cn.hutool.core.collection.CollectionUtil

集合创建

List<User> userList = CollectionUtil.list(true);
//true:ArrayList，false：LinkedList

List<String> list1 = Arrays.asList("1", "2", "3", "4", "5");//不可变集合
List<String> list2 = CollectionUtil.toList("1", "2", "3", "4", "5");//可变集合
System.out.println(list2);//[1, 2, 3, 4，5]

集合清空

List<String> list = CollectionUtil.toList("1", "2", "3", "4", "5");
CollectionUtil.clear(list);
System.out.println(list);//[]

集合判空

List<String> list1 = null;
List<String> list2 = new ArrayList<>();
boolean isEmpty = CollectionUtils.isEmpty(list1); // true
boolean isEmpty = CollectionUtils.isEmpty(list2); // true

List<String> list = Arrays.asList("a", "b", "c");
boolean isNotEmpty = CollectionUtils.isNotEmpty(list); // true

List<String> list1 = null;
List<String> list2 = Arrays.asList("1","2","3");
List<String> result1 = CollectionUtil.emptyIfNull(list1);//[]
List<String> result2 = CollectionUtil.emptyIfNull(list2);//["1","2","3"]

Set<String> set1 = null;
Set<String> set2 = new HashSet<>(Arrays.asList("1","2","3"));
Set<String> result3 = CollectionUtil.emptyIfNull(set1);//[]
Set<String> result4 = CollectionUtil.emptyIfNull(set2);//["1","2","3"]

//空集合给默认值
List<String> list = new ArrayList<>();
List<String> result = CollectionUtil.defaultIfEmpty(list, Arrays.asList("1", "2", "3"));//["1", "2", "3"]

//判断是否有空元素
CollectionUtil.hasNull(list);

集合去重

List<String> list = Arrays.asList("a", "b", "a", "c");
List<String> distinctList = CollectionUtil.distinct(list); // ["a", "b", "c"]

集合过滤

//List<String> list =Arrays.asList("a", "b", "c");//Arrays.asList()返回的集合是不可变长度的，在这里这么用会报错：UnsupportedOperationException
List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c"));
List<String> filteredList = CollectionUtil.filter(list, e -> e.equals("a")); // ["a"]

集合转换

List<String> list = Arrays.asList("1", "2", "3");
List<Integer> mappedList = CollectionUtil.map(list, Integer::valueOf,true); // [1, 2, 3]

集合合并

List<String> list1 = Arrays.asList("a", "b");
List<String> list2 = Arrays.asList("b", "c");

Set<String> unionDistinctList = CollectionUtil.unionDistinct(list1, list2); // ["a", "b", "c"]

List<String> unionAllList = (List<String>)CollectionUtil.unionAll(list1, list2); // ["a", "b", "c"]

集合交集

List<String> list1 = Arrays.asList("a", "b");
List<String> list2 = Arrays.asList("b", "c");
List<String> intersectionList = (List<String>)CollectionUtil.intersection(list1, list2); // ["b"]

集合差集

List<String> list1 = Arrays.asList("a", "b");
List<String> list2 = Arrays.asList("b", "c");
List<String> disjunctionList = (List<String>)CollectionUtil.disjunction(list1, list2); // ["a", "c"]

集合是否包含元素

List<String> list = Arrays.asList("a", "b", "c");
boolean contains = CollectionUtil.contains(list, "a"); // true

List<String> list2 = Arrays.asList("a", "b", "c");
List<String> subList = Arrays.asList("a", "b");
boolean containsAll = CollectionUtil.containsAll(list2, subList); // true

集合是否包含指定元素（自定义条件）

List<String> list = Arrays.asList("a", "b", "c");
boolean contains = CollectionUtil.contains(list, e -> e.equals("a")); // true

集合分页

List<String> list = Arrays.asList("a", "b", "c", "d", "e");
List<String> pageList = CollectionUtil.page(1, 2,list); // ["c", "d"]，页码索引从0开始

集合分组

List<User> list = Arrays.asList(
    new User("张三", 18, 1),
    new User("李四", 18, 2),
    new User("王五", 18, 3),
    new User("赵六", 18, 4),
    new User("老七", 18, 1),
    new User("勾八", 18, 1),
    new User("孙九", 18, 3),
    new User("郑十", 18, 2),
    new User("root", 18, 2));
List<List<User>> groupedList = Co
System.out.println(groupedList);
//[[User(name=张三, age=18, grade=1), User(name=老七, age=18, grade=1), User(name=勾八, age=18, grade=1)], 
//[User(name=李四, age=18, grade=2), User(name=郑十, age=18, grade=2), User(name=root, age=18, grade=2)], 
//[User(name=王五, age=18, grade=3), User(name=孙九, age=18, grade=3)], 
//[User(name=赵六, age=18, grade=4)]]

