String类
首先，String是引用类型，也是唯一一个可以不使用关键字new创建对象的引用类型，因为有字符串常量值，使用英文格式的双引号""括起来就是字符串常量值。创建对象的两种方式
方式一、String s1 = “abc”;
方式二、String s2 = new String(“abc”);
这两种方式都是创建了一个字符串对象，值都是字符串abc；
不同的是
方式一是直接使用字符串常量abc赋值给变量s1，此时系统先到常量池中查找有没有字符串常量abc，如果有将字符串常量abc在常量池中的地址赋值给变量s1，如果没有会将字符串常量abc加入到常量池中，并把地址赋值给变量s1。
而方式二是在堆内存中创建一个String类型的对象，在这个对象中存储着字符串abc
其次，String类型被设计成不可变的，即对象不可变；一旦对象被创建，这个对象将无法被修改，引用（变量）是可以修改；如上例中s1是引用（变量），它可以存储其他字符串的地址，但是字符串abc本身被创建后无法再改变了。
实例：
//创建一个String类型的对象
String s = "abc";
//如果要将变量s的修改为"abcxyz"，代码如下
s = s+"xyz";
//输出结果为：abcxyz
System.out.println(s);
//由于这里使用两个字符串常量相连接，系统会默认当成一个字符串，即"abcxyz"
String s1 = "abc" + "xyz";
String s2 = "abcxyz";
其中第二行代码会先到常量池中查找有没有字符串xyz，如果没有将字符串xyz加入到常量池中，此时常量池中有abc和xyz两个字符串常量；
然后系统底层会默认在堆中创建一个StringBuilder类型的对象；然后用StringBuilder对象的append方法，将两个字符串连接到一起，最后再调用toString方法返回连接后的字符串abcxyz，并将返回的字符串abcxyz的地址赋值给变量s。
接下来验证实例中变量s、s1和s2三个变量，引用的是不是一个对象。可以使用双等号来进行判断。
//输出结果为：false
System.out.println(s == s1);
//输出结果为：true
System.out.println(s1 == s2);
由以上结果可以看出，变量s1和s2中存储的是同一字符串对象的地址，而变量s中存储的是另一字符串对象的地址，但这两个字符串对象的内容是相同的（都是abcxyz）；那么如何判断两个字符串的内容是否相同呢？
在String类中已经重写了父类Object中的equals方法，通过equals方法可以判断两个字符串内容是否相同
//输出结果为：true
System.out.println(s.equals(s1));
总结：
当使用字符串常量时，系统会先到常量池中寻找，如果没找到，将字符串常量加入到常量池中。
使用关键字new创建的字符串对象保存在堆内存中。
使用字符串连接符+连接两个字符串常量时，系统会自动将它们当成一个字符串常量，并将其存储在常量池中
使用字符串连接符+字符串变量时，连接后的字符串存储在堆内存中
比较两个字符串的内容是否相同使用equals方法
所有引用类型使用双等号==比较时，比较的是对象地址是否相同，即是否是同一对象
以下内容初学者选读
//类A的源码public class A{
String s = "abc" + "d";
}
//打开字节码文件的命令：javap -verbose A
//以下是类A编译后class文件的内容
//关键行#2和5: ldc，可以看到Java编译器是将"abc" + "d"当做"abcd"来处理的
Classfile /H:/A.class
Last modified 2015-5-29; size 239 bytes
MD5 checksum 271095a9d37f742c9bdefa883c6b9a00
Compiled from "A.java"
public class A
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #5.#14 // java/lang/Object."<init>":()V
#2 = String #15 // abcd
#3 = Fieldref #4.#16 // A.s:Ljava/lang/String;
#4 = Class #17 // A
#5 = Class #18 // java/lang/Object
#6 = Utf8 s
#7 = Utf8 Ljava/lang/String;
#8 = Utf8 <init>
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 SourceFile
#13 = Utf8 A.java
#14 = NameAndType #8:#9 // "<init>":()V
#15 = Utf8 abcd
#16 = NameAndType #6:#7 // s:Ljava/lang/String;
#17 = Utf8 A
#18 = Utf8 java/lang/Object
{
java.lang.String s;
descriptor: Ljava/lang/String;
flags:
public A();
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: ldc #2 // String abcd
7: putfield #3 // Field s:Ljava/lang/String;
10: return
LineNumberTable:
line 1: 0
line 2: 4
}
SourceFile: "A.java"
//类B源码public class B{
String s = "abc";
String s1 = s+"d";
}
//类B的class文件
//首先看#2和#7，这里不在是当做一个字符串来处理的了
//然后看5: ldc和25: ldc，这两行是将两个字符串常量压栈
//11: new，这里创建了一个StringBuilder类的对象
//22: invokevirtual和27: invokevirtual调用了StringBuilder对象的append方法，将两个字符串追加到一起
//最后30: invokevirtual，调用了StringBuilder对象的toString方法，获得连接后的字符串"abcd"
Classfile /H:/B.class
Last modified 2015-5-29; size 443 bytes
MD5 checksum 3eaa63a1c104ccfe26e2a6f9df85d93c
Compiled from "B.java"
public class B
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #11.#21 // java/lang/Object."<init>":()V
#2 = String #22 // abc
#3 = Fieldref #10.#23 // B.s:Ljava/lang/String;
#4 = Class #24 // java/lang/StringBuilder
#5 = Methodref #4.#21 // java/lang/StringBuilder."<init>":()V
#6 = Methodref #4.#25 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#7 = String #26 // d
#8 = Methodref #4.#27 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#9 = Fieldref #10.#28 // B.s1:Ljava/lang/String;
#10 = Class #29 // B
#11 = Class #30 // java/lang/Object
#12 = Utf8 s
#13 = Utf8 Ljava/lang/String;
#14 = Utf8 s1
#15 = Utf8 <init>
#16 = Utf8 ()V
#17 = Utf8 Code
#18 = Utf8 LineNumberTable
#19 = Utf8 SourceFile
#20 = Utf8 B.java
#21 = NameAndType #15:#16 // "<init>":()V
#22 = Utf8 abc
#23 = NameAndType #12:#13 // s:Ljava/lang/String;
#24 = Utf8 java/lang/StringBuilder
#25 = NameAndType #31:#32 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#26 = Utf8 d
