Java Stream流中的filter()使用方法举例详解

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);

List<Integer> evenNumbers = numbers.stream()
    .filter(n -> n % 2 == 0)  // 保留偶数
    .collect(Collectors.toList());  // 终端操作，将结果收集为 List

System.out.println(evenNumbers);  // 输出: [2, 4, 6]

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

List<Integer> greaterThanThree = numbers.stream()
    .filter(n -> n > 3)  // 保留大于 3 的数字
    .collect(Collectors.toList());

System.out.println(greaterThanThree);  // 输出: [4, 5]

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

List<Integer> filteredNumbers = numbers.stream()
    .filter(n -> n > 3)    // 第一个条件: 大于 3
    .filter(n -> n < 8)    // 第二个条件: 小于 8
    .collect(Collectors.toList());

System.out.println(filteredNumbers);  // 输出: [4, 5, 6, 7]

List<Integer> numbers = Arrays.asList(1, 2, 3);

List<Integer> result = numbers.stream()
    .filter(n -> n > 10)  // 没有元素满足条件
    .collect(Collectors.toList());

System.out.println(result);  // 输出: []

Stream<Integer> stream = Stream.of(1, 2, 3, 4)
    .filter(n -> {
        System.out.println("Filtering: " + n);
        return n % 2 == 0;
    });

System.out.println("Before terminal operation");

// 终端操作，触发 filter 执行
List<Integer> evenNumbers = stream.collect(Collectors.toList());

System.out.println("After terminal operation: " + evenNumbers);

List<String> words = Arrays.asList("apple", "banana", "cherry", "date");

List<String> result = words.stream()
    .filter(word -> word.length() > 5)  // 筛选长度大于 5 的单词
    .map(String::toUpperCase)           // 将符合条件的单词转换为大写
    .collect(Collectors.toList());

System.out.println(result);  // 输出: [BANANA, CHERRY]

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);

List<Integer> result = numbers.stream()
    .filter(n -> n > 2 && n % 2 == 0)  // 组合条件: 大于 2 且为偶数
    .collect(Collectors.toList());

System.out.println(result);  // 输出: [4, 6]

Predicate<Integer> greaterThanTwo = n -> n > 2;
Predicate<Integer> isEven = n -> n % 2 == 0;

List<Integer> result = numbers.stream()
    .filter(greaterThanTwo.and(isEven))  // 使用 Predicate 的 and() 方法
    .collect(Collectors.toList());

System.out.println(result);  // 输出: [4, 6]

class Person {
    String name;
    int age;

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

    @Override
    public String toString() {
        return name + " (" + age + ")";
    }
}

List<Person> people = Arrays.asList(
    new Person("Alice", 22),
    new Person("Bob", 17),
    new Person("Charlie", 20)
);

List<Person> adults = people.stream()
    .filter(person -> person.age > 18)  // 筛选年龄大于 18 的人
    .collect(Collectors.toList());

System.out.println(adults);  // 输出: [Alice (22), Charlie (20)]