java8的新特性,快速实现List转Map,分组,过滤

1.创建实体

public class Apple { private Integer id; private String name; private BigDecimal money; private Integer num; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public BigDecimal getMoney() { return money; } public void setMoney(BigDecimal money) { this.money = money; } public Integer getNum() { return num; } public void setNum(Integer num) { this.num = num; } @Override public String toString() { return "Apple{" + "id=" + id + ", name='" + name + '\'' + ", money=" + money + ", num=" + num + '}'; } public Apple(){} public Apple(Integer id, String name, BigDecimal money, Integer num) { this.id = id; this.name = name; this.money = money; this.num = num; } }

2.给List赋值

List<Apple> appleList = new ArrayList<>();//存放apple对象集合 Apple apple11 = 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 applel3 = new Apple(2,"香蕉",new BigDecimal("2.89"),30); Apple applel4 = new Apple(3,"荔枝",new BigDecimal("9.99"),40); appleList.add(apple11); appleList.add(apple12); appleList.add(applel3); appleList.add(applel4);

3.根据ID进行分组

//List 以ID分组 Map<Integer,List<Apple>> Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId)); System.out.println("groupBy:" + groupBy);

4.List -> Map

/** * List -> Map * toMap 如果集合对象有重复的key，会报错Duplicate key .... * 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));

5.Map的三种遍历方式

//map的遍历第一种,二次取值(常用) Set<Integer> integers = appleMap.keySet(); for (Integer integer : integers) { System.out.println("第一种->" + appleMap.get(integer)); } //map的遍历第二种,迭代器 Iterator<Map.Entry<Integer, Apple>> iterator = appleMap.entrySet().iterator(); while (iterator.hasNext()){ Map.Entry<Integer, Apple> next = iterator.next(); System.out.println("第二种->" + next.getKey() + ":" + next.getValue()); } //map的遍历第三种,适合大容量遍历 for (Map.Entry<Integer, Apple> integerAppleEntry : appleMap.entrySet()) { System.out.println("第三种->" + integerAppleEntry.getKey() + ":" + integerAppleEntry.getValue()); } //.values()不赘述

6.Filter

//过滤出符合条件的数据 List<Apple> filterList = appleList.stream().filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList()); System.out.println("filterList:"+filterList);

7.BigDecimal的统计

//计算 总金额 BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add); System.out.println("totalMoney:"+totalMoney);

8.去重

// 根据id去重 List<Apple> collect = appleList.stream().collect(collectingAndThen(toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new)); //遍历集合 collect.stream().forEach(ret ->{ System.out.println(ret); });

人若成就一道景致，则无关春夏秋冬。