String的+号操作到底做了什么事情

用户4283147

发布于 2022-10-08 12:45:37

3370

发布于 2022-10-08 12:45:37

文章被收录于专栏：对线JAVA面试

前言

在之前的面试经历中，对于String的考察还是挺频繁的，大致考察以下几个知识点：

String 常量池
new String()
== 和 equals 的区别
native 方法 String.intern()

虽然面试中大体答对了，但是今天早上微信群里的一个问题我却答不上来，这个问题是这样的：

String str3 = "what";
String str4 = str3 + " a nice day";
//运行时， + 相当于 new，所以堆中会有 "what a nice day"对象，常量池中会有"what"," a nice day"两个对象，而不会有 "what a nice day"对象。
//这句话大佬们看看对不对啊，我怎么感觉不对啊
//常量池不会有"what a nice day" 对象吗？

看完这个问题，说实话我也是有点懵的，我只是知道 "what a nice day"不会在常量池，但是不知道具体的原因，后来群里的同学说 + 号是调用了 StringBuffer 的append 方法。我去证实了，发现确实调用了 append 方法，但是当时没有调用toString()方法，我很疑惑。（最后经过证实，是StringBuilder的append 方法，不是StringBuffer）。

代码验证

 public static void main(String[] args) {
        //#1
        String str1 = "what";
        //#2
        String str2 = str1 + " a nice day";
        //#3
        System.out.println("what a nice day".equals(str2));
        //#4
        System.out.println("what a nice day" == str2);
    }

现在有以下几个问题，小伙伴们看看是否能答出来，即使答出来了，你知道为什么吗？

解答分析（基于JDK1.8）

下面也不靠猜，我们直接查看生成的字节码：

localhost:test didi$ javap -verbose -p Main.class
Classfile /develop/project/string-test/out/production/classes/com/fanpan26/string/test/Main.class
  Last modified --; size  bytes
  MD5 checksum d1f1a23bfe85c2f88d2f767e8aac314
  Compiled from "Main.java"
public class com.fanpan26.string.test.Main
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #.#        // java/lang/Object."<init>":()V
   # = String             #            // what
   # = Class              #            // java/lang/StringBuilder
   # = Methodref          #.#         // java/lang/StringBuilder."<init>":()V
   # = Methodref          #.#         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   # = String             #            //  a nice day
   # = Methodref          #.#         // java/lang/StringBuilder.toString:()Ljava/lang/String;
   # = Fieldref           #.#        // java/lang/System.out:Ljava/io/PrintStream;
   # = String             #            // what a nice day
  # = Methodref          #.#        // java/lang/String.equals:(Ljava/lang/Object;)Z
  # = Methodref          #.#        // java/io/PrintStream.println:(Z)V
  # = Class              #            // com/fanpan26/string/test/Main
  # = Class              #            // java/lang/Object
  # = Utf8               <init>
  # = Utf8               ()V
  # = Utf8               Code
  # = Utf8               LineNumberTable
  # = Utf8               LocalVariableTable
  # = Utf8               this
  # = Utf8               Lcom/fanpan26/string/test/Main;
  # = Utf8               main
  # = Utf8               ([Ljava/lang/String;)V
  # = Utf8               args
  # = Utf8               [Ljava/lang/String;
  # = Utf8               str1
  # = Utf8               Ljava/lang/String;
  # = Utf8               str2
  # = Utf8               StackMapTable
  # = Class              #            // "[Ljava/lang/String;"
  # = Class              #            // java/lang/String
  # = Class              #            // java/io/PrintStream
  # = Utf8               SourceFile
  # = Utf8               Main.java
  # = NameAndType        #:#        // "<init>":()V
  # = Utf8               what
  # = Utf8               java/lang/StringBuilder
  # = NameAndType        #:#        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  # = Utf8                a nice day
  # = NameAndType        #:#        // toString:()Ljava/lang/String;
  # = Class              #            // java/lang/System
  # = NameAndType        #:#        // out:Ljava/io/PrintStream;
  # = Utf8               what a nice day
  # = Class              #            // java/lang/String
  # = NameAndType        #:#        // equals:(Ljava/lang/Object;)Z
  # = Class              #            // java/io/PrintStream
  # = NameAndType        #:#        // println:(Z)V
  # = Utf8               com/fanpan26/string/test/Main
  # = Utf8               java/lang/Object
  # = Utf8               java/lang/String
  # = Utf8               java/io/PrintStream
  # = Utf8               append
  # = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  # = Utf8               toString
  # = Utf8               ()Ljava/lang/String;
  # = Utf8               java/lang/System
  # = Utf8               out
  # = Utf8               Ljava/io/PrintStream;
  # = Utf8               equals
  # = Utf8               (Ljava/lang/Object;)Z
  # = Utf8               println
  # = Utf8               (Z)V
{
  public com.fanpan26.string.test.Main();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=, locals=, args_size=
         : aload_0
         : invokespecial #                  // Method java/lang/Object."<init>":()V
         : return
      LineNumberTable:
        line : 
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
                          this   Lcom/fanpan26/string/test/Main;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=, locals=, args_size=
         : ldc           #                  // String what
         : astore_1
         : new           #                  // class java/lang/StringBuilder
         : dup
         : invokespecial #                  // Method java/lang/StringBuilder."<init>":()V
        : aload_1
        : invokevirtual #                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        : ldc           #                  // String  a nice day
        : invokevirtual #                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        : invokevirtual #                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        : astore_2
        : getstatic     #                  // Field java/lang/System.out:Ljava/io/PrintStream;
        : ldc           #                  // String what a nice day
        : aload_2
        : invokevirtual #                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
        : invokevirtual #                 // Method java/io/PrintStream.println:(Z)V
        : getstatic     #                  // Field java/lang/System.out:Ljava/io/PrintStream;
        : ldc           #                  // String what a nice day
        : aload_2
        : if_acmpne     
        : iconst_1
        : goto          
        : iconst_0
        : invokevirtual #                 // Method java/io/PrintStream.println:(Z)V
        : return
      LineNumberTable:
        line : 
        line : 
        line : 
        line : 
        line : 
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
                         args   [Ljava/lang/String;
                         str1   Ljava/lang/String;
                        str2   Ljava/lang/String;
      StackMapTable: number_of_entries = 
        frame_type =  /* full_frame */
          offset_delta = 
          locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String ]
          stack = [ class java/io/PrintStream ]
        frame_type = 255 /* full_frame */
          offset_delta = 0
          locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String ]
          stack = [ class java/io/PrintStream, int ]
}
SourceFile: "Main.java"

