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
      
        appleList = new ArrayList<>();//存放apple对象集合Apple apple1 =  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 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转Mapid为key，apple对象为value，可以这么做：/** * List -> Map * 需要注意的是： * 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
      
        appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));打印appleMap：{1=Apple{id=1, name='苹果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. 分组 List里面的对象元素，以某个属性来分组，例如，以id分组，将id相同的放在一起：//List 以ID分组 Map
      
       
        >Map
        
         > groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));System.err.println("groupBy:"+groupBy);{1=[Apple{id=1, name='苹果1', money=3.25, num=10}, Apple{id=1, name='苹果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. 过滤filter: 从集合中过滤出来符合条件的元素(map只是覆盖属性，filter根据判断属性来collect宿主bean)：//过滤出符合条件的数据List
      
        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: 将集合中的数据按照某个属性求和:BigDecimal://计算 总金额BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);System.err.println("totalMoney:"+totalMoney); //totalMoney:17.48Integer://计算 数量int sum = appleList.stream().mapToInt(Apple::getNum).sum();System.err.println("sum:"+sum); //sum:100List
      
        cc = new ArrayList<>();cc.add(1);cc.add(2);cc.add(3);int sum = cc.stream().mapToInt(Integer::intValue).sum();//6
      

 

 

★5    Collectors.groupingBy进行分组、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.分组计数        List
      
        list1= Arrays.asList(new Student(1,"one","zhao"),new Student(2,"one","qian"),new Student(3,"two","sun"));        //1.1根据某个属性分组计数        Map
       
         result1=list1.stream().collect(Collectors.groupingBy(Student::getGroupId,Collectors.counting()));        System.out.println(result1);        //1.2根据整个实体对象分组计数,当其为String时常使用        Map
        
          result2=list1.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));        System.out.println(result2);        //1.3根据分组的key值对结果进行排序、放进另一个map中并输出        Map
         
           xMap=new HashMap<>();        result1.entrySet().stream().sorted(Map.Entry.
          
           comparingByKey().reversed()) //reversed不生效                .forEachOrdered(x->xMap.put(x.getKey(),x.getValue()));        System.out.println(xMap);        //2.分组，并统计其中一个属性值得sum或者avg:id总和        Map
           
             result3=list1.stream().collect( Collectors.groupingBy(Student::getGroupId,Collectors.summingInt(Student::getId)) ); System.out.println(result3); }}