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Java 5.0中的范型(Generics)

Justfly Shi — Tue, 13 Sep 2005 15:50:00 GMT

范型有两�U�，�c�范型和�Ҏ��范型。他们可以用于一些类��g��C�Q�＋中的模板之类的作用�?br />�q�里主要有这么几点要注意�Q?br />0、范型类之间的�{�?br />1、范型类的��?br />2、范型方法的override
3、关键字 super 和exends的��用�?br />

package cn.justfly.study.tiger.generics;

import java.util.Collection;

/**
* Sample of defining generics type
*
* @author Justfly Shi
* created at 2005-8-25 22:03:09
*/
public class Defination<G_T, G_B> {
  private G_T _t = null;

  private G_B _b = null;

  public G_B getB() {
    return _b;
  }

  public void setB(G_B b) {
    _b = b;
  }

  //Generics Method
  <G_A> G_A abc(G_A a,Defination<G_T, ? extends G_B> f)//keyword extends
  {
    return a;
  }

  //class Generics Method
  static <G_F,G_F2> void test(G_F[] a, Collection super G_F> b)// keyword super
  {
    for (G_F o : a) {
      b.add(o);
    }
  }

  public G_T getT() {
    return _t;
  }

  public void setT(G_T t) {
    _t = t;
  }

  public Defination(G_T t, G_B b) {
    super();
    _b = b;
    _t = t;
  }

  /**
   * @param args
   */
  public static void main(String[] args) {
    Defination<A, A> d = new Defination<A, A>(new AImp2(), new AImp1());
    printDefination(d);

    // about extends
    Defination<A, A> right = new SubDefination<A, A>(new AImp2(), new AImp1());
    printDefination(right);
    // Type mismatch: cannot convert from SubDefination// Defination// Defination// AImp1());

    // Type mismatch: cannot convert from Defination// Defination// Defination// AImp1());
  }

  private static void printDefination(Defination<A, A> defination) {
    A t = defination.getT();
    A b = defination.getB();
    System.out.println(t.getValue());
    System.out.println(b.getValue());
  }
}

class SubDefination<G_T, G_J> extends Defination<G_T, G_J> {

  public SubDefination(G_T t, G_J b) {
    super(t, b);
  }

  @Override
  <G_A> G_A abc(G_A a, Defination<G_T, ? extends G_J> f) {
    return super.abc(a, f);
  }

}

class A {
  String getValue() {
    return "class A";
  }
}

class AImp1 extends A {
  public String getValue() {
    return "class AImp1";
  }
}

class AImp2 extends A {
  public String getValue() {
    return "class AImp2";
  }
}

最后再推荐一��中文的范型学习资料
在Eclipse 3.1中体验J2SE 5.0的新�Ҏ�?: �W�三部分 �Q�范�?/a>

Justfly Shi 2005-09-13 23:50 发表评论

Java 5.0中的列�D�c�d��

Justfly Shi — Mon, 12 Sep 2005 16:02:00 GMT

Enum也是java中我比较喜欢的一个改�q�，虽然使用到的地方�q�不多�?br />每一个enum�c�d��都默认的�l�承了java.lang.Enum虚拟�c�R�?br />每一个列丑֮�例都是改enum�c�d��的一个实例�?/p>

package cn.justfly.study.tiger.enums;

/**
* Sample code of enum
*
* @author Justfly Shi created at 2005-9-12 23:59:59
*/
public enum Gentle {
  WOMAN(":)"), MAN(":|");
  Gentle(String hello) {
    _hello = hello;
  }

  String _hello;

  String sayHello() {
    return _hello;
  }

  public static void main(String[] args) {
    System.out.println(Gentle.MAN.getDeclaringClass());

    Gentle[] allGentles = Gentle.values();
    System.out.println("There are " + allGentles.length + " Gentles");
    for (Gentle g : allGentles) {
      System.out.println("index: " + g.ordinal() + " name: " + g.name()
          + " HelloSmile: " + g.sayHello());
    }
  }
}

Justfly Shi 2005-09-13 00:02 发表评论

Java 5.0中改�q�的for循环

Justfly Shi — Mon, 12 Sep 2005 15:44:00 GMT

改进的for循环是Java 5.0中的一个让我很喜欢的改�q�。它只对数组和实��C��java.util.Iterable接口的容器类有效�?br />

package cn.justfly.study.tiger.enhancedfor;

import java.util.ArrayList;
import java.util.Collection;

/**
* The demo of enhanced for statement
* it can only used for array and classes implements java.util.Iterable interface
* @author Justfly Shi
* created at 2005-8-28 21:42:12
*/
public class EnhancedFor {

  /**
   * @param args
   */
  public static void main(String[] args) {
    //for array
    int[] intArray={1,2,3,4};
    System.out.println("printing ints:");
    for(int i:intArray){
      System.out.println(i);
    }

    //for Collection
   Collection<String> list=new ArrayList<String>();
    for(int i=0;i<4;i++){
      list.add("String"+i);
    }
    System.out.println("print Strings in an Collection:");
    for(String i:list){
      System.out.println(i);
    }

    //self-define Iterable
    MyIterable<String> myIte=new MyIterable<String>(list);
    System.out.println("print Stings in an Iterable");
    for(String i:myIte){
      System.out.println(i);
    }

  }

}

package cn.justfly.study.tiger.enhancedfor;

import java.util.Collection;
import java.util.Iterator;

