从javaEye上转过来的文章,看完对一些以前的代码理解的更透彻了。转到自己博客上保存下。嘿。。
引言
Java中的匿名类是比较有意思的一种编程方式,在swing中关于监听器的注册时,经常可见到这样的代码:
- iexit.addActionListener( new ActionListener(){
- public void actionPerformed(ActionEvent e) {
- int y = JOptionPane.showConfirmDialog(
- null ,
- "Confirm exit" ,
- "Confirm Exit Dialog" ,
- JOptionPane.YES_NO_OPTION);
- if (y == JOptionPane.YES_OPTION){System.exit( 0 );}
- }
- });
iexit.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
int y = JOptionPane.showConfirmDialog(
null,
"Confirm exit",
"Confirm Exit Dialog",
JOptionPane.YES_NO_OPTION);
if(y == JOptionPane.YES_OPTION){System.exit(0);}
}
});
通常使用匿名类的场景是,临时的创建一个类,实现指定的接口 ,使得调用者可以通过接口的方法来实现一定的操作。比如上边这个例子中,exit这个菜单项,需要一个动作监听器,如果用常规写法,大概如此:
- class XListener implements ActionListener{
- public void actionPerformed(ActionEvent e){
-
- }
- }
- XListener l = new XListener();
- iexit.addActionListener(l);
class XListener implements ActionListener{
public void actionPerformed(ActionEvent e){
//action here
}
}
XListener l = new XListener();
iexit.addActionListener(l);
虽然完成了同样的工作,但是代码量明显比刚才要多。
实例
在javascript,或者其他函数式编程语言中,常常可以见到诸如此类的写法:
- $( "#element" ).click( function (){
-
- });
-
- $("div.type" ).each( function (index){
-
- });
$("#element").click(function(){
//do something
});
$("div.type").each(function(index){
//do something else with the index
});
将一个匿名函数传递给另一个函数,实现一些列定制的动作,正好工作中有个地方要用到类似的东东,就用java的匿名类实现了一套类似的机制:
- Integer[] is = new Integer[]{
- 4 , 13 , 65 , 64 , 2 , 4 , 5 , 9 , 10 , 25 , 20 , 32 , 30
- };
-
-
- FPList<Integer> p = new FPList<Integer>(is);
- p.append(100 );
-
-
- FPList<Integer> even = p.filter(new Filter(){
- public boolean isLegal(Object item) {
- int x = ((Integer)item).intValue();
- return x % 2 == 0 ? true : false ;
- }
- }).filter(new Filter(){
- public boolean isLegal(Object item){
- int x = ((Integer)item).intValue();
- return x % 10 == 0 ? true : false ;
- }
- });
Integer[] is = new Integer[]{
4,13,65,64,2,4,5,9,10,25,20,32,30
};
//用数组构造一个integer 的 list
FPList<Integer> p = new FPList<Integer>(is);
p.append(100);//再添加一个元素
//做一次过滤,只有偶数被留下来
FPList<Integer> even = p.filter(new Filter(){
public boolean isLegal(Object item) {
int x = ((Integer)item).intValue();
return x % 2 == 0 ? true : false;
}
}).filter(new Filter(){//再做一次过滤,10的倍数被留下来
public boolean isLegal(Object item){
int x = ((Integer)item).intValue();
return x % 10 == 0 ? true : false;
}
});
这样可以将很多的条件AND到一起,对数据做条件过滤。
设计与实现
首先,需要定义一个接口(这是匿名类的基础),在这个例子中,我定义了一个过滤器接口,其中只有一个方法,即isLegal(Object item), 这个方法接受一个Obejct参数,返回一个boolean值,调用者根据这个boolean值来对最终结果做过滤:
- package org.free.fplist;
-
-
-
-
-
-
-
- public interface Filter {
-
-
-
-
-
-
- boolean isLegal(Object item);
- }
package org.free.fplist;
/**
* defined what condition should be used in <code>FilterableList.filter()</code>
*
* @author [email protected]
*
*/
public interface Filter {
/**
* this is a condition definition, pass a object in, and then
* a <code>true</code> or <code>false</code> will be returned.
* @param item
* @return
*/
boolean isLegal(Object item);
}
另外,我们需要一个接口,用来表示一个链表具有被过滤的能力(FPable):
- package org.free.fplist;
-
-
-
-
-
-
-
- public interface FPable<E> {
-
-
-
-
-
-
- FPable<E> append(E e);
-
-
-
-
-
-
-
-
-
- FPable<E> filter(Filter f);
-
-
-
-
-
-
-
-
- FPable<E> mapping(Action act);
-
-
-
-
-
-
-
- FPable<E> distinct();
-
-
-
-
-
- void print();
- }
package org.free.fplist;
/**
* This is the interface defined Function-programmable support
*
* @author [email protected]
*
*/
public interface FPable<E> {
/**
* append a new element to list, and then return <code>this</code> object
*
* @param e element you want to insert into
* @return
*/
FPable<E> append(E e);
/**
* do a filter by the given rule, the <code>Filter</code>
* object passed in is defined as a interface, and you need
* to implement the condition.
