对Java中匿名类的一些思考

2009-12-04
对Java中匿名类的一些思考
关键字: 对java中匿名类的一些思考
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：

Java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);}   
    }   
});  
Java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：

Java代码 
class XListener implements ActionListener{   
    public void actionPerformed(ActionEvent e){   
        //action here   
    }   
}   
XListener l = new XListener();   
iexit.addActionListener(l);  
Java代码
class XListener implements ActionListener{  
    public void actionPerformed(ActionEvent e){  
        //action here  
    }  
}  
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：

Js代码 
$("#element").click(function(){   
    //do something   
});   
  
$("div.type").each(function(index){   
    //do something else with the index   
});  
Js代码
$("#element").click(function(){  
    //do something  
});  
 
$("div.type").each(function(index){  
    //do something else with the 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：

Java代码 
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;   
    }   
});  
Java代码
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;  
    }  
}); 

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：

Java代码 
package org.free.fplist;   
  
/**  
* defined what condition should be used in <code>FilterableList.filter()</code>  
*   
* @author [e-mail]  
*  
*/  
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);   
}  
Java代码
package org.free.fplist;  
 
/** 
* defined what condition should be used in <code>FilterableList.filter()</code> 
*  
* @author [e-mail] 
* 
*/ 
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);  
} 

package org.free.fplist;

/**
* defined what condition should be used in <code>FilterableList.filter()</code>
*
* @author [e-mail]
*
*/
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)：

Java代码 
package org.free.fplist;   
  
/**  
* This is the interface defined Function-programmable support  
*   
* @author [e-mail]  
*  
*/  
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();   
}  
Java代码
package org.free.fplist;  
 
/** 
* This is the interface defined Function-programmable support 
*  
* @author [e-mail] 
* 
*/ 
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();  
} 

package org.free.fplist;

/**
* This is the interface defined Function-programmable support
*
* @author [e-mail]
*
*/
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：

Java代码 
package org.free.fplist;   
  
public interface Action {   
    public Object doAction(Object item);   
}  
Java代码
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。



Java代码 
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));   
        }   
    }   
}  
Java代码
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));  
        }  
    }  
} 

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。



Java代码 
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;   
            }   
        });   
  
    }   
}  
Java代码
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。