之前将"虚拟机必须为每个被装载的类型维护一个常量池"这句话理解成了不同的类型的类声明中如果有相同类型的对象域，即使对象域持有的字面量一样，也应该不是同一个对象，因为他们在不同的类型中被声明的，所以在不同的常量池中，事实证明我的理解是错的。
如：
public class A{
public String s = "x";
}
public class B{
public String
s = "x";
}
之前的理解是，因为类A与类B类型不一样（A,B分别维护一个常量池，各自的常量池中的s应该不是指向同一个对象的），那么用"=="去比较两个类里面的字符串s时，应该返回false。但实际上返回的是true。因为虚拟机实际上是对String这一类型维护了一个常量池，所以常量池中常量"x"就只有一份对象存在。
下面的例子可以作为以后实验时理解用。
public class Test
{
public static void printStr(String
s1, String s2) {
if (s1 == s2) {//判断
System.out.println(s1 + "
" + s2 + " True");
} else {
System.out.println(s1 + "
" + s2 + " False");
}
}
// 测试两个线程中的字符串是否是同一个字符串
public static void main(String
a[]) throws InterruptedException {
Object lock = new Object();//
线程锁对象
StringThreadable fs = new StringThreadable();
fs.setStrclass(HoldString1.class);
fs.setLock(lock);
Thread t1 = new Thread(fs);//
第一个线程
StringThreadable ss = new StringThreadable();
ss.setStrclass(HoldString2.class);
ss.setLock(lock);
Thread t2 = new Thread(ss);//
第二个线程
// 执行第一个线程
synchronized (lock) {
t1.start();
lock.wait();
}
// 第一个线程执行完后，执行第二个线程
synchronized (lock) {
t2.start();
lock.wait();
}
String s1 = fs.getStr(), s2 = ss.getStr();
printStr(s1, s2);
String ts1 = fs.getThreadStr(),ts2 = ss.getThreadStr();
printStr(ts1, ts2);
}
}
public class StringThreadable implements Runnable{
private String str;
private Object lock;
private Class strclass;
private String threadStr="ThreadStr";
public void setLock(Object
lock) {
this.lock =
lock;
}
public void run()
{
synchronized(lock){
if (strclass.equals(HoldString1.class)){
this.str=(new HoldString1()).getStr();
System.out.println("HoldString1:"+str);
}
else{
this.str=(new HoldString2()).getStr();
System.out.println("HoldString2:"+str);
}
lock.notifyAll();
}
}
public String
getStr(){
return this.str;
}
public void setStrclass(Class strclass)
{
this.strclass =
strclass;
}
public String
getThreadStr() {
return threadStr;
}
}
//持有String的对象
public class HoldString1
{
private String str = "Gulshan";
public String
getStr(){
return this.str;
}
}
//持有String的对象
public class HoldString2
{
private String str = "Gulshan";
public String
getStr(){
return this.str;
}
}
-
if String objects having the same data are created using a constant expression, a string literal, a reference to an existing string, or by explicitly using the intern() method, their references will be the same
- if String objects having the same data are created explicitly with the new operator or their values are computed at runtime, their references will be different
- 参考：http://www.janeg.ca/scjp/lang/strLiteral.html
- 参考：《What is String literal pool?》http://www.xyzws.com/Javafaq/what-is-string-literal-pool/3