java.lang.String 的 + 号操作到底做了什么事情？
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？
#1str1 存放位置？#2str2 存放位置？#3结果是 true 还是 false？#4结果是 true 还是 false？#5"what a nice day" 存放在哪个位置呢？
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 2019-11-29; size 972 bytes
MD5 checksum 1d1f1a23bfe85c2f88d2f767e8aac314
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 #13.#34 // java/lang/Object."<init>":()V
#2 = String #35 // what
#3 = Class #36 // java/lang/StringBuilder
#4 = Methodref #3.#34 // java/lang/StringBuilder."<init>":()V
#5 = Methodref #3.#37 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#6 = String #38 // a nice day
#7 = Methodref #3.#39 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#8 = Fieldref #40.#41 // java/lang/System.out:Ljava/io/PrintStream;
#9 = String #42 // what a nice day
#10 = Methodref #43.#44 // java/lang/String.equals:(Ljava/lang/Object;)Z
#11 = Methodref #45.#46 // java/io/PrintStream.println:(Z)V
#12 = Class #47 // com/fanpan26/string/test/Main
#13 = Class #48 // java/lang/Object
#14 = Utf8 <init>
#15 = Utf8 ()V
#16 = Utf8 Code
#17 = Utf8 LineNumberTable
#18 = Utf8 LocalVariableTable
#19 = Utf8 this
#20 = Utf8 Lcom/fanpan26/string/test/Main;
#21 = Utf8 main
#22 = Utf8 ([Ljava/lang/String;)V
#23 = Utf8 args
#24 = Utf8 [Ljava/lang/String;
#25 = Utf8 str1
#26 = Utf8 Ljava/lang/String;
#27 = Utf8 str2
#28 = Utf8 StackMapTable
#29 = Class #24 // "[Ljava/lang/String;"
#30 = Class #49 // java/lang/String
#31 = Class #50 // java/io/PrintStream
#32 = Utf8 SourceFile
#33 = Utf8 Main.java
#34 = NameAndType #14:#15 // "<init>":()V
#35 = Utf8 what
#36 = Utf8 java/lang/StringBuilder
#37 = NameAndType #51:#52 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#38 = Utf8 a nice day
#39 = NameAndType #53:#54 // toString:()Ljava/lang/String;
#40 = Class #55 // java/lang/System
#41 = NameAndType #56:#57 // out:Ljava/io/PrintStream;
#42 = Utf8 what a nice day
#43 = Class #49 // java/lang/String
#44 = NameAndType #58:#59 // equals:(Ljava/lang/Object;)Z
#45 = Class #50 // java/io/PrintStream
#46 = NameAndType #60:#61 // println:(Z)V
#47 = Utf8 com/fanpan26/string/test/Main
#48 = Utf8 java/lang/Object
#49 = Utf8 java/lang/String
#50 = Utf8 java/io/PrintStream
#51 = Utf8 append
#52 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#53 = Utf8 toString
#54 = Utf8 ()Ljava/lang/String;
#55 = Utf8 java/lang/System
#56 = Utf8 out
#57 = Utf8 Ljava/io/PrintStream;
#58 = Utf8 equals
#59 = Utf8 (Ljava/lang/Object;)Z
#60 = Utf8 println
#61 = Utf8 (Z)V
{
public com.fanpan26.string.test.Main();
descriptor: ()V
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 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 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=3, locals=3, args_size=1
0: ldc #2 // String what
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 a nice day
16: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
22: astore_2
23: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
26: ldc #9 // String what a nice day
28: aload_2
29: invokevirtual #10 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
32: invokevirtual #11 // Method java/io/PrintStream.println:(Z)V
35: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
38: ldc #9 // String what a nice day
40: aload_2
41: if_acmpne 48
44: iconst_1
45: goto 49
48: iconst_0
49: invokevirtual #11 // Method java/io/PrintStream.println:(Z)V
52: return
LineNumberTable:
line 9: 0
line 11: 3
line 13: 23
line 15: 35
line 16: 52
LocalVariableTable:
Start Length Slot Name Signature
0 53 0 args [Ljava/lang/String;
3 50 1 str1 Ljava/lang/String;
23 30 2 str2 Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 48
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
11: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
14: ldc #6 // String a nice day
16: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: 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, 0, 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());
}
多学点，总不会吃亏的。
