How to realize List to map, grouping, filtering and other operations by java8

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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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