulkar
   #3 = Class              #22            //  StringConcatInspection
   #4 = Class              #23            //  java/lang/Object
   #5 = Utf8               <init>
   #6 = Utf8               ()V
   #7 = Utf8               Code
   #8 = Utf8               LineNumberTable
   #9 = Utf8               LocalVariableTable
  #10 = Utf8               this
  #11 = Utf8               LStringConcatInspection;
  #12 = Utf8               main
  #13 = Utf8               ([Ljava/lang/String;)V
  #14 = Utf8               args
  #15 = Utf8               [Ljava/lang/String;
  #16 = Utf8               s
  #17 = Utf8               Ljava/lang/String;
  #18 = Utf8               SourceFile
  #19 = Utf8               StringConcatInspection.java
  #20 = NameAndType        #5:#6          //  "<init>":()V
  #21 = Utf8               Sachin Tendulkar
  #22 = Utf8               StringConcatInspection
  #23 = Utf8               java/lang/Object
{
  public StringConcatInspection();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0       
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return        
      LineNumberTable:
        line 4: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0       5     0  this   LStringConcatInspection;

  public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=2, args_size=1
         0: ldc           #2                  // String Sachin Tendulkar
         2: astore_1      
         3: return        
      LineNumberTable:
        line 6: 0
        line 7: 3
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0       4     0  args   [Ljava/lang/String;
               3       1     1     s   Ljava/lang/String;
}

编译器版本:1.7.0_60-ea

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