String它是一個(gè)引用數(shù)據(jù)類型，不是一個(gè)基礎(chǔ)數(shù)據(jù)類型潮模。
先思考一個(gè)問(wèn)題：String為什么是不可更改的。
查看String類的簽名如下：

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

然后再看看String到底是怎么存儲(chǔ)字符串的：

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

String類的簽名，和存儲(chǔ)String的char數(shù)組都被final修飾师逸，它們確保了String對(duì)象是永遠(yuǎn)不會(huì)被修改的。

一豆混、內(nèi)存存儲(chǔ)

下面我們繼續(xù)String之旅吧篓像，先貼一段學(xué)習(xí)代碼动知。

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));
    }
}

運(yùn)行結(jié)果也如大家所想：

true
false
true

我們先看看String的內(nèi)存分布情況吧

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

編譯后，我們通過(guò)javap -v LearnString來(lái)查看內(nèi)存的分布情況员辩，可以看到在字節(jié)碼的Constant pool區(qū)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

可見不管是成員變量還是局部變量盒粮，只要String一開始就被賦值了，那么它的值就會(huì)被保存在字節(jié)碼的常量池中奠滑。其實(shí)你會(huì)發(fā)現(xiàn)如果把之前的：

String d = "ab" + "d";

重新編譯后再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

這時(shí)候我們可以發(fā)現(xiàn)結(jié)果是一樣的丹皱，也就是說(shuō)compiler發(fā)現(xiàn)這些"+"操作完全可以在編譯階段優(yōu)化掉，compiler就會(huì)進(jìn)行一定的優(yōu)化操作宋税。
接下來(lái)我們可以討論開篇的那個(gè)例子了摊崭。“abc”字符串按照上面所說(shuō)杰赛，在編譯的時(shí)候就被保存在字節(jié)碼的常量池中了呢簸，所以a 和 b都是拿的常量池中的“abc”值（指向了同一個(gè)堆地址），故a==b為true乏屯；c = new String("abc") 根时，它其實(shí)創(chuàng)建了兩個(gè)對(duì)象，一個(gè)new出來(lái)的另一個(gè)就是常量“abc”,c的引用指向了new出來(lái)的對(duì)象辰晕，故a!=c啸箫。

    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;
    }

這是String重寫Object的equals方法，由此可以發(fā)現(xiàn)equals方法是比較的是兩個(gè)String對(duì)象里頭的字符數(shù)組（char[]）伞芹，比較的是堆中值而非堆地址忘苛，故a.equals(c)是相等。

二唱较、注意的問(wèn)題

先看看下面代碼

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

同樣用javap反編譯字節(jié)碼

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"

在執(zhí)行"+"時(shí)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

在這個(gè)循環(huán)里每一次對(duì)String執(zhí)行"+"操作扎唾，都會(huì)創(chuàng)建一個(gè)StringBuilder對(duì)象，可見這多么消耗性能南缓。為了避免這種事情發(fā)生只要你在執(zhí)行循環(huán)之前創(chuàng)建一個(gè)StringBuilder對(duì)象然后將之后的"+"操作換成StringBuilder.append（）就可以胸遇。