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

在之前的面試經歷中，對于String的考察還是挺頻繁的，大致考察以下幾個知識點：

• String 常量池

• new String()

• == 和 equals 的區(qū)別

• native 方法 String.intern()

雖然面試中大體答對了，但是今天早上微信群里的一個問題我卻答不上來，這個問題是這樣的：

String?str3?=?"what"; String?str4?=?str3?+?"?a?nice?day"; //運行時，?+?相當于 new，所以堆中會有?"what a nice day"對象，常量池中會有"what"," a nice day"兩個對象，而不會有?"what a nice day"對象。 //這句話大佬們看看對不對啊，我怎么感覺不對啊 //常量池不會有"what a nice day"?對象嗎？

看完這個問題，說實話我也是有點懵的，我只是知道 "what a nice day"不會在常量池，但是不知道具體的原因，后來群里的同學說 + 號是調用了 StringBuffer 的append 方法。我去證實了，發(fā)現(xiàn)確實調用了 append 方法，但是當時沒有 調用toString()方法，我很疑惑。（最后經過證實，是StringBuilder的append 方法，不是StringBuffer）。

代碼驗證

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); }

現(xiàn)在有以下幾個問題，小伙伴們看看是否能答出來，即使答出來了，你知道為什么嗎？

str1 存放位置？

str2 存放位置？

結果是 true 還是 false？

結果是 true 還是 false？

"what a nice day" 存放在哪個位置呢？

解答分析（基于JDK1.8）

下面也不靠猜，我們直接查看生成的字節(jié)碼：

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"

從Constant pool: 中的信息可以看到，#2 、#6、#9 可以解答上文中的1,5兩個問題。

• str1 是存放在常量池的

• "what a nice day" （非str2）也是存放在常量池的

下面我們看一下 + 操作做了什么事情，可以在Code中看到，該操作調用了 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;

那么到這里一切都答案都出來了

• str2 是存放在堆中

• equals 為 true

• == 為 false

所以說其實 str1 + " a nice day" 就相當于 new StringBuilder().append(str1).append(" a nice day");

//這兩種寫法生成的字節(jié)碼是一樣的。 //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()?{//?所以說 str2 其實是一個 new String，是不在常量池里面的。return?new?String(value,?0,?count); }

總結

通過類的字節(jié)碼可以查看底層具體用什么方式實現(xiàn)，所以說雖然看似一個簡單的String問題，其實往深處挖掘還是考察了對生成的字節(jié)碼的理解。還有，遇到一個問題，不能死記答案，有些人告訴你，+ 操作就是 new 對象，但是具體到底是不是或者為什么是有沒有思考過呢？上文中如有錯誤，歡迎指出。

試一試

/***?以下程序輸出的結果是什么？*?*/ 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); } /***?以下程序輸出的結果是什么？*?*/ 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); } /***?以下程序輸出的結果是什么？*?*/ 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()); }

?

總結

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