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This article mainly explains the "java8 how to achieve List to map, grouping, filtering and other operations", the content of the article is simple and clear, easy to learn and understand, the following please follow the editor's ideas slowly in depth, together to study and learn "java8 how to achieve List to map, grouping, filtering and other operations" bar!

With the new features of java8, you can use concise 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;}}

Add some test data:

List appleList = new ArrayList (); / / Store the collection of apple objects 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)

1. Grouping

Object elements in List, grouped by certain attributes, for example, grouped by id, put the same id together:

/ List is grouped by ID MapMap 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=' Litchi', money=9.99, num=40}]}

2. List to Map

Id is key,apple object is value, you can do this:

/ * List-> Map * it should be noted that: * toMap if the collection object has a duplicate key, it will report an error Duplicate key. * the id of apple1,apple12 is 1. * it can be set with (K11=Apple K2)-> K1. If there are duplicates of key, keep key1 and discard key2 * / Map appleMap = appleList.stream (). Collect (Collectors.toMap (Apple::getId, a-> a, (K1PowerK2)-> K1); print appleMap {1=Apple {id=1, name=' apple 1x, money=3.25, num=10}, 2=Apple {id=2, name=' banana', money=2.89, num=30}, 3=Apple {id=3, name=' litchi', money=9.99, num=40}}

3. Filter Filter

Filter out eligible elements from the collection:

/ / filter out eligible data List filterList = appleList.stream (). Filter (a-> a.getName (). Equals ("banana")) .Collectors.toList (); System.err.println ("filterList:" + filterList); [Apple {id=2, name=' banana', money=2.89, num=30}]

4. Summation

Sum the data in the collection according to a property:

/ / calculate the total amount BigDecimal totalMoney = appleList.stream (). Map (Apple::getMoney). Reduce (BigDecimal.ZERO, BigDecimal::add); System.err.println ("totalMoney:" + totalMoney); / / totalMoney:17.48

5. Find the maximum and minimum value in the stream

Collectors.maxBy and Collectors.minBy to calculate the maximum or minimum values in the flow.

Optional maxDish = Dish.menu.stream (). Collect (Collectors.maxBy (Comparator.comparing (Dish::getCalories); maxDish.ifPresent (System.out::println); Optional minDish = Dish.menu.stream (). Collect (Collectors.minBy (Comparator.comparing (Dish::getCalories); minDish.ifPresent (System.out::println)

6. Deweighting

Import static java.util.Comparator.comparingLong;import static java.util.stream.Collectors.collectingAndThen;import static java.util.stream.Collectors.toCollection; / / remove weight according to id List unique = appleList.stream () .collect (collectingAndThen (toCollection (()-> new TreeSet (comparingLong (Apple::getId), ArrayList::new)

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