java8 Collectors

本文轉載自查看原文 2018-06-26 15:45 1308

記錄一下 java8 Collectors 收集器使用詳解。

1.Collectors.toList()

List<String> result = givenList.stream()
  .collect(toList());

2.Collectors.toSet()

Set<String> result = givenList.stream()
  .collect(toSet());

3.Collectors.toMap()

toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction, Supplier mapSupplier)

The arguments are as follows.

Function keyMapper: It generates key for Map.

Function valueMapper: It generates value for Map.

BinaryOperator mergeFunction: This is optional. The usability of merge function is to handle the situation of duplicate Map keys. Using BinaryOperator we can merge the values of duplicate keys. If we do not pass this argument, then by default it throws IllegalStateException in case of duplicate keys.

Supplier mapSupplier: This is optional. It returns a Map in which data is filled as key/value. If we do not pass map supplier then the default supplier will return HashMap. If we want to get any other instance such as LinkedHashMap, we need to pass supplier as LinkedHashMap::new.

3.1 toMap(Function keyMapper, Function valueMapper

ListToMap1.java

package com.concretepage;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class ListToMap1 {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Mohan");
        list.add("Sohan");
        list.add("Mahesh");
        Map<String, Object> map = list.stream().collect(Collectors.toMap(Function.identity(), s->s));
        map.forEach((x, y) -> System.out.println("Key: " + x +", value: "+ y));
    }
}

output

Key: Mohan, value: Mohan
Key: Mahesh, value: Mahesh
Key: Sohan, value: Sohan

ListToMap2.java

package com.concretepage;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class ListToMap2 {
    public static void main(String[] args) {
        List<Person> list = new ArrayList<>();
        list.add(new Person(100, "Mohan"));
        list.add(new Person(200, "Sohan"));
        list.add(new Person(300, "Mahesh"));
        Map<Integer, String> map = list.stream()
                .collect(Collectors.toMap(Person::getId, Person::getName));
        map.forEach((x, y) -> System.out.println("Key: " + x +", value: "+ y));
    }
}

output

Key: 100, value: Mohan
Key: 200, value: Sohan
Key: 300, value: Mahesh

3.2 `toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction)`

ListToMapWithBinaryOperator.java

package com.concretepage;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class ListToMapWithBinaryOperator {
    public static void main(String[] args) {
        List<Person> list = new ArrayList<>();
        list.add(new Person(100, "Mohan"));
        list.add(new Person(100, "Sohan"));
        list.add(new Person(300, "Mahesh"));
        Map<Integer, String> map = list.stream()
                .collect(Collectors.toMap(Person::getId, Person::getName, (x, y) -> x+", "+ y));
        map.forEach((x, y) -> System.out.println("Key: " + x +", value: "+ y));
    }
}

output

Key: 100, value: Mohan, Sohan
Key: 300, value: Mahesh

3.3 `toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction, Supplier mapSupplier)`

ListToMapWithSupplier.java

package com.concretepage;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.stream.Collectors;
public class ListToMapWithSupplier {
    public static void main(String[] args) {
        List<Person> list = new ArrayList<>();
        list.add(new Person(100, "Mohan"));
        list.add(new Person(100, "Sohan"));
        list.add(new Person(300, "Mahesh"));
        LinkedHashMap<Integer, String> map = list.stream()
                .collect(Collectors.toMap(Person::getId, Person::getName, 
                        (x, y) -> x+", "+ y, LinkedHashMap::new));
        map.forEach((x, y) -> System.out.println("Key: " + x +", value: "+ y));
    }
}

output

Key: 100, value: Mohan, Sohan
Key: 300, value: Mahesh

4. Collectors.toCollection()

可以轉成具體的實現

List<String> result = givenList.stream()
  .collect(toCollection(LinkedList::new))

5. Collectors.partitioningBy()

判斷true、false 返回兩個集合

 Map<Boolean, List<Developer>> youngerThan30 = team.
                stream().
                collect(Collectors.partitioningBy(d -> d.getAge() < 30));
        System.out.println("Developers younger than thirty are using: " + youngerThan30.get(true).stream().
        map(d -> d.getFavoriteLanguage()).
        collect(Collectors.toSet()));
        //Output: Developers younger than thirty are using: [java]
        System.out.println("Developers older than thirty are using: " + youngerThan30.get(false).stream().
        map(d -> d.getFavoriteLanguage()).
        collect(Collectors.toSet()));
        //Output: Developers older than thirty are using: [logo, javascript]

6.Collectors.groupingBy()

<T,K,A,D> Collector<T,?,Map<K,D>> groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)

Map<Integer, List<Person>> peropleByAge = people.filter(p -> p.age > 12).collect(Collectors.groupingBy(p -> p.age, Collectors.toList()));

