深入探讨 Java 8 集合操作：全面解析 Stream API 的强大功能

Java 8 引入的 Stream API 使得集合操作更加简洁和高效。本文通过详细示例，展示如何利用 Stream API 进行各种集合操作，包括遍历、转换、过滤、排序、分组、去重等。

1. 遍历集合

示例：使用 forEach 遍历列表

import java.util.Arrays;
import java.util.List;public class ForEachExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 使用 forEach 遍历列表names.forEach(name -> System.out.println(name));}
}

示例：使用 forEach 遍历 Map

import java.util.HashMap;
import java.util.Map;public class ForEachMapExample {public static void main(String[] args) {Map<String, Integer> ages = new HashMap<>();ages.put("Peter", 30);ages.put("Anna", 25);ages.put("Mike", 35);// 使用 forEach 遍历 Mapages.forEach((name, age) -> System.out.println(name + ": " + age));}
}

2. List 转 Map

示例：将 List 转换为 Map

import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;class Person {private String name;private int age;// 构造方法和 getter 方法public Person(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public int getAge() {return age;}
}public class ListToMapExample {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Peter", 30), new Person("Anna", 25), new Person("Peter", 28));// 将 List 转换为 MapMap<String, Integer> personMap = people.stream().collect(Collectors.toMap(Person::getName, Person::getAge, (existing, replacement) -> existing));personMap.forEach((name, age) -> System.out.println(name + ": " + age));// 有序写法Map<String, Person> personOrderlyMap = people.stream().collect(Collectors.toMap(Person::getName,a -> a, (oldValue, newValue) -> newValue, LinkedHashMap::new));// value为对象时，也可以这样写，注意ID相同时，会抛异常Map<String, Person> personMapOther = people.stream().collect(Collectors.toMap(Person::getName, person -> person));// 还可以这样写 输出跟输入一样的Lambda表达式对象，等价于person -> personMap<String, Person> personAscMap = people.stream().collect(Collectors.toMap(Person::getName, Function.identity()));// map按key排序 默认正序，反序为： Map.Entry.comparingByKey().reversed()Map<String, Person> personDescMap = personAscMap.entrySet().stream().sorted(Map.Entry.comparingByKey()).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new));}
}

3. 过滤和映射

示例：过滤列表

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class FilterExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 过滤名字以 "P" 开头的元素List<String> filteredNames = names.stream().filter(name -> name.startsWith("P")).collect(Collectors.toList());System.out.println(filteredNames); // 输出：[Peter]}
}

示例：映射列表

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class MapExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 将名字转换为大写List<String> upperCaseNames = names.stream().map(String::toUpperCase).collect(Collectors.toList());System.out.println(upperCaseNames); // 输出：[PETER, ANNA, MIKE, XENIA]}
}

4. 聚合操作

示例：求和

import java.util.Arrays;
import java.util.List;public class SumExample {public static void main(String[] args) {List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);// 使用 Stream API 计算总和int sum = numbers.stream().mapToInt(Integer::intValue).sum();System.out.println("Sum: " + sum); // 输出：Sum: 15}
}

示例：求平均值

import java.util.Arrays;
import java.util.List;
import java.util.OptionalDouble;public class AverageExample {public static void main(String[] args) {List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);// 使用 Stream API 计算平均值OptionalDouble average = numbers.stream().mapToInt(Integer::intValue).average();System.out.println("Average: " + (average.isPresent() ? average.getAsDouble() : "N/A"));}
}

5. 分组和分区

示例：按条件分组

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;public class GroupingByExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 按名字的长度分组Map<Integer, List<String>> groupedByLength = names.stream().collect(Collectors.groupingBy(String::length));groupedByLength.forEach((length, nameList) -> System.out.println(length + ": " + nameList));}
}

示例：按条件分区

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;public class PartitioningByExample {public static void main(String[] args) {List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);// 按是否为偶数分区Map<Boolean, List<Integer>> partitionedByEven = numbers.stream().collect(Collectors.partitioningBy(n -> n % 2 == 0));partitionedByEven.forEach((isEven, numList) -> System.out.println(isEven + ": " + numList));}
}

6. 并行流

示例：使用并行流提高性能

import java.util.Arrays;
import java.util.List;public class ParallelStreamExample {public static void main(String[] args) {List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);// 使用并行流计算总和int sum = numbers.parallelStream().mapToInt(Integer::intValue).sum();System.out.println("Sum: " + sum); // 输出：Sum: 55}
}

