org.apache.commons.collections包下的CollectionUtils工具类。

<!--CollectionUtils -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.4</version>
</dependency>

集合判空

通过CollectionUtils工具类的isEmpty方法可以轻松判断集合是否为空，isNotEmpty方法判断集合不为空。

List<Integer> list = new ArrayList<>();
list.add(2);
list.add(1);
list.add(3);
if (CollectionUtils.isEmpty(list)) {
    System.out.println("集合为空");
}
if (CollectionUtils.isNotEmpty(list)) {
    System.out.println("集合不为空");
}

对两个集合进行交、并、补、差集操作

List<Integer> list = new ArrayList<>();
list.add(2);
list.add(1);
list.add(3);

List<Integer> list2 = new ArrayList<>();
list2.add(2);
list2.add(4);
 
//获取并集
Collection<Integer> unionList = CollectionUtils.union(list, list2);
System.out.println(unionList);
 
//获取交集
Collection<Integer> intersectionList = CollectionUtils.intersection(list, list2);
System.out.println(intersectionList);
 
//获取交集的补集
Collection<Integer> disjunctionList = CollectionUtils.disjunction(list, list2);
System.out.println(disjunctionList);
 
//获取差集
Collection<Integer> subtractList = CollectionUtils.subtract(list, list2);
System.out.println(subtractList);
//运行结果：
//[1, 2, 3, 4]
//[2]
//[1, 3, 4]
//[1, 3]

从一个对象的集合中，获取某一属性的集合

从一个对象的集合中，获取某一属性的集合。如，从人员信息集合中获取人员姓名的集合。

List<Map<String, Object>> list = new ArrayList<>();
Map<String, Object> map1 = new HashMap<>();
map1.put("name", "张三");
map1.put("age", 1);
Map<String, Object> map2 = new HashMap<>();
map2.put("name", "李四");
map2.put("age", 2);
Map<String, Object> map3 = new HashMap<>();
map3.put("name", "王五");
map3.put("age", 3);
list.add(map1);
list.add(map2);
list.add(map3);
List<String> nameList = (List<String>) CollectionUtils.collect(
    list, new Transformer() {
        public Object transform(Object arg0) {
            Map<String, Object> map = (Map<String, Object>) arg0;
            return map.get("name");
        }
    });
System.out.println(nameList);
//运行结果：
//[张三, 李四, 王五]

从一个对象集合中过滤出只符合要求的对象

List<Map<String, Object>> list = new ArrayList<>();
Map<String, Object> map1 = new HashMap<>();
map1.put("name", "张三");
map1.put("age", 1);
Map<String, Object> map2 = new HashMap<>();
map2.put("name", "张三");
map2.put("age", 2);
Map<String, Object> map3 = new HashMap<>();
map3.put("name", "王五");
map3.put("age", 3);
list.add(map1);
list.add(map2);
list.add(map3);
CollectionUtils.filter(list, new Predicate() {
    public boolean evaluate(Object arg0) {
        Map<String, Object> map = (Map<String, Object>) arg0;
        return "张三".equals(map.get("name"))
            && 2 == Integer.parseInt(String.valueOf(map.get("age")));
    }
});
System.out.println(list);
//执行结果：
//[{name=张三, age=2}]