JDK 1.5 新功能使用实例之Generics (转)

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Generics 是JDK 1.5 一个最重要的特性,主要用来处理Collection。

  以下代码在JDK 1.5 调试通过。

  代码实例1: Demo.java

package maoxiang.examples.jdk15.generics;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

/**
* @author 毛翔
*
* 演示如何使用Generics 特性。代码来自于 Generics 教程:
* http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf
*
* Generics类似于C++中的模板。
* 区别:
* 1.
* 2.
*/

public class Demo {
 public static void main(String[] args) {}

 /**
 * 最简单的用法
 */

 public void Test1() {

  // 以前的用法
  //List myIntList = new LinkedList(); // 1
  //myIntList.add(new Integer(0)); // 2
  //Integer x = (Integer) myIntList.iterator().next(); // 3 需要强制转换
  // 1.5 的用法
  List<Integer> myIntList = new LinkedList<Integer>(); // 1’
  myIntList.add(new Integer(0)); //2’
  Integer x = myIntList.iterator().next(); // 3’
 }

 /**
 * 匿名字符的用法
 */

 public void Test2() {
  List<Circle> list = new ArrayList<Circle>();
  //通过匿名字符打印一个集合

  Wildcards(list);
  Wildcards1();
  /*
  * 如果 Wildcards2 定义为Wildcards2(List <Shape> shapes)
  * 以下调用错误
  */
  Wildcards2(list);
 }

 public void Wildcards(Collection< ? > c) {
  // 以前的用法
  //Iterator i = c.iterator();
  //for (int k = 0; k < c.size(); k++) {
  //
  log(i.next());
  //}

 //1.5 的用法
 //Collection<?> c 表示
 for (Object e : c) {
  log(e);
 }
}

public void Wildcards1() {
 //Collection<?> c = new ArrayList<String>();
 //c.add(new Object()); // compile time error

 //以上为错误的用法,因为不能确定 c 的类型 ,不能使用add ,但get可以 。正确的用法如下:

 ArrayList<String> c = new ArrayList<String>();
 c.add("test");
 List< ? > list = c;
 log(c.get(0));
}

public void Wildcards2(List< ? extends Shape> shapes) {
 //List<Shape> shapes 定义只能接受List<Shape> shapes,也不能接受 List<Circle>
 for (Shape s : shapes) {
  s.draw();
 }

 //以下写法错误,因为为参数申明为 extends Shpape,无法确定Rectangle 为Shape子类,属于不安全调用
 //shapes.add(0, new Rectangle());

 Map<String, Driver> allDrivers = new HashMap<String, Driver>();
 Census.addRegistry(allDrivers);
 //以下写法允许,因为drivers明确定义,
 List<Driver> drivers = new ArrayList<Driver>();
 Census.add(drivers);
}

/**
* Generic Methods 的用法
*
*/

public void Test3() {
 //适用于各种类型的函数
 Object[] oa = new Object[100];
 Collection<Object> co = new ArrayList<Object>();
 fromArrayToCollection(oa, co);// T inferred to be Object
 String[] sa = new String[100];
 Collection<String> cs = new ArrayList<String>();
 fromArrayToCollection(sa, cs);// T inferred to be String
 fromArrayToCollection(sa, co);// T inferred to be Object
 Integer[] ia = new Integer[100];
 Float[] fa = new Float[100];
 Number[] na = new Number[100];
 Collection<Number> cn = new ArrayList<Number>();
 fromArrayToCollection(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>[10]; // not really allowed

List< ? >[] lsa = new List< ? >[10]; // 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[1] = li; // correct
//String s = lsa[1].get(0); // run-time error - ClassCastException
//String s = lsa[1].get(0); // run time error, but we were warned
String s = (String) lsa[1].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< ? extends Person> persons) {

}

}

class Collections {

public static <T, S extends T> void copy(List<T> dest, List<S> src) {

}

}

  代码实例2。Sink.java

package maoxiang.examples.jdk15.generics;

/**

*

* @author 毛翔

*

* 定义一个接口模板,简化了接口的定义

*

*/

interface Sink<E> {

public void flush(E t);

}

/*

* 如果是以前的定义,则要定义要各种类型的接口,显然更麻烦

* interface Sink {

*

* public void flush(String str);

* public void flush(Object obj);

* public void flush(Integer test);

* ......

* }

*/

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