在使用Java的集合时，有些时候会需要比较两个集合是否相等，自己写方法其实也简单，但是既然有了好的实现，就不要自己造轮子了，只要了解这个轮子是什么原理就好了。

public static boolean isEqualCollection(final Collection
     a, final Collection
     b)

 

传入两个Collection就可以了，我们常用的List或者Set，根据源码发现：

这个方法比较的是集合中的元素以及元素的个数，不管是List或者是Set，不要求顺序相同。

下面是一个例子，其中的源码可能因为版本更迭与最新的不同，但是原理是一样的：

package collection;import java.util.ArrayList;import java.util.Collection;import java.util.HashMap;import java.util.Iterator;import java.util.Map;import org.apache.commons.collections4.CollectionUtils;import org.junit.Test;public class Test1 {	// CollectionUtils.isEqualCollection	/**	 * isEqualCollection	 * 	 * public static 
    
      boolean isEqualCollection(Collection
     
       a, Collection
      
        b) Returns true iff the given Collections contain exactly the same elements with exactly the same cardinalities.	 * That is, iff the cardinality of e in a is equal to the cardinality of e in b, for each element e in a or b.	 * 	 * Parameters: 	 * 		a - the first collection, must not be null 	 * 		b - the second collection, must not be null 	 * Returns: 	 * 		true iff the collections contain the same elements with the same cardinalities.	 * 	 */	@Test	public void test1() {		ArrayList
       
         arr1 = new ArrayList
        
         ();		arr1.add("1");		arr1.add("2");		arr1.add("2");		arr1.add("3");		arr1.add("4");		arr1.add("5");		ArrayList
         
           arr2 = new ArrayList
          
           (); arr2.add("1"); arr2.add("1"); arr2.add("2"); arr2.add("3"); arr2.add("5"); arr2.add("4"); System.out.println(CollectionUtils.isEqualCollection(arr1, arr2)); // copy的源码 System.out.println(isEqualCollection(arr1, arr2)); } public static boolean isEqualCollection(Collection a, Collection b) { if (a.size() != b.size()) return false; Map mapa = getCardinalityMap(a); Map mapb = getCardinalityMap(b); if (mapa.size() != mapb.size()) return false; for (Iterator it = mapa.keySet().iterator(); it.hasNext();) { Object obj = it.next(); if (getFreq(obj, mapa) != getFreq(obj, mapb)) return false; } return true; } private static Integer INTEGER_ONE = new Integer(1); public static Map getCardinalityMap(Collection coll) { Map count = new HashMap(); for (Iterator it = coll.iterator(); it.hasNext();) { Object obj = it.next(); Integer c = (Integer) count.get(obj); if (c == null) count.put(obj, INTEGER_ONE); else count.put(obj, new Integer(c.intValue() + 1)); } return count; } private static final int getFreq(Object obj, Map freqMap) { Integer count = (Integer) freqMap.get(obj); if (count != null) return count.intValue(); else return 0; }}