lamda表达式对List进行分组，List按条件过滤_lambda list 分组

示例对象

    private Integer id;
    private String name;
    private BigDecimal money;
    private Integer num;
    public Apple(Integer id, String name, BigDecimal money, Integer num) {
        this.id = id;
        this.name = name;
        this.money = money;
        this.num = num;
}
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数据

List<Apple> appleList = new ArrayList<>();//存放apple对象集合
 
Apple apple1 =  new Apple(1,"苹果1",new BigDecimal("3.25"),10);
Apple apple12 = new Apple(1,"苹果2",new BigDecimal("1.35"),20);
Apple apple2 =  new Apple(2,"香蕉",new BigDecimal("2.89"),30);
Apple apple3 =  new Apple(3,"荔枝",new BigDecimal("9.99"),40);
 
appleList.add(apple1);
appleList.add(apple12);
appleList.add(apple2);
appleList.add(apple3);
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1、分组
List里面的对象元素，以某个属性来分组，例如，以id分组，将id相同的放在一起：

//List 以ID分组 Map<Integer,List<Apple>>
Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));
 
System.err.println("groupBy:"+groupBy);
{1=[Apple{id=1, name='苹果1', money=3.25, num=10}, Apple{id=1, name='苹果2', money=1.35, num=20}], 2=[Apple{id=2, name='香蕉', money=2.89, num=30}], 3=[Apple{id=3, name='荔枝', money=9.99, num=40}]}
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List<User> users = new LinkedList<>();
users.add(new User("Jim", 12));
users.add(new User("John", 18));
users.add(new User("Tom", 21));
users.add(new User("Leo", 30));
users.add(new User("Kate", 44));
users.add(new User("Lio", 50));
 
Map<String, List<User>> tripleUsers = users.stream()
        .collect(Collectors.groupingBy((Function<User, String>) user -> {
    String key;
    if (user.getAge() <= 20) {
        key = "less20";
    } else if (user.getAge() <= 40) {
        key = "less40";
    } else {
        key = "more40";
    }
    return key;
}, Collectors.toList()));
 
System.out.println(tripleUsers);
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2、List转Map
id为key，apple对象为value

/**
 * List -> Map
 * 需要注意的是：
 * toMap 如果集合对象有重复的key，会报错Duplicate key .... 解决办法见我的另一篇文章。。。
 *  apple1,apple12的id都为1。
 *  可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2
 */
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));
 
{1=Apple{id=1, name='苹果1', money=3.25, num=10}, 2=Apple{id=2, name='香蕉', money=2.89, num=30}, 3=Apple{id=3, name='荔枝', money=9.99, num=40}}
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3、过滤Filter
从集合中过滤出来符合条件的元素：

//过滤出符合条件的数据
示例一：

List<Apple> filterList = appleList.stream().filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList());
 
System.err.println("filterList:"+filterList);
[Apple{id=2, name='香蕉', money=2.89, num=30}]


示例二：

List<user> filterAges = users.stream().filter(user->ages.contains(user.getAge())).collect(Collectors.toList());

以下代码就相当于一个foreach
filterAges.stream().forEach(user -> {
    System.out.println(user.getName()+"======"+user.getEmail());
});
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多条件过滤

List<user> filter= users.stream()
                          .filter(user -> user.getName().indexOf("月") > -1 || user.getEmail().indexOf("mu") > -1)
                          .collect(Collectors.toList());
collect.stream().forEach(user -> {
    System.out.println(user.getName()+"======"+user.getEmail());
});
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4、求和
将集合中的数据按照某个属性求和

//计算 总金额
BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.err.println("totalMoney:"+totalMoney);  //totalMoney:17.48
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5、查找流中最大 最小值
Collectors.maxBy 和 Collectors.minBy 来计算流中的最大或最小值。

Optional<Dish> maxDish = Dish.menu.stream().
      collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));
maxDish.ifPresent(System.out::println);
 
Optional<Dish> minDish = Dish.menu.stream().
      collect(Collectors.minBy(Comparator.comparing(Dish::getCalories)));
minDish.ifPresent(System.out::println);
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6、去重
Java8 lambda自带的去重为 distinct 方法, 但是只能过滤整体对象和基本数据类型, 不能实现对象里的某个值进行判定去重, 比如:

List<Integer> numbers = Arrays.asList(8, 2, 8, 2, 6, 5, 9, 5, 6, 7);
List<Integer> distinctNumbers = numbers.stream()
.distinct()
.collect(Collectors.toList());
System.out.println(distinctNumbers);
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如果对一个实体对象List , 对指定的Student的某个字段作为条件去重就不行了,例如name字段，以下代码达不到效果

List<Student> distinctUsers = users.stream()
.distinct(Student::getName)
.collect(Collectors.toList());
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可以通过一下代码实现根据实体类的字段去重

import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toCollection;
 
// 根据id去重
  List<Person> unique = appleList.stream().collect(
              collectingAndThen(
                      toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new)
      );
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根据多个字段去除重复

List<User> userList = users.stream() .collect(
        Collectors.collectingAndThen(
                Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(user -> user.getName()+";"+user.getId()))), ArrayList::new));
List去重两个相同的实体类对象或者相同的单个对象字段
 
List<String> classNameList = new ArrayList(new HashSet(userList));
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7、获取实体类对象中某一个字段转为List

List<Long> userIds = users.stream().map(User::getId).collect(Collectors.toList());

User::getId  就相当于  a —> a.getId()
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8、排序
可通过Sort对单字段多字段排序

//单字段排序，根据id排序
  userList.sort(Comparator.comparing(User::getId));
 
//多字段排序，根据id，年龄排序
  userList.sort(Comparator.comparing(User::getId).thenComparing(User::getAge));
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