7. 去重操作

示例：去除列表中的重复元素

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class DistinctExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Anna", "Xenia", "Peter");// 使用 distinct 去重List<String> uniqueNames = names.stream().distinct().collect(Collectors.toList());System.out.println(uniqueNames); // 输出：[Peter, Anna, Mike, Xenia]// 对象，根据ID去重List<User> duplicateRemoval = zxrMaps.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getId))), ArrayList::new));}
}

8. 收集结果

示例：将列表转换为 Set

import java.util.Arrays;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;public class ToSetExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 将列表转换为 SetSet<String> nameSet = names.stream().collect(Collectors.toSet());System.out.println(nameSet);}
}

示例：连接字符串

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class JoiningExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 连接字符串，使用逗号分隔String joinedNames = names.stream().collect(Collectors.joining(", "));System.out.println(joinedNames); // 输出：Peter, Anna, Mike, Xenia}
}

9. 映射和扁平化

示例：扁平化嵌套列表

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class FlatMapExample {public static void main(String[] args) {List<List<String>> nestedList = Arrays.asList(Arrays.asList("Peter", "Anna"),Arrays.asList("Mike", "Xenia"));// 扁平化嵌套列表List<String> flatList = nestedList.stream().flatMap(List::stream).collect(Collectors.toList());System.out.println(flatList); // 输出：[Peter, Anna, Mike, Xenia]}
}

10. 生成流

示例：生成数值范围

import java.util.stream.IntStream;public class IntStreamRangeExample {public static void main(String[] args) {// 生成 1 到 10 的数值范围IntStream.rangeClosed(1, 10).forEach(System.out::println);}
}

11. 归约操作

示例：使用 reduce 求和

import java.util.Arrays;
import java.util.List;public class ReduceSumExample {public static void main(String[] args) {List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);// 使用 reduce 求和int sum = numbers.stream().reduce(0, Integer::sum);System.out.println("Sum: " + sum); // 输出：Sum: 15}
}

示例：使用 reduce 求乘积

import java.util.Arrays;
import java.util.List;public class ReduceProductExample {public static void main(String[] args) {List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);// 使用 reduce 求乘积int product = numbers.stream().reduce(1, (a, b) -> a * b);System.out.println("Product: " + product); // 输出：Product: 120}
}

12. 排序和查找

示例：排序列表

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class SortExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 排序列表List<String> sortedNames = names.stream().sorted().collect(Collectors.toList());System.out.println(sortedNames); // 输出：[Anna, Mike, Peter, Xenia]}
}

示例：查找第一个匹配元素

import java.util.Arrays;
import java.util.List;
import java.util.Optional;public class FindFirstExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 查找第一个以 "P" 开头的元素Optional<String> firstMatchingName = names.stream().filter(name -> name.startsWith("P")).findFirst();System.out.println("First matching name: " + firstMatchingName.orElse("No match"));}
}

13. 自定义收集器

示例：自定义收集器

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collector;
import java.util.stream.Collectors;public class CustomCollectorExample {public static void main(String[] args) {List<String> names = Arrays.asList("Peter", "Anna", "Mike", "Xenia");// 自定义收集器String result = names.stream().collect(Collector.of(StringBuilder::new,(sb, name) -> sb.append(name).append("|"),StringBuilder::append,StringBuilder::toString));System.out.println(result); // 输出：Peter|Anna|Mike|Xenia|}
}

14. List 并集、差集、交集

// list1
List<String> list1 = Arrays.asList("a", "b", "c");// list2
List<String> list2 = Arrays.asList("b", "c", "d");// 交集 ("b", "c")
List<String> intersection = list1.stream().filter(item -> list2.contains(item)).collect(toList());// 差集 (list1 - list2)
List<String> reduce1 = list1.stream().filter(item -> !list2.contains(item)).collect(toList());// 差集 (list2 - list1)
List<String> reduce2 = list2.stream().filter(item -> !list1.contains(item)).collect(toList());// 并集 (list1 + list2)
List<String> listAll = new ArrayList();
listAll.addAll(list1);
listAll.addAll(list2);
listAll.parallelStream().forEach(System.out :: println);// 并集去重
List<String> listAllDistinct = listAll.stream().distinct().collect(toList());