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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