Java集合工具类（Java集合转换、去重、过滤、分组）

Java8中的Lambda+Stream简化了我们对集合类的操作。

通过Lambda+Stream，可以很简洁地实现集合转换、去重、过滤、分组等操作。

以下为封装好的工具类：

package utils;

import java.util.*;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class CollectionKit {
    private CollectionKit() {
    }

    
    public static  List convertToList(Collection collection,
                                               Function mapper) {
        if (collection == null || collection.isEmpty()) {
            return new ArrayList<>();
        }
        return collection.stream().map(mapper).collect(Collectors.toList());
    }

    
    public static  Set convertToSet(Collection collection,
                                             Function mapper) {
        if (collection == null || collection.isEmpty()) {
            return new HashSet<>();
        }
        return collection.stream().map(mapper).collect(Collectors.toSet());
    }

    
    public static  Map convertToMap(Collection collection,
                                                Function keyMapper) {
        if (collection == null || collection.isEmpty()) {
            return new HashMap<>();
        }
        return collection.stream().collect(Collectors.toMap(keyMapper, Function.identity(), (key1, key2) -> key2));
    }

    
    public static  Map convertToMap(Collection collection,
                                                   Function keyMapper,
                                                   Function valueMapper) {
        if (collection == null || collection.isEmpty()) {
            return new HashMap<>();
        }
        return collection.stream().collect(Collectors.toMap(keyMapper, valueMapper, (key1, key2) -> key2));
    }

    
    public static  List filter(Collection collection,
                                     Predicate filterPredicate) {
        if (collection == null || collection.isEmpty()) {
            return new ArrayList<>();
        }
        return collection.stream().filter(filterPredicate).collect(Collectors.toList());
    }

    
    public static  Map> group(Collection collection,
                                               Function classifier) {
        if (collection == null || collection.isEmpty()) {
            return new HashMap<>();
        }
        return collection.stream().collect(Collectors.groupingBy(classifier));
    }

    
    public static  List distinct(Collection collection,
                                       Comparator comparing) {
        return collection.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(comparing)), ArrayList::new));
    }
}

使用详解：

数据准备：

    class User {
        String name;
        int age;

        public String getName() { return name; }

        public void setName(String name) { this.name = name; }

        public int getAge() { return age; }

        public void setAge(int age) { this.age = age; }

        public User(String name, int age) {
            this.name = name;
            this.age = age;
        }
    }

    List users = new ArrayList<>();
    users.add(new User("张三", 10));
    users.add(new User("李四", 10));
    users.add(new User("王五", 20));
    users.add(new User("张三", 25));

1、转换为新的list，每个用户转换为名字+年龄格式的字符串

List list = CollectionKit.convertToList(users, item -> item.name + "_" + item.age);

运行结果：[张三_10, 李四_10, 王五_20, 张三_25]

2、使用convertToSet得到所有用户的年龄（去除重复项）

Set set = CollectionKit.convertToSet(users, User::getAge);

运行结果：[20, 25, 10]

3、使用convertToMap转换为key=name，value=user格式的map

Map map = CollectionKit.convertToMap(users, User::getName);

运行结果：
{
     李四=User{name='李四', age=10},
     张三=User{name='张三', age=25},
     王五=User{name='王五', age=20}
}

4、使用convertToMap转换为key=name，value=age格式的map

Map map1 = CollectionKit.convertToMap(users, User::getName, User::getAge);

运行结果：
{
     李四=10,
     张三=25,
     王五=20
}

5、使用filter筛选年龄为10的用户

List filter = CollectionKit.filter(users, item -> item.age == 10);

运行结果：[User{name='张三', age=10}, User{name='李四', age=10}]

6、使用distinct根据名字或年龄去重

List distinct = CollectionKit.distinct(users, Comparator.comparing(o -> o.name));
List distinct1 = CollectionKit.distinct(users, Comparator.comparing(o -> o.age));

运行结果：
[User{name='张三', age=10}, User{name='李四', age=10}, User{name='王五', age=20}]
[User{name='张三', age=10}, User{name='王五', age=20}, User{name='张三', age=25}]

7、使用group根据年龄分组

Map> group = CollectionKit.group(users, User::getAge);

运行结果：
{
     20=[User{name='王五', age=20}],
     25=[User{name='张三', age=25}],
     10=[User{name='张三', age=10}, User{name='李四', age=10}]
}