java Stream collect()使用

Stream的collect()方法是一个非常强大的终端操作，用于将流中的元素收集到一个结果容器中。collect()方法通常接收一个Collector接口的实现作为参数。

toList()：将流中的元素收集到一个List中

 import java.util.Arrays;
   import java.util.List;
   import java.util.stream.Collectors;

   public class CollectToListExample {
       public static void main(String[] args) {
           List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
           List<Integer> evenNumbers = numbers.stream()
                  .filter(n -> n % 2 == 0)
                  .collect(Collectors.toList());
           System.out.println(evenNumbers);
       }
   }

toSet()：将流中的元素收集到一个Set中，去除重复元素。

 import java.util.Arrays;
   import java.util.Set;
   import java.util.stream.Collectors;

   public class CollectToSetExample {
       public static void main(String[] args) {
           List<Integer> numbers = Arrays.asList(1, 2, 3, 2, 4, 3, 5);
           Set<Integer> distinctNumbers = numbers.stream()
                  .collect(Collectors.toSet());
           System.out.println(distinctNumbers);
       }
   }

toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)：将流中的元素收集到一个Map中，通过指定键和值的映射函数。

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

   public class CollectToMapExample {
       public static void main(String[] args) {
           List<Person> people = Arrays.asList(
                   new Person("Alice", 25),
                   new Person("Bob", 30),
                   new Person("Charlie", 28)
           );
           Map<String, Integer> nameToAgeMap = people.stream()
                  .collect(Collectors.toMap(Person::getName, Person::getAge));
           System.out.println(nameToAgeMap);
       }

       static class Person {
           private String name;
           private int age;

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

           public String getName() {
               return name;
           }

           public int getAge() {
               return age;
           }
       }
   }

joining()：将流中的元素连接成一个字符串

 import java.util.Arrays;
   import java.util.stream.Collectors;

   public class CollectJoiningExample {
       public static void main(String[] args) {
           List<String> words = Arrays.asList("Hello", "World", "Java");
           String joinedString = words.stream()
                  .collect(Collectors.joining(", "));
           System.out.println(joinedString);
       }
   }

summingInt(ToIntFunction<? super T> mapper)、summingLong(ToLongFunction<? super T> mapper)、summingDouble(ToDoubleFunction<? super T> mapper)：分别用于计算流中元素的整数、长整数和双精度浮点数属性的总和。

import java.util.Arrays;
   import java.util.stream.Collectors;

   public class CollectSummingExample {
       public static void main(String[] args) {
           List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
           int sum = numbers.stream()
                  .collect(Collectors.summingInt(Integer::intValue));
           System.out.println(sum);
       }
   }

averagingInt(ToIntFunction<? super T> mapper)、averagingLong(ToLongFunction<? super T> mapper)、averagingDouble(ToDoubleFunction<? super T> mapper)：分别用于计算流中元素的整数、长整数和双精度浮点数属性的平均值。

 import java.util.Arrays;
   import java.util.stream.Collectors;

   public class CollectAveragingExample {
       public static void main(String[] args) {
           List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
           double average = numbers.stream()
                  .collect(Collectors.averagingInt(Integer::intValue));
           System.out.println(average);
       }
   }

groupingBy(Function<? super T,? extends K> classifier)：根据指定的分类函数对流中的元素进行分组，返回一个Map<K, List>。

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

   public class CollectGroupingByExample {
       public static void main(String[] args) {
           List<Person> people = Arrays.asList(
                   new Person("Alice", 25),
                   new Person("Bob", 30),
                   new Person("Charlie", 28),
                   new Person("David", 25)
           );
           Map<Integer, List<Person>> ageGroupMap = people.stream()
                  .collect(Collectors.groupingBy(Person::getAge));
           System.out.println(ageGroupMap);
       }

       static class Person {
           private String name;
           private int age;

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

           public String getName() {
               return name;
           }

           public int getAge() {
               return age;
           }
       }
   }

partitioningBy(Predicate<? super T> predicate)：根据指定的谓词对流中的元素进行分区，返回一个Map<Boolean, List>，其中键为true和false，分别对应满足和不满足谓词的元素列表。

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

   public class CollectPartitioningByExample {
       public static void main(String[] args) {
           List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
           Map<Boolean, List<Integer>> evenOddMap = numbers.stream()
                  .collect(Collectors.partitioningBy(n -> n % 2 == 0));
           System.out.println(evenOddMap);
       }
   }