public interface List<E> {
 void add(E);
 Iterator<E> iterator();
}

List<String> ls = new ArrayList<String>(); //1
List<Object> lo = ls; //2
lo.add(new Object()); // 3
String s = ls.get(0); // 4，将object转换为string将失败。

void printCollection(Collection<Object> c) {
 for (Object o: c) {
 // do something
 }
}

void printCollection(Collection<?> c) {
 for (Object o: c) { // 1
 // do something
 }
} // ok

void add(Collection<? extends MyClass> c) {
 c.add(new MyClass()); // wrong
} // ok

<T> add(Collection<T> c, T t) {
 c.add(t);
}

List<String> l1 = new ArrayList<String>();
List<Integer> l2 = new ArrayList<Integer>();
System.out.println(l1.getClass() == l2.getClass()); // true

Object o = lsa;
Object []oa = (Object[])o;
List<Integer> li = new ArrayList<Integer>();
li.add(new Integer(3));
oa[1] = li;
String s = lsa[1].get(0); // runtime error