java.lang.String 的 + 号操作，这个谜终于要解开了！

作者：丶Pz
来源：https://www.cnblogs.com/panzi/p/11956782.html

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) {//#1String str1 = "what";//#2String str2 = str1 + " a nice day";//#3System.out.println("what a nice day".equals(str2));//#4System.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？

str1 存放位置？
str2 存放位置？
结果是 true 还是 false？
结果是 true 还是 false？
"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.classLast modified 2019-11-29; size 972 bytesMD5 checksum 1d1f1a23bfe85c2f88d2f767e8aac314Compiled from "Main.java"
public class com.fanpan26.string.test.Mainminor version: 0major version: 52flags: 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: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1                  // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 6: 0LocalVariableTable:Start  Length  Slot  Name   Signature0       5     0  this   Lcom/fanpan26/string/test/Main;public static void main(java.lang.String[]);descriptor: ([Ljava/lang/String;)Vflags: ACC_PUBLIC, ACC_STATICCode:stack=3, locals=3, args_size=10: ldc           #2                  // String what2: astore_13: new           #3                  // class java/lang/StringBuilder6: dup7: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V10: aload_111: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;14: ldc           #6                  // String  a nice day16: 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_223: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;26: ldc           #9                  // String what a nice day28: aload_229: invokevirtual #10                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z32: invokevirtual #11                 // Method java/io/PrintStream.println:(Z)V35: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;38: ldc           #9                  // String what a nice day40: aload_241: if_acmpne     4844: iconst_145: goto          4948: iconst_049: invokevirtual #11                 // Method java/io/PrintStream.println:(Z)V52: returnLineNumberTable:line 9: 0line 11: 3line 13: 23line 15: 35line 16: 52LocalVariableTable:Start  Length  Slot  Name   Signature0      53     0  args   [Ljava/lang/String;3      50     1  str1   Ljava/lang/String;23      30     2  str2   Ljava/lang/String;StackMapTable: number_of_entries = 2frame_type = 255 /* full_frame */offset_delta = 48locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String ]stack = [ class java/io/PrintStream ]frame_type = 255 /* full_frame */offset_delta = 0locals = [ 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());
}