JAVA-8流收集高级用法

Question

package streams;

import java.util.Arrays;
import java.util.List;

class Student{
    String name;
    int age;
    String type;

    public Student(){}

    public Student(String name, int age, String type) {
        this.name = name;
        this.age = age;
        this.type = type;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", type='" + type + '\'' +
                '}';
    }

    public static List<Student> generateData() {

        List<Student> st = Arrays.asList(new Student("Ashish", 27, "College"),
                new Student("Aman", 24, "School"),
                new Student("Rahul", 18, "School"),
                new Student("Ajay", 29, "College"),
                new Student("Mathur", 25, "College"),
                new Student("Modi", 28, "College"),
                new Student("Prem", 15, "School"),
                new Student("Akash", 17, "School"));
        return st;
    }
}

---

//AdvancedStreamingP2 class uses the Student class

package streams;

import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;

import static java.util.Comparator.comparing;
import static java.util.stream.Collectors.*;

public class AdvancedStreamingP2 {

    public static void main(String[] args) {

        List<Student> studentList = Student.generateData();
        System.out.println("\n---------- Extracting Student Name with Max Age by Type -----------");
        Map<String, Optional<Student>> stuMax = studentList.stream().collect(groupingBy(Student::getType, maxBy(comparing(Student::getAge))));
        stuMax.forEach((k, v) -> System.out.println("Key : " + k + ", Value :" + v.get()));
    }
}

我想提取学生姓名，最大年龄，按类型分组学生。 是否可以在“收集”中使用任何组合？

我希望输出像：

---------- Extracting Student Name with Max Age by Type -----------

Key : School, Value : Aman
Key : College, Value : Ajay

Answer 1

是的，你可以使用Collectors.collectingAndThen 。 此收集器调整现有收集器以执行其他修整器操作。 在这种情况下，整理器操作只返回学生的名称。

Map<String, String> stuMax = 
    studentList.stream()
               .collect(groupingBy(
                    Student::getType, 
                    collectingAndThen(maxBy(comparing(Student::getAge)), v -> v.get().getName()) 
               ));

输出：

---------- Extracting Student Name with Max Age by Type -----------
Key : School, Value :Aman
Key : College, Value :Ajay

边注：而不是使用comparing ，你可以使用comparingInt因为Person.getAge()返回一个int ：这避免了不必要的拳。

Answer 2

结帐以下是java 8中消费者的例子： -

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Collectors;
import java.util.stream.Stream;


public class Java8ConsumerExample1 {
private static List<Integer> QUERY_RESULTSET_INTEGER = Arrays.asList(new   Integer(1), new Integer(10), new Integer(200), new Integer(101), new Integer(-10), new Integer(0));
private static List<String> QUERY_RESULTSET_STRING = Stream.of("A", "B", "C", "D", "E", "F").collect(Collectors.toList());

public static void main(String[] args) {

    // EXAMPLE : 1

    /**
     * Iterator over the Query generated integer list and print on console.
     */
    StubsUtils.forEach(new ExecutorContext() {
        List<Integer> consumerList = new ArrayList<Integer>();

        /**
         * Let's assume that our query returns set of Integer that would be consume by some caller class or print on console.
         * @return
         */
        @Override
        public List<Integer> getQuery() {
            return QUERY_RESULTSET_INTEGER;
        }

        @Override
        public Consumer<Integer> getConsumer() {
            return x -> {
                System.out.println(x);
                consumerList.add(x);
            };
        }
    });

    // EXAMPLE : 2
    /**
     * Iterator over the Query generated String list and print on console.
     */
    StubsUtils.forEach(new ExecutorContext() {
        List<String> consumerList = new ArrayList<String>();

        /**
         * Let's assume that our query returns set of Integer that would be consume by some caller class or print on console.
         * @return
         */
        @Override
        public List<String> getQuery() {
            return QUERY_RESULTSET_STRING;
        }

        @Override
        public Consumer<String> getConsumer() {
            return x -> {
                System.out.println(x);
                consumerList.add(x);
            };
        }
    });

}
}

StubsUtils.Java

 /**
   * Utility class
 */
 class StubsUtils {

 public static void forEach(ExecutorContext executorContext) {
    executorContext.getQuery().forEach(executorContext.getConsumer());
}

}

ExecutorContext.Java

interface ExecutorContext<E> {

List<Integer> getQuery();

Consumer<E> getConsumer();
}

Answer 3

这是另一个消费者示例： -

    package com;

    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Consumer;


    public class AdvanceConsumerTest {

        public static void main(String[] args) {

            TestLookupService testLookupService = new TestLookupService();
            testLookupService.forEach("A",val->{
                System.out.println(" Count of 'A' is "+ val);
            });
            System.out.println("******************************************");
            testLookupService.forEach("B",val->{
                System.out.println(" Count of 'B' is "+ val);
            });
            System.out.println("******************************************");
            testLookupService.forEach("C",val->{
                System.out.println(" Count of 'C' is "+ val);
            });

        }
    }

    class TestLookupService {
        void forEach(String parameter, Consumer<Long> stringConsumer) {
            LocalRepository.forEach(new QueryExecutionContext() {
                @Override
                public String getQuery() {
                    return parameter;
                }

                @Override
                public Consumer<Long> getConsumer() {
                    return stringConsumer;
                }
            });
        };
    }

    class LocalRepository {
        static DataSetRepo dataSetRepo = new DataSetRepo();

        static void forEach(QueryExecutionContext executionContext) {
            executionContext.getConsumer().accept(dataSetRepo.queryResults(executionContext));
        }

    }

    interface QueryExecutionContext {

        String getQuery();

        Consumer<Long> getConsumer();
    }

    class DataSetRepo {
        List<String> cacheOf = Arrays.asList("A", "B", "C", "A", "C", "C", "B");

        long queryResults(QueryExecutionContext context) {
            return cacheOf.stream().filter(s -> s.equalsIgnoreCase(context.getQuery())).count();
        }
    }