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CollectionUtils是日常经常会用到的一个工具类，在包org.apache.commons.collections中。
目前最常用到的两个方法是CollectionUtils.isEmpty()以及CollectionUtils.isNotEmpty()。
还有待补充一些使用实例
其中有一个私有的静态变量INTEGER_ONE：
private static Integer INTEGER_ONE = new Integer(1);
一个不可修改的空集合：
public static final Collection EMPTY_COLLECTION = UnmodifiableCollection.decorate(new ArrayList());
取并集方法：
public static Collection union(Collection a, Collection b) {ArrayList list = new ArrayList();Map mapa = getCardinalityMap(a);// 该方法以Collection中的元素作为key，出现次数作为value。Map mapb = getCardinalityMap(b);Set elts = new HashSet(a);elts.addAll(b);Iterator it = elts.iterator();while(it.hasNext()) {Object obj = it.next();int i = 0;for(int m = Math.max(getFreq(obj, mapa), getFreq(obj, mapb)); i < m; ++i) {// getFrep是从指定map中获取该元素出现的次数list.add(obj);}}return list;}使用方法：
List<Integer> list1 = Lists.newArrayList(1, 3, 5, 7, 9, 9, 9);List<Integer> list2 = Lists.newArrayList(1, 1, 2, 4, 5, 7, 9, 9, 9, 9);Collection<Integer> result = CollectionUtils.union(list1, list2);for (Integer i : result) {System.out.print(i + " ");}输出如下：
1 1 2 3 4 5 7 9 9 9 9
取交集：
public static Collection intersection(final Collection a, final Collection b) {ArrayList list = new ArrayList();Map mapa = getCardinalityMap(a);Map mapb = getCardinalityMap(b);Set elts = new HashSet(a);elts.addAll(b);Iterator it = elts.iterator();while(it.hasNext()) {Object obj = it.next();for(int i=0,m=Math.min(getFreq(obj,mapa),getFreq(obj,mapb));i<m;i++) {list.add(obj);}}return list;}用法：
List<Integer> list1 = Lists.newArrayList(1, 3, 5, 7, 9, 9, 9);List<Integer> list2 = Lists.newArrayList(1, 1, 2, 4, 5, 7, 9, 9, 9, 9);Collection<Integer> result = CollectionUtils.intersection(list1, list2);for (Integer i : result) {System.out.print(i + " ");}输出：
1 5 7 9 9 9
取得交集之外的部分：
public static Collection disjunction(final Collection a, final Collection b) {ArrayList list = new ArrayList();Map mapa = getCardinalityMap(a);Map mapb = getCardinalityMap(b);Set elts = new HashSet(a);elts.addAll(b);Iterator it = elts.iterator();while(it.hasNext()) {Object obj = it.next();for(int i=0,m=((Math.max(getFreq(obj,mapa),getFreq(obj,mapb)))-(Math.min(getFreq(obj,mapa),getFreq(obj,mapb))));i<m;i++) {list.add(obj);}}return list;}用法：
List<Integer> list1 = Lists.newArrayList(1, 3, 5, 7, 9, 9, 9);List<Integer> list2 = Lists.newArrayList(1, 1, 2, 4, 5, 7, 9, 9, 9, 9);Collection<Integer> result = CollectionUtils.disjunction(list1, list2);for (Integer i : result) {System.out.print(i + " ");}
输出：
1 2 3 4 9
从一个集合中移除另一个集合中存在的元素：
public static Collection subtract(final Collection a, final Collection b) {ArrayList list = new ArrayList( a );for (Iterator it = b.iterator(); it.hasNext();) {list.remove(it.next());}return list;}
判断两个集合是否存在交集：
public static boolean containsAny(final Collection coll1, final Collection coll2) {if (coll1.size() < coll2.size()) {for (Iterator it = coll1.iterator(); it.hasNext();) {if (coll2.contains(it.next())) {return true;}}} else {for (Iterator it = coll2.iterator(); it.hasNext();) {if (coll1.contains(it.next())) {return true;}}}return false;}是否是子集：
public static boolean isSubCollection(final Collection a, final Collection b) {Map mapa = getCardinalityMap(a);Map mapb = getCardinalityMap(b);Iterator it = a.iterator();while (it.hasNext()) {Object obj = it.next();if (getFreq(obj, mapa) > getFreq(obj, mapb)) {return false;}}return true;}是否是真子集：
public static boolean isProperSubCollection(final Collection a, final Collection b) {return (a.size() < b.size()) && CollectionUtils.isSubCollection(a,b);}
