说说String

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原创最新推荐文章于 2026-07-22 15:21:07 发布 354 阅读 AI 写同款· GEO 优化 ›

C++:string类 1.字符串是表示字符序列的类2.标准的字符串类提供了对此类对象的支持，其接口类似于标准字符容器的接口，但添加了专门用于操作单字节字符字符串的设计特性。3. string类是使用char(即作为它的字符类型，使用它的默认和分配器类型关于模板的更多信息，请参阅basic_string)。4. string类是模板类的一个实例，它使用char来实例化模板类，并用char_traits和allocator作为的默认参数根于更多的模板信息请参考。5. 阅读详情

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：

true
false
true

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_SUPER
Constant 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