#27 = NameAndType #33:#34 // toString:()Ljava/lang/String;
#28 = NameAndType #14:#13 // s1:Ljava/lang/String;
#29 = Utf8 B
#30 = Utf8 java/lang/Object
#31 = Utf8 append
#32 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#33 = Utf8 toString
#34 = Utf8 ()Ljava/lang/String;
{
java.lang.String s;
descriptor: Ljava/lang/String;
flags:
java.lang.String s1;
descriptor: Ljava/lang/String;
flags:
public B();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=3, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String abc
7: putfield #3 // Field s:Ljava/lang/String;
10: aload_0
11: new #4 // class java/lang/StringBuilder
14: dup
15: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
18: aload_0
19: getfield #3 // Field s:Ljava/lang/String;
22: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
25: ldc #7 // String d
27: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
30: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
33: putfield #9 // Field s1:Ljava/lang/String;
36: return
LineNumberTable:
line 1: 0
line 2: 4
line 3: 10
}
SourceFile: "B.java"
//类C源码public class C{
String s = new String("abc");
String s1 = s+"d";
}
//C.class文件内容
//这里主要演示类B中String s = "abc";和类C中String s = new String("abc");的区别
//首先5: new 这里创建了一个String对象
//然后9: ldc 字符串常量"abc"被压栈
//最后11: invokespecial 初始化String对象
//而类B只有ldc将常量压栈这一步
Classfile /H:/C.class
Last modified 2015-5-29; size 506 bytes
MD5 checksum 97026d45729fe35e1f470e17e1b5f0e0
Compiled from "C.java"
public class C
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #13.#23 // java/lang/Object."<init>":()V
#2 = Class #24 // java/lang/String
#3 = String #25 // abc
#4 = Methodref #2.#26 // java/lang/String."<init>":(Ljava/lang/String;)V
#5 = Fieldref #12.#27 // C.s:Ljava/lang/String;
#6 = Class #28 // java/lang/StringBuilder
#7 = Methodref #6.#23 // java/lang/StringBuilder."<init>":()V
#8 = Methodref #6.#29 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#9 = String #30 // d
#10 = Methodref #6.#31 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#11 = Fieldref #12.#32 // C.s1:Ljava/lang/String;
#12 = Class #33 // C
#13 = Class #34 // java/lang/Object
#14 = Utf8 s
#15 = Utf8 Ljava/lang/String;
#16 = Utf8 s1
#17 = Utf8 <init>
#18 = Utf8 ()V
#19 = Utf8 Code
#20 = Utf8 LineNumberTable
#21 = Utf8 SourceFile
#22 = Utf8 C.java
#23 = NameAndType #17:#18 // "<init>":()V
#24 = Utf8 java/lang/String
#25 = Utf8 abc
#26 = NameAndType #17:#35 // "<init>":(Ljava/lang/String;)V
#27 = NameAndType #14:#15 // s:Ljava/lang/String;
#28 = Utf8 java/lang/StringBuilder
#29 = NameAndType #36:#37 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#30 = Utf8 d
#31 = NameAndType #38:#39 // toString:()Ljava/lang/String;
#32 = NameAndType #16:#15 // s1:Ljava/lang/String;
#33 = Utf8 C
#34 = Utf8 java/lang/Object
#35 = Utf8 (Ljava/lang/String;)V
#36 = Utf8 append
#37 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#38 = Utf8 toString
#39 = Utf8 ()Ljava/lang/String;
{
java.lang.String s;
descriptor: Ljava/lang/String;
flags:
java.lang.String s1;
descriptor: Ljava/lang/String;
flags:
public C();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=4, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: new #2 // class java/lang/String
8: dup
9: ldc #3 // String abc
11: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V
14: putfield #5 // Field s:Ljava/lang/String;
17: aload_0
18: new #6 // class java/lang/StringBuilder
21: dup
22: invokespecial #7 // Method java/lang/StringBuilder."<init>":()V
25: aload_0
26: getfield #5 // Field s:Ljava/lang/String;
29: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
32: ldc #9 // String d
34: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
37: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
40: putfield #11 // Field s1:Ljava/lang/String;
43: return
LineNumberTable:
line 1: 0
line 2: 4
line 3: 17
}
SourceFile: "C.java"