*
* @param f
* @return
*/
FPable<E> filter(Filter f);
/**
* mapping the action to each item of <code>function-programming-list</code>
* and will not affect the original list
*
* @param act the Action will used to mapping
* @return
*/
FPable<E> mapping(Action act);
/**
* distinct the <code>FilterableList</code>, keep one same elements only, and
* does not affect the List itself.
*
* @return
*/
FPable<E> distinct();
/**
* for debug only, print the <code>index</code> and <code>content</code>
* of each item of a list.
*/
void print();
}
附加的,我需要对这个链表有函数映射 (map)的支持,上面这个接口中的Action,为另一个接口,同样会被很多的匿名类使用到:
- package org.free.fplist;
-
- public interface Action {
- public Object doAction(Object item);
- }
package org.free.fplist;
public interface Action {
public Object doAction(Object item);
}
好了,我们现在来看一个FPable的实现FPList,FPList继承了LinkedList,并且实现了FPable,可以对其中的数据进行过滤(前提是传入一个过滤器 ),或者对其中的元素进行映射(传入一个动作 ),FPList会自动的将过滤器和动作作用到List中的每一个元素。
- package org.free.fplist;
-
- import java.util.HashSet;
- import java.util.Iterator;
- import java.util.LinkedList;
- import java.util.Set;
-
- public class FPList<E> extends LinkedList<E> implements FPable<E>{
- private static final long
- serialVersionUID = 348375840291148300L;
-
- public FPList(){
-
- }
-
-
-
-
-
-
- public FPList(E[] es){
- for ( int i = 0 ;i < es.length;i++){
- add(es[i]);
- }
- }
-
- public FPList<E> filter(Filter f){
- FPList<E> filtered = new FPList<E>();
- for ( int i = 0 ; i < size();i++){
- E o = get(i);
- if (f.isLegal(o)){
- filtered.add(o);
- }
- }
- return filtered;
- }
-
- public FPList<E> append(E e){
- add(e);
- return this ;
- }
-
- public FPList<E> distinct(){
- FPList<E> filtered = this ;
- Set<E> set = new HashSet<E>();
- for ( int i = 0 ; i < filtered.size();i++){
- set.add(filtered.get(i));
- }
- filtered.clear();
- Iterator<E> it = set.iterator();
- while (it.hasNext()){
- filtered.add(it.next());
- }
- return filtered;
- }
-
- public FPList<E> mapping(Action act){
- FPList<E> mapped = this ;
- for ( int i = 0 ;i < size();i++){
- mapped.add(i, (E)act.doAction(get(i)));
- mapped.remove(i+1 );
- }
- return mapped;
- }
-
- public void print(){
- for ( int i = 0 ;i < size();i++){
- System.err.println("index : " +i+ ", content : " +get(i));
- }
- }
- }
package org.free.fplist;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Set;
public class FPList<E> extends LinkedList<E> implements FPable<E>{
private static final long
serialVersionUID = 348375840291148300L;
public FPList(){
}
/**
* construct a fp-list by given array.
*
* @param es
*/
public FPList(E[] es){
for(int i = 0;i < es.length;i++){
add(es[i]);
}
}
public FPList<E> filter(Filter f){
FPList<E> filtered = new FPList<E>();
for(int i = 0; i < size();i++){
E o = get(i);
if(f.isLegal(o)){
filtered.add(o);
}
}
return filtered;
}
public FPList<E> append(E e){
add(e);
return this;
}
public FPList<E> distinct(){
FPList<E> filtered = this;
Set<E> set = new HashSet<E>();
for(int i = 0; i < filtered.size();i++){
set.add(filtered.get(i));
}
filtered.clear();
Iterator<E> it = set.iterator();
while(it.hasNext()){
filtered.add(it.next());
}
return filtered;
}
public FPList<E> mapping(Action act){
FPList<E> mapped = this;
for(int i = 0;i < size();i++){
mapped.add(i, (E)act.doAction(get(i)));
mapped.remove(i+1);
}
return mapped;
}
public void print(){
for(int i = 0;i < size();i++){
System.err.println("index : "+i+", content : "+get(i));
}
}
}
使用匿名类
匿名类的使用是比较方便的,为了代码更简洁,我使用了jQuery中的链机制,其实,大家平时使用的StringBuffer就提供这样的能力。
- package org.free.fplist;
-
- public class Main {
- public static void main(String[] args){
- String[] as = new String[]{
- "Apple" ,
- "Borland" ,
- "Cisco" ,
- "Dell" ,
- "Epson" ,
- "Flick" ,
- "Google"
- };
-
- FPList<String> k = new FPList<String>(as);
-
- k.distinct().filter(new Filter(){
- public boolean isLegal(Object item) {
- return ((String)item).indexOf( "e" ) >= 0 ? true : false ;