/**
* an self-defined Iterable ,that can be used in enhanced-for statement
* @author Justfly Shi
* created at 2005-8-28 22:09:05
* @param
*/
public class MyIterable<G_E> implements Iterable<G_E> {
  private Collection<G_E> _list;

  public Iterator<G_E> iterator() {
    return new MyIterator<G_E>(_list.iterator());
  }

  public MyIterable(Collection<G_E> list) {
    _list = list;
  }

  class MyIterator<G_I> implements Iterator<G_I> {
    private Iterator<G_I> _ite;

    MyIterator(Iterator<G_I> ite) {
      _ite = ite;
    }

    public boolean hasNext() {
      return _ite.hasNext();
    }

    public G_I next() {
      return _ite.next();
    }

    public void remove() {
      _ite.remove();

    }
  }
}

输出�l�果
printing ints:
1
2
3
4
print Strings in an Collection:
String0
String1
String2
String3
print Stings in an Iterable
String0
String1
String2
String3

Justfly Shi 2005-09-12 23:44 发表评论

Justfly Shi — Mon, 12 Sep 2005 15:17:00 GMT

如下列代码所�C?br />
1、传入的可变参数��做��Z��个数�l�处�?br />2、一个函��C��只能有一个可变参数列�Q��ƈ且只能在函数参数定义的最后�?br />

package cn.justfly.study.tiger;

/**
* Sample of using varargs
* @author Justfly Shi
* created at 2005-8-31 0:38:19
*/
public class Varargs {
  public void printObjectArgs(Object objects ){
    for(Object o:objects)// objects is an Object Array
    {
      System.out.println(o);
    }
    System.out.println("var count:"+ objects.length);
  }
  public void printFloats(float fs ){
    for(float f:fs){
      System.out.println(f);
    }
    System.out.println("var count:"+ fs.length);
  }
// only one param can be var args,and should be placed at the end of param list
//  public void printArgs(Integer integers,String objects  ){}

  /**
   * @param args
   */
  public static void main(String[] args) {
    Varargs vars=new Varargs();
    vars.printObjectArgs(new Integer(1),new Integer(2),3,"abc");
    vars.printFloats(1.2f,1.3f);
  }

}

输出�l�果�Q?br />1
2
3
abc
var count:4
1.2
1.3
var count:2

Justfly Shi 2005-09-12 23:17 发表评论

Justfly Shi — Sun, 11 Sep 2005 12:15:00 GMT

在java5中添加的一个新�Ҏ��就是static import(静态导�?)通过静态导入我们可以很方便的��用在其他�c�M��定义的函数。如下面的代码所�C�，我们可以直接的��用java.lang.Math 的min和max以及其他在java.lang.Math中所定义的静态方法，只需要在import中添加一�?font style="background-color: rgb(127, 255, 212);">import static java.lang.Math.*�?/font>�q�是一个很方便的功能�?br />

package cn.justfly.study.tiger;

import static java.lang.Math.max;
import static java.lang.Math.min;
/**
* Sample of Static Import
* @author Justfly Shi
* created at 2005-9-3 23:41:50
*/
public class StaticImport {

  /**
   * @param args
   */
  public static void main(String[] args) {
   /*
    * min() and max() are defined in java.lang.Math as static method.
    * but they can now be easily access.
    * */
   int min=min(3,4);
   System.out.println("min is : "+min);
   int max=max(3,4);
   System.out.println("max is : "+max);

  }

}

但是�q�个功能却不能滥用。因为它会导致代码的可读性变得很差。考虑一下一个未曾接触过java.lang.Math�cȝ��读者来看这�D�代码。当他读�?ldquo;int min=min(3,4)”�Q�他会很�q�h��Q�这个min函数到底是在哪里定义的呢�Q�于是他��得��d��析import�q�里。这�D�代码还好说�Q�只有一个类被静态导入，只需要打开 java.lang.Math的文档就可以直接了解�q�些�Ҏ��的相关信息了。但是如果同旉��态导入了10个类的情况下呢？如果�q�些�c�M��有着名字�c�M��Q�相同）但是行�ؓ却不一致的�Ҏ��的时候呢�Q�比如Person.eat(Food food)�?Animal.eat(Food food)�?/p>

那么�q�个功能该如何用呢？我认�?strong>一些常用的工具�c�R��全局变量�cȝ��当需要在一个类中多�ơ��用的时候可以导�q�来�Q�但�?font color="#ff0000">对于�pȝ��中的模型�c?/strike>或者是~~用的�ơ数不多的工��L��~~�q�是不要导入的好。我们需要在自己写代码时的方便和代码本��n的可��L��间做个权衡�?/p>

~~Justfly Shi 2005-09-11 20:15 发表评论~~

基本语法: Sample of autoboxing and unboxing

Justfly Shi — Sat, 10 Sep 2005 18:07:00 GMT

要注意的是，当要被unboxing的封装类为null的时候或未被初始化时会抛��Z��个nullpoint错误

package cn.justfly.study.tiger;

import java.util.ArrayList;
import java.util.Collection;

/**//**
* Sample code of autoBoxing and auto-unboxing
* @author Justfly Shi
* created at 2005-8-28
*/
public class Boxing {

  /**//**
   * @param args
   */
  public static void main(String[] args) {
    Collection<Integer> c = new ArrayList<Integer>();

    for (int i = 0; i < 5; i++) {
      c.add(i);// autoboxing
    }
    System.out.println("iterate with Interger");
    for (Integer i : c) {

      System.out.println(i + 100);// unboxing
    }

    System.out.println("iterate with int");
    for (int i : c)// unboxing in enhanced for
    {

      System.out.println(i + 100);
    }

    Integer i=null;
    int j=i;//NullPointerException will be throw here
    System.out.println(j);
  }

}

Justfly Shi 2005-09-11 02:07 发表评论