4个线程,其中两个线程每次对j增加1,另外两个线程对j每次减少

public class Maintest {
private int j=0;

public static void main (String [] args){
Maintest mt = new Maintest();
add add = mt.new add();
dec dec = mt.new dec();
for (int i=0;i<2;i++){
Thread t = new Thread(add);
t.start();
t = new Thread(dec);
t.start();
}
}

private synchronized void add(){
j++;
System.out.println(Thread.currentThread().getName() +" _add is " +j);
}

private synchronized void dec(){
j--;
System.out.println(Thread.currentThread().getName() +" _dec is " +j);
}

class add implements Runnable{
public void run() {
for(int i = 0;i<10;i++){
add();
}
}
}

class dec implements Runnable{
public void run() {
for(int i = 0;i<10;i++){
dec();
}
}
}

}