Map<Integer, List<String>> peropleByAge = people.collect(Collectors.groupingBy(p -> p.age, Collectors.mapping((Person p) -> p.name, Collectors.toList())));

Map<String, Integer> sumAgeByName = people.collect(Collectors.groupingBy(p -> p.name, Collectors.reducing(0, (Person p) -> p.age, Integer::sum)));
/* 或者使用summingInt方法 */
sumAgeByName = people.collect(Collectors.groupingBy(p -> p.name, Collectors.summingInt((Person p) -> p.age)));

        Map<String,Long> map = list.stream().collect(groupingBy(Person::getName,counting()));

7.Collectors.joininng()

String result = givenList.stream()
  .collect(joining(" "));

String result = givenList.stream()
  .collect(joining(" ", "PRE-", "-POST"));

8.Collectors.counting()

Long result = givenList.stream()
  .collect(counting());

9.Collectors.reduce()

9.1 `reduce(BinaryOperator accumulator)`

package com.concretepage;
import java.util.Arrays;
public class ReduceDemo1 {
    public static void main(String[] args) {
          int[] array = {23,43,56,97,32};
          Arrays.stream(array).reduce((x,y) -> x+y).ifPresent(s -> System.out.println(s));
          Arrays.stream(array).reduce(Integer::sum).ifPresent(s -> System.out.println(s));
          Arrays.stream(array).reduce(StatisticsUtility::addIntData).ifPresent(s -> System.out.println(s));
    }
}

package com.concretepage;
public class StatisticsUtility {
    public static int addIntData(int num1, int num2) {
        return num1 + num2;
    }
}

9.2 `reduce(T identity, BinaryOperator<T> accumulator)`

ackage com.concretepage;
import java.util.Arrays;
public class ReduceDemo2 {
    public static void main(String[] args) {
          int[] array = {23,43,56,97,32};
          //Set start value. Result will be start value + sum of array. 
          int startValue = 100;
          int sum = Arrays.stream(array).reduce(startValue, (x,y) -> x+y);
          System.out.println(sum);
          sum = Arrays.stream(array).reduce(startValue, Integer::sum);
          System.out.println(sum);
          sum = Arrays.stream(array).reduce(startValue, StatisticsUtility::addIntData);
          System.out.println(sum);
    }
}

9.3 `reduce(U identity, BiFunction<U,? super T,U> accumulator, BinaryOperator<U> combiner)`

package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class ReduceDemo3 {
    public static void main(String[] args) {
      List<Integer> list2 = Arrays.asList(2, 3, 4);
      //Here result will be 2*2 + 2*3 + 2*4 that is 18. 
      int res = list2.parallelStream().reduce(2, (s1, s2) -> s1 * s2, (p, q) -> p + q);
      System.out.println(res);
      
      List<String> list1 = Arrays.asList("Mohan", "Sohan", "Ramesh");
      String result = list1.parallelStream().reduce("-", (s1, s2) -> s1 + s2, (p, q) -> p + q);
      System.out.println(result);
   }
}

output

18
-Mohan-Sohan-Ramesh

10.Collectors.collectingAndThen()

將收集到的結果轉換成另一種形式

public class CollectingAndThenExample {
    public static void main (String[] args) {
        Stream<String> s = Stream.of("apple", "banana", "orange");
        List<String> synchronizedList = s.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::synchronizedList));
        System.out.println(synchronizedList);
    }
}

public class CollectingAndThenExample {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.collectingAndThen(Collectors.averagingLong(v->v*2),
                s-> s*s));
        System.out.println(result);
    }
}

11.Collectors.summarizingDouble/Long/Int()

統計數據時常用

getAverage(): It returns the average of all accepted value. 
getCount(): It calculates the count of all element. 
getMax(): It returns the maximum value. 
getMin(): It returns the minimum value. 
getSum(): It returns the sum of all elements. 
accept(): It adds the element into the summary information. 
combine(): It combines two summary statistics.

11.1 primitive data type

public class SummaryStatisticsWithPrimitiveDataType {
    public static void main(String[] args) {
        System.out.println("---DoubleSummaryStatistics---");        
        DoubleSummaryStatistics dstats = DoubleStream.of(5.33d,2.34d,5.32d,2.31d,3.51d).
                collect(DoubleSummaryStatistics::new, DoubleSummaryStatistics::accept, 
                        DoubleSummaryStatistics::combine);
        System.out.println("Max:"+dstats.getMax()+", Min:"+dstats.getMin());
        System.out.println("Count:"+dstats.getCount()+", Sum:"+dstats.getSum());
        System.out.println("Average:"+dstats.getAverage());        
        
