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：

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){
        //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：

$("#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：

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：

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)：

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：

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(){

    }

    /**
     * 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。

 

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。

 

来源:http://www.iteye.com/topic/539515