HashMap中的对象根据成员进行自定义排序

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Map是Java中最常用的存储对象的集合类之一，存储在HashMap中的对象在取出时是无序的，下文以示例介绍了如果对HashMap中存储的对象根据成员进行自定义排序。

应用说明：计算缓存对象的点击次数按次数排序输出。

1. 定义CacheObj类

定义了一个简单的对象，这个对象将存储在一个HashMap中，希望根据CacheObj中的整型成员变量cachedCounter进行排序输出。




/*
* Created on 2003-3-5
*/
package com.cache;
/**
* @author Weidong
*
*/
public class CacheObj {
int cachedCounter;
Object cacheObj = null;
/**
* @return Returns the cacheObj.
*/
public Object getCacheObj() {
return cacheObj;
}
/**
* @param cacheObj
* The cacheObj to set.
*/
public void setCacheObj(Object cacheObj) {
this.cacheObj = cacheObj;
}
/**
* @return Returns the cachedCounter.
*/
public int getCachedCounter() {
return cachedCounter;
}
/**
* @param cachedCounter
* The cachedCounter to set.
*/
public void setCachedCounter(int cachedCounter) {
this.cachedCounter = cachedCounter;
}
}


2. 自定义HotItemComparator

HotItemComparator实现了Comparator 接口， 实现的方法非常简单就是根据cachedCounter进行比较返回比较结果

正序：return obj1.getCachedCounter() - obj2.getCachedCounter();

倒序：return obj2.getCachedCounter() - obj1.getCachedCounter();




package com.cache;
import java.util.Comparator;
/**
* @author Weidong
*/
public final class HotItemComparator implements Comparator {
/*
* @see java.util.Comparator#compare(java.lang.Object, java.lang.Object)
*/
public int compare(Object arg0, Object arg1) {
CacheObj obj1 = (CacheObj) arg0;
CacheObj obj2 = (CacheObj) arg1;
return obj1.getCachedCounter() - obj2.getCachedCounter();
}
}
3. 测试用例

定义变量：public static final Comparator HOT_ITEM = new HotItemComparator();
将HashMap追加到List对象sortList中：sortList.addAll(caches.values());
对sorList应用HOT_ITEM排序：Collections.sort(sortList, HOT_ITEM);


package com.cache;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
/**
* @author Weidong
*/
public class TestBean {
public static final Comparator HOT_ITEM = new HotItemComparator();

public static void sortMap() {
Map caches = new HashMap();
CacheObj s1 = new CacheObj();
s1.setCachedCounter(2);
CacheObj s2 = new CacheObj();
s2.setCachedCounter(3);
CacheObj s3 = new CacheObj();
s3.setCachedCounter(1);
caches.put("1", s1);
caches.put("2", s2);
caches.put("3", s3);

List sortList = new ArrayList();
sortList.addAll(caches.values());
Collections.sort(sortList, HOT_ITEM);
Iterator iter = sortList.iterator();
while (iter.hasNext()) {
CacheObj s = (CacheObj) iter.next();
System.out.println("counter is "+s.getCachedCounter());
}
}
public static void main(String[] args) {
sortMap();
}
}
输出：文中用正序

counter is 1
counter is 2
counter is 3

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