        System.out.println("---LongSummaryStatistics---");            
        LongSummaryStatistics lstats = LongStream.of(51l,23l,53l,23l,35l).
                collect(LongSummaryStatistics::new, LongSummaryStatistics::accept, 
                        LongSummaryStatistics::combine);
        System.out.println("Max:"+lstats.getMax()+", Min:"+lstats.getMin());
        System.out.println("Count:"+lstats.getCount()+", Sum:"+lstats.getSum());
        System.out.println("Average:"+lstats.getAverage());        
        
        System.out.println("---IntSummaryStatistics---");            
        IntSummaryStatistics istats = IntStream.of(51,22,50,27,35).
                collect(IntSummaryStatistics::new, IntSummaryStatistics::accept, 
                        IntSummaryStatistics::combine);
        System.out.println("Max:"+istats.getMax()+", Min:"+istats.getMin());
        System.out.println("Count:"+istats.getCount()+", Sum:"+istats.getSum());
        System.out.println("Average:"+istats.getAverage());        
    }
}

11.2 with objects

public class SummaryStatisticsDemoWithObject {
    public static void main(String[] args) {
        System.out.println("--DoubleSummaryStatistics--");
        List<Rectangle> list = Rectangle.getRectangle();
        DoubleSummaryStatistics dstats = list.stream()
                 .collect(Collectors.summarizingDouble(Rectangle::getWidth));
        System.out.println("Max:"+dstats.getMax()+", Min:"+dstats.getMin());
        System.out.println("Count:"+dstats.getCount()+", Sum:"+dstats.getSum());
        System.out.println("Average:"+dstats.getAverage());
        
        System.out.println("--IntSummaryStatistics--");
        list = Rectangle.getRectangle();
        IntSummaryStatistics istats = list.stream()
                 .collect(Collectors.summarizingInt(Rectangle::getLength));
        System.out.println("Max:"+istats.getMax()+", Min:"+istats.getMin());
        System.out.println("Count:"+istats.getCount()+", Sum:"+istats.getSum());
        System.out.println("Average:"+istats.getAverage());
        
        System.out.println("--LongSummaryStatistics--");
        list = Rectangle.getRectangle();
        LongSummaryStatistics lstats = list.stream().
                         collect(Collectors.summarizingLong(Rectangle::getId));
        System.out.println("Max:"+lstats.getMax()+", Min:"+lstats.getMin());
        System.out.println("Count:"+lstats.getCount()+", Sum:"+lstats.getSum());
        System.out.println("Average:"+lstats.getAverage());
    }
}

12. Collectors.averagingDouble/Long/Int()

求平均值

12.1 averagingDouble

public class AveragingDoubleExample {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.averagingDouble(d->d*2));
        System.out.println(result);
    }
}

12.2 averagingInt

public class AveragingIntExample {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.averagingInt(v->v*2));
        System.out.println(result);
    }
}

12.3 averagingLong

public class AveragingLongExample {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4);
        Double result = list.stream().collect(Collectors.averagingLong(v->v*2));
        System.out.println(result);
    }
}

13. maxBy/minBy

求最大值、最小值

public class MaxByMinBy {
    public static void main(String[] args) {
        Student s1 = new Student("Shyam", 22,"A");
        Student s2 = new Student("Ram",23,"A");
        Student s3 = new Student("Mohan",22,"B");
        Student s4 = new Student("Ramesh",21,"B");
        List<Student> list = Arrays.asList(s1,s2,s3,s4);
        Comparator<Student> ageComparator = Comparator.comparing(Student::getAge); 
        //Using BinaryOperator.maxBy        
        System.out.println("---BinaryOperator.maxBy---");
        Map<String, Optional<Student>> eldestByClass = list.stream().collect(Collectors.groupingBy(Student::getClassName, 
                Collectors.reducing(BinaryOperator.maxBy(ageComparator))));
        eldestByClass.forEach((k,v)->System.out.println("Class:"+k+" Age:"+
                ((Optional<Student>)v).get().getAge()+" Name:"+((Optional<Student>)v).get().getName()));
        
        //Using BinaryOperator.minBy        
        System.out.println("---BinaryOperator.minBy---");
        Map<String, Optional<Student>> youngestByClass = list.stream().collect(Collectors.groupingBy(Student::getClassName, 
                Collectors.reducing(BinaryOperator.minBy(ageComparator))));
        youngestByClass.forEach((k,v)->System.out.println("Class:"+k+" Age:"+
                ((Optional<Student>)v).get().getAge()+" Name:"+((Optional<Student>)v).get().getName()));
    }
}

有道詞典

Here we will pa ...

詳細 X

在這里我們會通過BinaryOperator蓄電池。數字BinaryOperator,開始值是0。在字符串的情況下,開始值將是一個空字符串。

averagingDouble 詳細 X

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