判断两个集合是否相等：
public static boolean isEqualCollection(final Collection a, final Collection b) {if(a.size() != b.size()) {return false;} else {Map mapa = getCardinalityMap(a);Map mapb = getCardinalityMap(b);if(mapa.size() != mapb.size()) {return false;} else {Iterator it = mapa.keySet().iterator();while(it.hasNext()) {Object obj = it.next();if(getFreq(obj,mapa) != getFreq(obj,mapb)) {return false;}}return true;}}}对象在集合中出现的次数：
public static int cardinality(Object obj, final Collection coll) {if (coll instanceof Set) {return (coll.contains(obj) ? 1 : 0);}if (coll instanceof Bag) {return ((Bag) coll).getCount(obj);}int count = 0;if (obj == null) {for (Iterator it = coll.iterator();it.hasNext();) {if (it.next() == null) {count++;}}} else {for (Iterator it = coll.iterator();it.hasNext();) {if (obj.equals(it.next())) {count++;}}}return count;}找到集合中第一个满足条件的元素：
public static Object find(Collection collection, Predicate predicate) {if (collection != null && predicate != null) {for (Iterator iter = collection.iterator(); iter.hasNext();) {Object item = iter.next();if (predicate.evaluate(item)) {return item;}}}return null;}对集合中每个元素执行指定的闭包：
public static void forAllDo(Collection collection, Closure closure) {if (collection != null && closure != null) {for (Iterator it = collection.iterator(); it.hasNext();) {closure.execute(it.next());}}}过滤掉某些不满足指定条件的元素：
public static void filter(Collection collection, Predicate predicate) {if (collection != null && predicate != null) {for (Iterator it = collection.iterator(); it.hasNext();) {if (predicate.evaluate(it.next()) == false) {it.remove();}}}}对集合中每个元素进行指定的操作，这个操作会影响传入的集合：
public static void transform(Collection collection, Transformer transformer) {if (collection != null && transformer != null) {if (collection instanceof List) {List list = (List) collection;for (ListIterator it = list.listIterator(); it.hasNext();) {it.set(transformer.transform(it.next()));}} else {Collection resultCollection = collect(collection, transformer);collection.clear();collection.addAll(resultCollection);}}}返回满足指定条件的元素个数：
public static int countMatches(Collection inputCollection, Predicate predicate) {int count = 0;if (inputCollection != null && predicate != null) {for (Iterator it = inputCollection.iterator(); it.hasNext();) {if (predicate.evaluate(it.next())) {count++;}}}return count;}判断集合中是否存在满足某个指定条件的元素：
public static boolean exists(Collection collection, Predicate predicate) {if (collection != null && predicate != null) {for (Iterator it = collection.iterator(); it.hasNext();) {if (predicate.evaluate(it.next())) {return true;}}}return false;}找出集合中满足指定条件的所有元素：
public static Collection select(Collection inputCollection, Predicate predicate) {ArrayList answer = new ArrayList(inputCollection.size());select(inputCollection, predicate, answer);return answer;}找出集合中所有不满足条件的元素：
public static Collection selectRejected(Collection inputCollection, Predicate predicate) {ArrayList answer = new ArrayList(inputCollection.size());selectRejected(inputCollection, predicate, answer);return answer;}
添加一个可能为空的元素：
public static boolean addIgnoreNull(Collection collection, Object object) {return (object == null ? false : collection.add(object));}向集合中添加一个迭代器、枚举、数组：
public static void addAll(Collection collection, Iterator iterator) {while (iterator.hasNext()) {collection.add(iterator.next());}}
其余还有获取第几个元素的index()方法、获取集合大小的size()方法、获取集合为空、不为空、反转数组reverseArray()、集合是否已经满的isFull()方法、保留全部retainAll()、移除全部removeAll()、获取同步集合、不可修改集合等方法。
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