轉自:https://www.cnblogs.com/yangweiqiang/p/6934671.html

利用java8新特性，可以用簡潔高效的程式碼來實現一些資料處理~

定義1個Apple物件:

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;
    }
}

新增一些測試資料：

List<Apple> appleList = new ArrayList<>();//存放apple物件集合

Apple apple1 =  new Apple(1,"蘋果1",new BigDecimal("3.25"),10);
Apple apple12 = new Apple(1,"蘋果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);

1. List轉Map

id為key，apple物件為value，可以這麼做：

/**
 * List -> Map
 * 需要注意的是：
 * toMap 如果集合物件有重複的key，會報錯Duplicate key ....
 *  apple1,apple12的id都為1。
 *  可以用 (k1,k2)->k1 來設定，如果有重複的key,則保留key1,捨棄key2
 
 
*/
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));

列印appleMap：

{1=Apple{id=1, name='蘋果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}}

 2. 分組

List裡面的物件元素，以某個屬性來分組，例如，以id分組，將id相同的放在一起：

//List 以ID分組 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='蘋果1', money=3.25, num=10}, Apple{id=1, name='蘋果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}]}

 3. 過濾filter

從集合中過濾出來符合條件的元素：

//過濾出符合條件的資料
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}]

 4. 求和

將集合中的資料按照某個屬性求和:

BigDecimal:

//計算 總金額
BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.err.println("totalMoney:"+totalMoney);  //totalMoney:17.48

Integer:

//計算 數量
int sum = appleList.stream().mapToInt(Apple::getNum).sum();
System.err.println("sum:"+sum);  //sum:100