public class StringTest {
  public static void main(String[] args) {
    //part 1
    String s1 = "i love china";
    String s2 = "i love china";
    System.out.println("result:" + s1 == s2);//程序运行结果为true
    //part 2
    String s3 = new String("i love china");
    String s4 = new String("i love china");
    System.out.println("result:" + s3 == s4);//程序运行结果为false
  }

}

Constant pool:
  #1 = Class       #2       // StringTest
  #2 = Utf8        StringTest
  #3 = Class       #4       // java/lang/Object
  #4 = Utf8        java/lang/Object
  #5 = Utf8        <init>
  #6 = Utf8        ()V
  #7 = Utf8        Code
  #8 = Methodref     #3.#9     // java/lang/Object."<init>":()V
  #9 = NameAndType    #5:#6     // "<init>":()V
 #10 = Utf8        LineNumberTable
 #11 = Utf8        LocalVariableTable
 #12 = Utf8        this
 #13 = Utf8        LStringTest;
 #14 = Utf8        main
 #15 = Utf8        ([Ljava/lang/String;)V
 #16 = String       #17      // i love china 字符串地址的引用
 #17 = Utf8        i love china
 #18 = Fieldref      #19.#21    // java/lang/System.out:Ljava/io/PrintStream;
 #19 = Class       #20      // java/lang/System
 #20 = Utf8        java/lang/System
 #21 = NameAndType    #22:#23    // out:Ljava/io/PrintStream;
 #22 = Utf8        out
 #23 = Utf8        Ljava/io/PrintStream;
 #24 = Class       #25      // java/lang/StringBuilder
 #25 = Utf8        java/lang/StringBuilder
 #26 = String       #27      // result:
 #27 = Utf8        result:

#28 = Methodref #24.#29 // java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
#29 = NameAndType #5:#30 // "<init>":(Ljava/lang/String;)V
#30 = Utf8 (Ljava/lang/String;)V
#31 = Methodref #24.#32 // java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;
#32 = NameAndType #33:#34 // append:(Z)Ljava/lang/StringBuilder;
#33 = Utf8 append
#34 = Utf8 (Z)Ljava/lang/StringBuilder;
#35 = Methodref #24.#36 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#36 = NameAndType #37:#38 // toString:()Ljava/lang/String;
#37 = Utf8 toString
#38 = Utf8 ()Ljava/lang/String;
#39 = Methodref #40.#42 // java/io/PrintStream.println:(Ljava/lang/String;)V
#40 = Class #41 // java/io/PrintStream
#41 = Utf8 java/io/PrintStream
#42 = NameAndType #43:#30 // println:(Ljava/lang/String;)V
#43 = Utf8 println
#44 = Class #45 // java/lang/String
#45 = Utf8 java/lang/String
#46 = Methodref #44.#29 // java/lang/String."<init>":(Ljava/lang/String;)V
#47 = Utf8 args
#48 = Utf8 [Ljava/lang/String;
#49 = Utf8 s1
#50 = Utf8 Ljava/lang/String;
#51 = Utf8 s2
#52 = Utf8 s3
#53 = Utf8 s4
#54 = Utf8 StackMapTable
#55 = Class #48 // "[Ljava/lang/String;"
#56 = Utf8 SourceFile
#57 = Utf8 StringTest.java
...........
//对应的方法的字节码指令，由jvm运行时解释执行。
 public static void main(java.lang.String[]);
  descriptor: ([Ljava/lang/String;)V
  flags: ACC_PUBLIC, ACC_STATIC
  Code:
   stack=4, locals=5, args_size=1
     0: ldc      #16         // String i love china，该指令是将常量池的#16处符号引用，在这里为字符串“ilove china”符号引用push到栈顶。该指令与底下的指令2对应于程序中的String s1 = "i love china"语句
     2: astore_1             //将栈顶的对象引用赋值给局部变量1.
    3: ldc      #16         // String i love china,同0处的指令，指向的是同一个符号引用处的常量。该指令与底下的指令5对应于程序中的 String s2 = "i love china"语句。
     5: astore_2             //将栈顶的对象引用赋值给局部变量2.
    6: getstatic   #18         // Field java/lang/System.out:Ljava/io/PrintStream;
     9: new      #24         // class java/lang/StringBuilder
    12: dup
    13: ldc      #26         // String result:
    15: invokespecial #28         // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
    18: aload_1
    19: aload_2
    20: if_acmpne   27         //弹出栈顶两个对象引用进行比较其是否相等，不等，转到指令27处，执行，相等执行下一条指令
    23: iconst_1
    24: goto     28
    27: iconst_0
    28: invokevirtual #31         // Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;
    31: invokevirtual #35         // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
    34: invokevirtual #39         // Method java/io/PrintStream.println:(Ljava/lang/String;)V
    37: new      #44         // class java/lang/String,创建一个对象，该对象位于常量池#44引用处，这里为String对象，并将对象引用push到栈顶。
    40: dup                //拷贝栈顶一份对象引用push到栈顶。
    41: ldc      #16         // String i love china，同0，3处指令。
    43: invokespecial #46         // Method java/lang/String."<init>":(Ljava/lang/String;)V
    46: astore_3
    47: new      #44         // class java/lang/String//创建一个对象，并将对象引用push到栈顶
    50: dup
    51: ldc      #16         // String i love china,  将字符串的符号引用push到栈顶。
    53: invokespecial #46         // Method java/lang/String."<init>":(Ljava/lang/String;)V，根据栈顶的对应的对象引用及字符串引用调用对象的init初始化方法，对字符串对象初始化
    56: astore    4           //将栈顶对象引用赋值给变量4.
    58: getstatic   #18         // Field java/lang/System.out:Ljava/io/PrintStream;
    61: new      #24         // class java/lang/StringBuilder
    64: dup
    65: ldc      #26         // String result:
    67: invokespecial #28         // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
    70: aload_3
    71: aload     4
    73: if_acmpne   80
    76: iconst_1
    77: goto     81
    80: iconst_0
    81: invokevirtual #31         // Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;
    84: invokevirtual #35         // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
    87: invokevirtual #39         // Method java/io/PrintStream.println:(Ljava/lang/String;)V
    90: return
.........


