作者:丶Pz
https://www.cnblogs.com/panzi/p/11956782.html
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?
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.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 StringBuffer 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( 3种骚操作,教你查看 Java 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()); }