java 多线程顺序打印ABC

public class Main {

public static String lock = "A";
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Thread a = new A();
Thread b = new B();
Thread c = new C();
a.start();
b.start();
c.start();
}
}

public class A extends Thread {

@Override
public void run() {
// TODO Auto-generated method stub
int k = 0;
while (k < 10) {
synchronized (Main.lock) {
if (Main.lock.equals("A")) {
System.out.print("A");
k++;
Main.lock = "B";
}
}
}
}

}

public class B extends Thread {

@Override
public void run() {
// TODO Auto-generated method stub
int k = 0;
while (k < 10) {
synchronized (Main.lock) {
if (Main.lock.equals("B")) {
System.out.print("B");
k++;
Main.lock = "C";
}
}
}
}
}

public class C extends Thread{
@Override
public void run() {
// TODO Auto-generated method stub
int k = 0;
while (k < 10) {
synchronized (Main.lock) {
if (Main.lock.equals("C")) {
System.out.print("C");
k++;
Main.lock = "A";
}
}
}
}

}