ArrayList和Vector线程安全问题（新手提问） threadJava

我有一段关于ArrayLisy和Vector线程安全的代码，如下：


import java.util.ArrayList;
import java.util.List;
import java.util.Vector;

public class SynchronizedCollectionsTest {

static List<Integer> list = new ArrayList<Integer>(20);
static List<Integer> vector = new Vector<Integer>();

public static void main(String[] args) {
Thread thread1 = new Thread() {
public void run() {
for (int i = 0; i < 10; i++) {
list.add(list.size(), i);
vector.add(vector.size(), i);
try {
Thread.sleep(10);
} catch (InterruptedException e) {
System.out.println(e);
}
}
}
};

Thread thread2 = new Thread() {
public void run() {
for (int i = 0; i < 10; i++) {
list.add(list.size(), i);
vector.add(vector.size(), i);
try {
Thread.sleep(10);
} catch (InterruptedException e) {
System.out.println(e);
}
}
}
};

thread1.start();
thread2.start();

try {
thread1.join();
thread2.join();
} catch (InterruptedException e) {
System.out.println(e);
}
System.out.println(list);
System.out.println(vector);
}

}


现在想请教各位，为什么list中会有null输出？想知道list是如何产生出null的？