从 Constant pool: 中的信息可以看到，#2 #6 #9 可以解答上文中的#1,#5两个问题。

str1 是存放在常量池的
"what a nice day" （非str2）也是存放在常量池的.

下面我们看一下 + 操作做了什么事情，可以在Code中看到，该操作调用了 StringBuilder.append 方法

: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
: ldc           #6                  // String  a nice day
: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

那么到这里一切都答案都出来了

str2 是存放在堆中。
equals 为 true
== 为 false

所以说其实 str1 + " a nice day" 就相当于 new StringBuilder().append(str1).append(" a nice day");

//这两种写法生成的字节码是一样的。
//String str2 = str1 + " a nice day";
String str2 = new StringBuilder().append(str1).append(" a nice day").toString();

而StringBuilder 的toString 方法如下：

    @Override
    public String toString() {
        // 所以说 str2 其实是一个 new String，是不在常量池里面的。
        return new String(value, , count);
    }

总结

通过类的字节码可以查看底层具体用什么方式实现，所以说虽然看似一个简单的String问题，其实往深处挖掘还是考察了对生成的字节码的理解。还有，遇到一个问题，不能死记答案，有些人告诉你，+ 操作就是 new 对象，但是具体到底是不是或者为什么是有没有思考过呢？上文中如有错误，欢迎指出。

试一试

/**
     * 以下程序输出的结果是什么？
     * */
    public static void main(String[] args) {
        String str1 = "what";
        String str2 = str1 + " a nice day";
        System.out.println("what a nice day".equals(str2));
        System.out.println("what a nice day" == str2);
    }

/**
     * 以下程序输出的结果是什么？
     * */
    public static void main(String[] args) {
        String str1 = "what a nice day";
        String str2 = new String("what a nice day");
        System.out.println(str1.equals(str2));
        System.out.println(str1 == str2);
    }

/**
     * 以下程序输出的结果是什么？
     * */
    public static void main(String[] args) {
        String str1 = "what";
        String str2 = str1.concat(" a nice day");
        System.out.println("what a nice day".equals(str2));
        System.out.println("what a nice day" == str2);
        System.out.println("what a nice day"==str2.intern());
    }

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原始发表：2021-06-16，如有侵权请联系 cloudcommunity@tencent.com 删除

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