Java8 quickly converts lists to maps, groups, and filters

With the new features of java8, you can use simple and efficient code to achieve some data processing. \

Define 1 Apple object:

public class Apple { 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; }}Copy the code

Add some test data:

List<Apple> appleList = new ArrayList<>();// Store the apple object collection
 
Apple apple1 =  new Apple(1."Apple 1".new BigDecimal("3.25"),10);
Apple apple12 = new Apple(1."Apple 2".new BigDecimal("1.35"),20);
Apple apple2 =  new Apple(2."Banana".new BigDecimal("2.89"),30);
Apple apple3 =  new Apple(3."Litchi".new BigDecimal("9.99"),40);
 
appleList.add(apple1);
appleList.add(apple12);
appleList.add(apple2);
appleList.add(apple3);
Copy the code

1, grouping

Group object elements in a List by an attribute, for example, by id, and group them together with the same ID:

//List is grouped by ID Map
      
       >
      ,list
Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));
 
System.err.println("groupBy:"+groupBy);
{1=[Apple{id=1, name=Apple '1', money=3.25, num=10}, Apple{id=1, name='apple 2', money=1.35, num=20}].2=[Apple{id=2, name='banana', money=2.89, num=30}].3=[Apple{id=3, name='lychee', money=9.99, num=40}}]Copy the code

2. Convert List to Map

Id = key and apple = value

/** * List -> Map * toMap Duplicate key.... * Apple1 and apple12 both have ids of 1. * Use (k1,k2)->k1. If there are duplicate keys, keep key1 and discard key2 */
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));
Copy the code

Print appleMap

{1=Apple{id=1, name=Apple '1', money=3.25, num=10}, 2=Apple{id=2, name='banana', money=2.89, num=30}, 3=Apple{id=3, name='lychee', money=9.99, num=40}}
Copy the code

3. Filter

Filter out qualifying elements from the collection:

// Filter out the data that meets the criteria
List<Apple> filterList = appleList.stream().filter(a -> a.getName().equals("Banana")).collect(Collectors.toList());
 
System.err.println("filterList:"+filterList);
[Apple{id=2, name='banana', money=2.89, num=30}]
Copy the code

4, sum

Sum the data in the collection according to an attribute:

// Calculate the total amount
BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.err.println("totalMoney:"+totalMoney);  / / totalMoney: 17.48
Copy the code

5. Find the maximum and minimum values in the stream

MaxBy and Collectors. MinBy to calculate the maximum or minimum value in the flow. Search Java bosom friend public account, reply “backend interview”, send you a Java interview questions treasure book

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);
Copy the code

6, to heavy

import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toCollection;
 
// Remove weight by id
     List<Person> unique = appleList.stream().collect(
                collectingAndThen(
                        toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new));Copy the code

The following table shows static factory methods for Collectors.

END

Source: suo. Im / 5 wpn61