Java8流式编程GroupBy和求最值示例

流的创建(演示常用的)

数组创建

Arrays.stream
Arrays的静态方法stream() 可以获取数组流

     String[] arr = { "a", "b", "c", "d", "e", "f", "g" };
     Stream<String> stream = Stream.of(arr);
     Stream<String> stream1 = Arrays.stream(arr);

集合创建

Collection接口提供了两个默认方法创建流：stream()、parallelStream()

    List<String> list = new ArrayList<String>();
    list.add("a");
    list.add("b");
    list.add("c");
    Stream<String> stream = list.stream();

值创建

Stream.of

使用静态方法 Stream.of(), 通过显示值创建一个流。它可以接收任意数量的参数。

    Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5, 6);

函数创建

Stream.iterate
Stream.generate
使用静态方法 Stream.iterate() 和 Stream.generate()创建无限流。

    Stream<Double> stream = Stream.generate(Math::random).limit(5);
    Stream<Integer> stream1 = Stream.iterate(0, i -> i + 1).limit(5);

注意：使用无限流一定要配合limit截断，不然会无限制创建下去。

GroupBy和求最值示例

实体类：

package com.demo;
 
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;
 
/** lombok的使用需要的依赖：
 <dependency>
     <groupId>org.projectlombok</groupId>
     <artifactId>lombok</artifactId>
     <version>1.18.16</version>
 </dependency>
 */
@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class Student {
    private long id;
    private String name;
    private int age;
    private int grade;
    private String major;
    private String school;
}

示例代码：

package com.demo;
 
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
 
public class FileTest {
    public static void main(String[] args) {
       /*------------------复杂对象演示------------------------*/
        List<Student> students = new ArrayList<Student>() {
            {
                add(new Student(20160001, "孔明", 20, 1, "土木工程", "武汉大学"));
                add(new Student(20160003, "玄德", 22, 3, "经济管理", "武汉大学"));
                add(new Student(20161001, "翼德", 21, 2, "机械与自动化", "华中科技大学"));
                add(new Student(20161003, "奉孝", 23, 4, "计算机科学", "华中科技大学"));
                add(new Student(20163001, "丁奉", 24, 5, "土木工程", "南京大学"));
            }
        };
 
        // group by 统计用法示例
        Map<String, Long> group2 = students.stream().collect(
                Collectors.groupingBy(
                        Student::getSchool,
                        Collectors.counting()
                )
        );
        //输出：{南京大学=1, 武汉大学=2, 华中科技大学=2}
        System.out.println(group2);
 
        TreeMap<String, Set<String>> collect = students.stream().collect(
                Collectors.groupingBy(
                        Student::getSchool,
                        TreeMap::new,
                        Collectors.mapping(Student::getName, Collectors.toSet())
                )
        );
        //输出：{华中科技大学=[翼德, 奉孝], 南京大学=[丁奉], 武汉大学=[孔明, 玄德]}
        System.out.println(collect);
 
 
        Map<String, List<Student>> collect3 = students.stream().collect(
                Collectors.groupingBy(
                        Student::getSchool
                )
        );
        /* 输出：{
                南京大学=[Student(id=20163001, name=丁奉, age=24, grade=5, major=土木工程, school=南京大学)],
                武汉大学=[
                    Student(id=20160001, name=孔明, age=20, grade=1, major=土木工程, school=武汉大学),
                    Student(id=20160003, name=玄德, age=22, grade=3, major=经济管理, school=武汉大学)
                 ],
                 华中科技大学=[
                    Student(id=20161001, name=翼德, age=21, grade=2, major=机械与自动化, school=华中科技大学),
                    Student(id=20161003, name=奉孝, age=23, grade=4, major=计算机科学, school=华中科技大学)
                  ]
              }
        *
        */
        System.out.println(collect3);
 
        // 求最大  最小值
        Student studentMax = students.stream().max((s1, s2) -> s1.getAge() - s2.getAge()).get();
        Student studentMin = students.stream().min((s1, s2) -> s1.getAge() - s2.getAge()).get();
        // 输出：Student(id=20163001, name=丁奉, age=24, grade=5, major=土木工程, school=南京大学)
        System.out.println(studentMax);
        // 输出：Student(id=20160001, name=孔明, age=20, grade=1, major=土木工程, school=武汉大学)
        System.out.println(studentMin);
 
 
 
 
 
        /*------------------基础数据类型演示------------------------*/
        List<Integer> list = new ArrayList<Integer>() {
            {
                add(20);
                add(18);
                add(18);
                add(17);
            }
        };
        Map<Integer, Long> collect1 = list.stream().collect(
                Collectors.groupingBy(
                        (integer) -> integer,
                        Collectors.counting()
                )
        );
        // 输出：{17=1, 18=2, 20=1}
        System.out.println(collect1);
 
        TreeMap<Integer, List<Integer>> collect2 = list.stream().collect(
                Collectors.groupingBy(
                        Integer::intValue,
                        TreeMap::new,
                        Collectors.mapping((integer) -> integer, Collectors.toList())
                )
        );
        // 输出：{17=[17], 18=[18, 18], 20=[20]}
        System.out.println(collect2);
 
        Integer max = list.stream().max(Integer::compareTo).get();
        Integer min = list.stream().min(Integer::compareTo).get();
        System.out.println(max);// 输出：20
        System.out.println(min);// 输出：17
 
 
        /*------------------map集合演示------------------------*/
        List<Map<String,Object>> list2 = new ArrayList<Map<String,Object>>() {
            {
                add(new HashMap<String,Object>(){{
                    put("prdTyp","lon");
                    put("typ","2.3");
                }});
                add(new HashMap<String,Object>(){{
                    put("prdTyp","lon");
                    put("typ","1.0");
                }});
                add(new HashMap<String,Object>(){{
                    put("prdTyp","lpr");
                    put("typ","88");
                }});
                add(new HashMap<String,Object>(){{
                    put("prdTyp","lpr");
                    put("typ","99");
                }});
 
            }
        };
 
        Map<Object, List<Map<String, Object>>> prdTyp = list2.stream().collect(
                Collectors.groupingBy( (map) -> map.get("prdTyp") )
        );
        /* 输出：{
                    lon=[{prdTyp=lon, typ=2.3}, {prdTyp=lon, typ=1.0}],
                    lpr=[{prdTyp=lpr, typ=88}, {prdTyp=lpr, typ=99}]
                }
        */
        System.out.println(prdTyp);
 
 
        TreeMap<Object, List<Object>> collect4 = list2.stream().collect(
                Collectors.groupingBy(
                        (map) -> map.get("prdTyp"),
                        TreeMap::new,
                        Collectors.mapping((map) -> map.get("prdTyp"), Collectors.toList())
                )
        );
        // 输出：{lon=[lon, lon], lpr=[lpr, lpr]}
        System.out.println(collect4);
 
        Optional<Map<String, Object>> mapMax = list2.stream().max(
                Comparator.comparing( (map) -> new BigDecimal((String) map.get("typ")) )
        );
        // mapMax直接输出：Optional[{prdTyp=lpr, typ=99}]
        System.out.println(mapMax.get().get("typ"));//99
 
        Optional<Map<String, Object>> mapMin = list2.stream().min(
                (map1,map2)->new BigDecimal((String) map1.get("typ")).compareTo(new BigDecimal((String) map2.get("typ")))
        );
        // mapMin直接输出：Optional[{prdTyp=lon, typ=1.0}]
        System.out.println(mapMin.get().get("typ"));//1.0
 
    }
 
 
}