说说String

来源：互联网发布：数据概况英文翻译编辑：程序博客网时间：2024/06/07 06:33

String它是一个引用数据类型，不是一个基础数据类型。
先思考一个问题：String为什么是不可更改的。
查看String类的签名如下：

public final class String      implements java.io.Serializable, Comparable<String>, CharSequence {}

然后再看看String到底是怎么存储字符串的：

/** The value is used for character storage. */      private final char value[];

String类的签名，和存储String的char数组都被final修饰，它们确保了String对象是永远不会被修改的。

一、内存存储

下面我们继续String之旅吧，先贴一段学习代码。

public class LearnString {    public static void main(String args[]) {        String a = "abc";        String b = "abc";        String c = new String("abc");        System.out.println(a == b);        System.out.println(a == c);        System.out.println(a.equals(c));    }}

运行结果也如大家所想：

truefalsetrue

我们先看看String的内存分布情况吧

public class LearnString {     String b = "abc";    public static void main(String args[]) {        String a = "abc";    }}

编译后，我们通过javap -v LearnString来查看内存的分布情况，可以看到在字节码的Constant pool区有如下一行：

Constant pool:   #1 = Methodref          #6.#23         // java/lang/Object."<init>":()V   #2 = String             #24            // ab   #3 = Fieldref           #5.#25         // com/example/learn/LearnString.d:Ljava/lang/String;   #4 = String             #26            // abc

可见不管是成员变量还是局部变量，只要String一开始就被赋值了，那么它的值就会被保存在字节码的常量池中。其实你会发现如果把之前的：

String d = "ab" + "d";

重新编译后再javap后如下:

Constant pool:   #1 = Methodref          #6.#23         // java/lang/Object."<init>":()V   #2 = String             #24            // ab   #3 = Fieldref           #5.#25         // com/example/learn/LearnString.d:Ljava/lang/String;   #4 = String             #26            // abc

这时候我们可以发现结果是一样的，也就是说compiler发现这些”+”操作完全可以在编译阶段优化掉，compiler就会进行一定的优化操作。
接下来我们可以讨论开篇的那个例子了。“abc”字符串按照上面所说，在编译的时候就被保存在字节码的常量池中了，所以a 和 b都是拿的常量池中的“abc”值（指向了同一个堆地址），故a==b为true；c = new String(“abc”) ，它其实创建了两个对象，一个new出来的另一个就是常量“abc”,c的引用指向了new出来的对象，故a!=c。

    public boolean equals(Object anObject) {        if (this == anObject) {            return true;        }        if (anObject instanceof String) {            String anotherString = (String)anObject;            int n = value.length;            if (n == anotherString.value.length) {                char v1[] = value;                char v2[] = anotherString.value;                int i = 0;                while (n-- != 0) {                    if (v1[i] != v2[i])                        return false;                    i++;                }                return true;            }        }        return false;    }

这是String重写Object的equals方法，由此可以发现equals方法是比较的是两个String对象里头的字符数组（char[]），比较的是堆中值而非堆地址，故a.equals(c)是相等。

二、注意的问题

先看看下面代码

String s = "";  for(int i = 0; !"end".equals(args[i]);){        s+=args[i];  }

同样用javap反编译字节码

Classfile /F:/workspaces/JavaLearn/out/production/JavaLearn/com/example/learn/LearnString.class  Last modified 2017-4-25; size 779 bytes  MD5 checksum 646b7bf02cd674af0bc5e55605ee3713  Compiled from "LearnString.java"public class com.example.learn.LearnString  minor version: 0  major version: 50  flags: ACC_PUBLIC, ACC_SUPERConstant pool:   #1 = Methodref          #10.#30        // java/lang/Object."<init>":()V   #2 = String             #31            //   #3 = String             #32            // end   #4 = Methodref          #33.#34        // java/lang/String.equals:(Ljava/lang/Object;)Z   #5 = Class              #35            // java/lang/StringBuilder   #6 = Methodref          #5.#30         // java/lang/StringBuilder."<init>":()V   #7 = Methodref          #5.#36         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;   #8 = Methodref          #5.#37         // java/lang/StringBuilder.toString:()Ljava/lang/String;   #9 = Class              #38            // com/example/learn/LearnString  #10 = Class              #39            // java/lang/Object  #11 = Utf8               <init>  #12 = Utf8               ()V  #13 = Utf8               Code  #14 = Utf8               LineNumberTable  #15 = Utf8               LocalVariableTable  #16 = Utf8               this  #17 = Utf8               Lcom/example/learn/LearnString;  #18 = Utf8               main  #19 = Utf8               ([Ljava/lang/String;)V  #20 = Utf8               i  #21 = Utf8               I  #22 = Utf8               args  #23 = Utf8               [Ljava/lang/String;  #24 = Utf8               s  #25 = Utf8               Ljava/lang/String;  #26 = Utf8               StackMapTable  #27 = Class              #40            // java/lang/String  #28 = Utf8               SourceFile  #29 = Utf8               LearnString.java  #30 = NameAndType        #11:#12        // "<init>":()V  #31 = Utf8  #32 = Utf8               end  #33 = Class              #40            // java/lang/String  #34 = NameAndType        #41:#42        // equals:(Ljava/lang/Object;)Z  #35 = Utf8               java/lang/StringBuilder  #36 = NameAndType        #43:#44        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;  #37 = NameAndType        #45:#46        // toString:()Ljava/lang/String;  #38 = Utf8               com/example/learn/LearnString  #39 = Utf8               java/lang/Object  #40 = Utf8               java/lang/String  #41 = Utf8               equals  #42 = Utf8               (Ljava/lang/Object;)Z  #43 = Utf8               append  #44 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;  #45 = Utf8               toString  #46 = Utf8               ()Ljava/lang/String;{  public com.example.learn.LearnString();    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/example/learn/LearnString;  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         2: astore_1         3: iconst_0         4: istore_2         5: ldc           #3                  // String end         7: aload_0         8: iload_2         9: aaload        10: invokevirtual #4                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z        13: ifne          40        16: new           #5                  // class java/lang/StringBuilder        19: dup        20: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V        23: aload_1        24: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;        27: aload_0        28: iload_2        29: aaload        30: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;        33: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;        36: astore_1        37: goto          5        40: return      LineNumberTable:        line 8: 0        line 9: 3        line 10: 16        line 12: 40      LocalVariableTable:        Start  Length  Slot  Name   Signature            5      35     2     i   I            0      41     0  args   [Ljava/lang/String;            3      38     1     s   Ljava/lang/String;      StackMapTable: number_of_entries = 2        frame_type = 253 /* append */          offset_delta = 5          locals = [ class java/lang/String, int ]        frame_type = 250 /* chop */          offset_delta = 34}SourceFile: "LearnString.java"

在执行”+”时候

        13: ifne          40        16: new           #5                  // class java/lang/StringBuilder        19: dup        20: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V        23: aload_1

在这个循环里每一次对String执行”+”操作，都会创建一个StringBuilder对象，可见这多么消耗性能。为了避免这种事情发生只要你在执行循环之前创建一个StringBuilder对象然后将之后的”+”操作换成StringBuilder.append（）就可以。

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