Java List过滤 Stream API filter() 应用

Java 8 前用 for-each 循环或 Iterator 处理集合，引入 Stream API 后可更简洁、声明式地处理集合，在复杂数据处理时更便捷
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1. Stream与Filter基础

Stream 是从支持数据源获取数据的序列，有强大 API 可执行中间和最终操作，能内部并行化提升大规模数据处理性能，基于函数式编程逻辑清晰，可利用并行计算提升大数据处理速度，惰性求值避免一次性加载整个集合

filter 是 Stream 的中间操作，接受谓词函数参数，返回新 Stream，包含满足条件元素，如以下代码展示如何用 filter 从 words 列表中筛选以“c”开头的单词。

java">List<String> words = Arrays.asList("apple", "banana", "cherry", "date", "elderberry");
List<String> filteredWords = words.stream().filter(word -> word.startsWith("c")).collect(Collectors.toList());

2. 多条件筛选

Stream API 可链式调用多个 filter 等中间操作实现多条件筛选，此示例展示了根据年龄和性别条件过滤 30 岁以下女性对象。

java">List<Person> people = ... // 假设Person类有age和gender属性
List<Person> youngFemales = people.stream().filter(p -> p.getAge() < 30).filter(p -> "female".equals(p.getGender())).collect(Collectors.toList());

3. 实战示例

假设我们有个Employee类，有name（姓名）、age（年龄）、salary（薪水）属性，有个员工列表，要筛选出年龄大于 30 岁且薪水超 50000 的员工。先定义Employee类

java">public class Employee {private String name;private int age;private double salary;public Employee(String name, int age, double salary) {this.name = name;this.age = age;this.salary = salary;}public String getName() {return name;}public int getAge() {return age;}public double getSalary() {return salary;}@Overridepublic String toString() {return "Employee{" +"name='" + name + '\'' +", age=" + age +", salary=" + salary +'}';}
}

然后创建员工列表，用 Stream API 过滤数据

java">import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class StreamFilterExample {public static void main(String[] args) {List<Employee> employees = Arrays.asList(new Employee("Alice", 32, 60000),new Employee("Bob", 28, 55000),new Employee("Charlie", 35, 70000),new Employee("David", 26, 45000),new Employee("Eve", 30, 55000));// 使用Stream API过滤数据List<Employee> filteredEmployees = employees.stream().filter(employee -> employee.getAge() > 30).filter(employee -> employee.getSalary() > 50000).collect(Collectors.toList());// 输出结果System.out.println("Filtered Employees: ");filteredEmployees.forEach(System.out::println);}
}

在代码中，先调用employees.stream()将列表转为 Stream，经两次链式调用.filter()按年龄和薪水条件筛选，再用.collect(Collectors.toList())将满足条件的员工对象收集到新列表，运行后输出年龄大于 30 岁且薪水超过 50000 的员工信息

4. 应用场景总结

数据清洗：筛选符合条件的数据。
报表统计：快速汇总特定条件统计数据。
业务逻辑处理：简化复杂业务场景循环和判断。
数据库查询结果处理：对接查询结果后过滤转换数据。

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