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;
    }
}

 

List<Apple> appleList = new ArrayList<>();//存放apple對象集合

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);

 

list.stream().mapToDouble(User::getHeight).sum()//和
list.stream().mapToDouble(User::getHeight).max()//最大
list.stream().mapToDouble(User::getHeight).min()//最小
list.stream().mapToDouble(User::getHeight).average()//平均值

 

1. List轉Map
id為key，apple對象為value，可以這么做：
/**
 * List -> Map
 * 需要注意的是：
 * toMap 如果集合對象有重復的key，會報錯Duplicate key ....
 *  apple1,apple12的id都為1。
 *  可以用 (k1,k2)->k1 來設置，如果有重復的key,則保留key1,舍棄key2
 */
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));
打印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里面的對象元素，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(map只是覆蓋屬性，filter根據判斷屬性來collect宿主bean)：
//過濾出符合條件的數據
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: 將集合中的數據按照某個屬性求和:
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

List<Integer> cc = new ArrayList<>();
cc.add(1);cc.add(2);cc.add(3);
int sum = cc.stream().mapToInt(Integer::intValue).sum();//6

 

 

★5    Collectors.groupingBy進行分組、排序等操作:
import javaX.Model.Student;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class FunctionX {
    public static void main(String[] args) {
        //1.分組計數
        List<Student> list1= Arrays.asList(new Student(1,"one","zhao"),new Student(2,"one","qian"),new Student(3,"two","sun"));
        //1.1根據某個屬性分組計數
        Map<String,Long> result1=list1.stream().collect(Collectors.groupingBy(Student::getGroupId,Collectors.counting()));
        System.out.println(result1);
        //1.2根據整個實體對象分組計數,當其為String時常使用
        Map<Student,Long> result2=list1.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
        System.out.println(result2);
        //1.3根據分組的key值對結果進行排序、放進另一個map中並輸出
        Map<String,Long> xMap=new HashMap<>();
        result1.entrySet().stream().sorted(Map.Entry.<String,Long>comparingByKey().reversed()) //reversed不生效
                .forEachOrdered(x->xMap.put(x.getKey(),x.getValue()));
        System.out.println(xMap);

        //2.分組，並統計其中一個屬性值得sum或者avg:id總和
        Map<String,Integer> result3=list1.stream().collect(
                Collectors.groupingBy(Student::getGroupId,Collectors.summingInt(Student::getId))
        );
        System.out.println(result3);

    }
}