集合转字符串

List<String> list = Arrays.asList("a", "b", "c");
String joinedString = CollectionUtil.join(list, ","); // "a,b,c"

元素添加

List<String> list = new ArrayList<>(Arrays.asList("1", "2"
CollectionUtil.addAll(list,"4");
CollectionUtil.addAll(list,new String[]{"5","6"});
CollectionUtil.addAll(list,Arrays.asList("7","8"));
CollectionUtil.addAll(list,new HashSet<>(Arrays.asList("9"
System.out.println(list);//[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

List<String> list = new ArrayList<>(Arrays.asList("1", "2", "3"));
CollectionUtil.addAllIfNotContains(list, Arrays.asList("1", "2", "4"));
System.out.println(list);//[1, 2, 3, 4]

List<String> list = new ArrayList<>(Arrays.asList("1", "2", "3"));
CollectionUtil.addIfAbsent(list,"2");//已存在不添加
CollectionUtil.addIfAbsent(list,"4");
System.out.println(list);//[1, 2, 3, 4]

元素删除

List<String> list = new ArrayList<>(Arrays.asList("1", "2", "3","","4",null,"5"));
CollectionUtil.removeEmpty(list);
//CollectionUtil.removeBlank(list);
System.out.println(list);//[1, 2, 3, 4，5]

List<String> list = new ArrayList<>(Arrays.asList("1", "2", "3","4","3","5","3","2"));
CollectionUtil.removeAny(list, "3","2");
System.out.println(list);//[1, 4，5]

List<String> list = new ArrayList<>(Arrays.asList("1", "2", "3","","4",null,"5"));
CollectionUtil.removeNull(list);
System.out.println(list);//[1, 2, 3, 4，5]

//自定义条件
List<String> list = new ArrayList<>(Arrays.asList("1", "2", "3","","4",null,"5"));
CollectionUtil.removeWithAddIf(list, item -> item == null || item.isEmpty());
System.out.println(list);//[1, 2, 3, 4，5]

根据属性转Map

List<User> list = Arrays.asList(
    new User(1,"张三", 18, 1),
    new User(2,"李四", 18, 2),
    new User(3,"王五", 18, 3),
    new User(4,"张三", 18, 4),
    new User(5,"李四", 18, 1));
Map<String, User> result = CollectionUtil.fieldValueMap(list, "id");

Map<String, User> result2 = CollectionUtil.fieldValueAsMap(list, "id", "name");

获取元素

List<User> list = Arrays.asList(
            new User(1,"张三", 18, 1),
            new User(2,"李四", 18, 2),
            new User(3,"王五", 18, 3),
            new User(4,"张三", 18, 4),
            new User(5,"李四", 18, 1));
User result = CollectionUtil.get(list,6);//自带判空效果
User result = CollectionUtil.getFirst(list);
User result = CollectionUtil.getLast(list);
List<User> result = CollectionUtil.getAny(list, 0,  2);
List<Object> result = CollectionUtil.getFieldValues(list, "name");//[张三, 李四, 王五, 张三, 李四]  User类要先实现Iterable<E>接口

//截取集合
List<User> result = CollectionUtil.sub(list, 0, 3);

获取最大/最小元素

List<User> list = Arrays.asList(
    new User(1,"张三", 19, 1),
    new User(2,"李四", 18, 2),
    new User(3,"王五", 16, 3),
    new User(4,"张三", 24, 4),
    new User(5,"李四", 17, 1));
//需要User先实现Comparable<e>接口
User result = CollectionUtil.max(list);//张三
User result = CollectionUtil.min(list);//王五

集合排序

List<User> list = Arrays.asList(
    new User(1,"张三", 19, 1),
    new User(2,"李四", 18, 2),
    new User(3,"王五", 16, 3),
    new User(4,"张三", 24, 4),
    new User(5,"李四", 17, 1));
//不需要User先实现Comparable<e>接口，默认从小到大排序
List<User> result = CollectionUtil.sort(list, Comparator.comparing(User::getAge));