- }
- }).filter(new Filter(){
- public boolean isLegal(Object item) {
- return ((String)item).indexOf( "p" ) >= 0 ? true : false ;
- }
- }).mapping(new Action(){
- public Object doAction(Object item) {
- return ((String)item)+ ", co" ;
- }
- }).print();
-
- Integer[] is = new Integer[]{
- 4 , 13 , 65 , 64 , 2 , 4 , 5 , 9 , 10 , 25 , 20 , 32 , 30
- };
-
- FPList<Integer> p = new FPList<Integer>(is);
- p.append(100 );
-
- FPList<Integer> even = p.filter(new Filter(){
- public boolean isLegal(Object item) {
- int x = ((Integer)item).intValue();
- return x % 2 == 0 ? true : false ;
- }
- }).filter(new Filter(){
- public boolean isLegal(Object item){
- int x = ((Integer)item).intValue();
- return x % 10 == 0 ? true : false ;
- }
- });
-
- even.mapping(new Action(){
- public Object doAction(Object item) {
- return ((Integer)item).intValue()* 10 ;
- }
- }).mapping(new Action(){
- public Object doAction(Object item){
- return ((Integer)item).intValue()/ 2 ;
- }
- }).print();
-
- Person[] person = new Person[]{
- new Person( "abruzzi" , 25 , "male" ),
- new Person( "smith" , 25 , "female" ),
- new Person( "json" , 26 , "female" ),
- new Person( "jet.lee" , 25 , "male" )
- };
-
- FPList<Person> fp = new FPList<Person>(person);
- fp.filter(new Filter(){
- public boolean isLegal(Object item) {
- Person p = (Person)item;
- return p.getAge() == 25 ? true : false ;
- }
- }).filter(new Filter(){
- public boolean isLegal(Object item) {
- Person p = (Person)item;
- return p.getSex().equals( "male" ) ? true : false ;
- }
- }).mapping(new Action(){
- public Object doAction(Object item) {
- System.err.println(((Person)item).getName());
- return null ;
- }
- });
-
- }
- }
package org.free.fplist;
public class Main {
public static void main(String[] args){
String[] as = new String[]{
"Apple",
"Borland",
"Cisco",
"Dell",
"Epson",
"Flick",
"Google"
};
FPList<String> k = new FPList<String>(as);
k.distinct().filter(new Filter(){
public boolean isLegal(Object item) {
return ((String)item).indexOf("e") >= 0 ? true : false;
}
}).filter(new Filter(){
public boolean isLegal(Object item) {
return ((String)item).indexOf("p") >= 0 ? true : false;
}
}).mapping(new Action(){
public Object doAction(Object item) {
return ((String)item)+", co";
}
}).print();
Integer[] is = new Integer[]{
4,13,65,64,2,4,5,9,10,25,20,32,30
};
FPList<Integer> p = new FPList<Integer>(is);
p.append(100);
FPList<Integer> even = p.filter(new Filter(){
public boolean isLegal(Object item) {
int x = ((Integer)item).intValue();
return x % 2 == 0 ? true : false;
}
}).filter(new Filter(){
public boolean isLegal(Object item){
int x = ((Integer)item).intValue();
return x % 10 == 0 ? true : false;
}
});
even.mapping(new Action(){
public Object doAction(Object item) {
return ((Integer)item).intValue()*10;
}
}).mapping(new Action(){
public Object doAction(Object item){
return ((Integer)item).intValue()/2;
}
}).print();
Person[] person = new Person[]{
new Person("abruzzi", 25, "male"),
new Person("smith", 25, "female"),
new Person("json", 26, "female"),
new Person("jet.lee", 25, "male")
};
FPList<Person> fp = new FPList<Person>(person);
fp.filter(new Filter(){
public boolean isLegal(Object item) {
Person p = (Person)item;
return p.getAge() == 25 ? true : false;
}
}).filter(new Filter(){
public boolean isLegal(Object item) {
Person p = (Person)item;
return p.getSex().equals("male") ? true : false;
}
}).mapping(new Action(){
public Object doAction(Object item) {
System.err.println(((Person)item).getName());
return null;
}
});
}
}
main的运行结果如下:
index : 0, content : Apple, co
index : 0, content : 50
index : 1, content : 100
index : 2, content : 150
index : 3, content : 500
abruzzi
jet.lee
上边的例子显示,匿名类在接口 中的方法不多的时候,整个匿名类整体作为一个对象传递给另外一个方法,可以很好的做到可定制性。比如第三个例子,使用Person bean的时候,可以定制多个过滤条件,依次将原始列表过滤成一个符合要求的列表。
另,文章中用到了比较多的函数式编程的概念,虽然java原生不支持,但是函数式编程作为一种思想,肯定可以在命令式的程序设计中有所体现。