JDK 1.5 新功能使用实例之Generics

ArrayToCollection(ia, cn);// T inferred to be Number
　fromArrayToCollection(fa, cn);// T inferred to be Number
　fromArrayToCollection(na, cn);// T inferred to be Number
　fromArrayToCollection(na, co);// T inferred to be Object
　//test.fromArrayToCollection(na, cs);// 错误用法
}
public <T> void fromArrayToCollection(T a, Collection<T> c) {
　for (T o : a) {
　　//如果参数定义为 Collection< ? > c 以下写法错误
　　c.add(o); // compile time error
　}
}
/**
* generics 嵌套用法
* @param shapes
*/
public void drawAll(List< ? extends Shape> shapes) {
　List<List< ? extends Shape>> history = new ArrayList<List< ? extends Shape>>();
　history.add(shapes);
　for (Shape s : shapes) {
　　s.draw();
　}
}
/**
*
*
*/
public void Test4() {
　List<String> l1 = new ArrayList<String>();
　List<Integer> l2 = new ArrayList<Integer>();
　System.out.print(l1.getClass() == l2.getClass());
　//打印为 true，
}
/**
* 错误用法
*/
public void Test5() {
　Collection cs = new ArrayList<String>();
　//以下为错误用法
　//if (cs instanceof Collection<String>) { } // illegal
　//以下为警告用法
　//Collection<String> cstr = (Collection<String>) cs; // unchecked
　// warning
}
public void Test6() {
//错误用法
//List<String> lsa = new List<String>; // not really allowed
List< ? > lsa = new List< ? >; // ok, array of unbounded wildcard
// type
Object o = lsa;
Object oa = (Object) o;
List<Integer> li = new ArrayList<Integer>();
li.add(new Integer(3));
oa = li; // correct
//String s = lsa.get(0); // run-time error - ClassCastException
//String s = lsa.get(0); // run time error, but we were warned
String s = (String) lsa.get(0); // run time error, but cast is
// explicit
}
public void Test7() {
　Sink<Object> s = null;
　Sink<String> s1 = null;
　Collection<String> cs = null;
　String str = writeAll(cs, s1);
　//String str = writeAll(cs, s); // 无效调用
　Object obj = writeAll1(cs, s); // 正确的调用
　str = writeAll2(cs, s1); // 正确的调用
}
public <T> T writeAll(Collection<T> coll, Sink<T> snk) {
　T last = null;
　for (T t : coll) {
　　last = t;
　　snk.flush(last);
　}
　return last;
}
public <T> T writeAll1(Collection< ? extends T> coll, Sink<T> snk) {
　T last = null;
　for (T t : coll) {
　　last = t;
　　snk.flush(last);
　}
　return last;
}
public <T> T writeAll2(Collection<T> coll, Sink< ? super T> snk) {
　T last = null;
　for (T t : coll) {
　　last = t;
　　snk.flush(last);
　}
　return last;
}
// 打印
private void log(Object ob) {
　System.out.print(ob);
}
}
//辅助定义
abstract class Shape {
　public abstract void draw();
}
class Circle extends Shape {
　private int x, y, radius;
　public void draw() {
　}
}
class Rectangle extends Shape {
　private int x, y, width, height;
　public void draw() {
　}
}
class Person {}
class Driver extends Person {}
class Census {
　public static void addRegistry(Map<String, ? extends Person> registry) {
}
public static void add(List< ? exten