LineNumberTable:
line 7: 0
line 8: 3
line 9: 6
line 11: 37
line 12: 47
line 13: 58
line 14: 90
LocalVariableTable:
Start Length Slot Name Signature
91 0 args [Ljava/lang/String;//局部变量0
88 1 s1 Ljava/lang/String; //局部变量1
85 2 s2 Ljava/lang/String;//局部变量2
44 3 s3 Ljava/lang/String;//局部变量3
33 4 s4 Ljava/lang/String;//局部变量4

<SPAN style="FONT-SIZE: 14pt">public final class String 
  implements java.io.Serializable, Comparable<String>, CharSequence { 
  /** The value is used for character storage. */
  private final char value[]; 
 
  /** Cache the hash code for the string */
  private int hash; // Default to 0 
 
  public String() { 
    this.value = new char[0]; 
  }</SPAN> 
  <SPAN style="BACKGROUND-COLOR: #ffffff; FONT-SIZE: 18pt"> public String(String original) { 
    this.value = original.value; 
    this.hash = original.hash; 
  } 
</SPAN>

public java.lang.String();
  descriptor: ()V
  flags: ACC_PUBLIC
  Code:
   stack=2, locals=1, args_size=1
     0: aload_0
     1: invokespecial #1         // Method java/lang/Object."<init>":()V
     4: aload_0
     5: iconst_0
     6: newarray    char
     8: putfield   #2         // Field value:[C
    11: return
   LineNumberTable:
    line 137: 0
    line 138: 4
    line 139: 11

 public java.lang.String(java.lang.String);
  descriptor: (Ljava/lang/String;)V
  flags: ACC_PUBLIC
  Code:
   stack=2, locals=2, args_size=2
     0: aload_0              //将局部变量0push到栈顶，自身对象的引用。
     1: invokespecial #1         // Method java/lang/Object."<init>":()V 弹出栈顶对象引用调用该对象的#1处的初始化方法。
     4: aload_0              //将自身对象引用push到栈顶。
     5: aload_1              //传递的字符串引用push到栈顶。
     6: getfield   #2         // Field value:[C // 弹出栈顶的字符串引用并将其赋值给#2处的实例变量，并将其存放到栈上。
     9: putfield   #2         // Field value:[C // 弹出栈顶的字符串引用及对象自身的引用并将字符串的引用赋值给本对象自身的实例变量。
    12: aload_0
    13: aload_1
    14: getfield   #3         // Field hash:I
    17: putfield   #3         // Field hash:I
    20: return