【设计模式】过滤器模式

过滤器顾名思义，定义一些过滤规则，将符合要求的内容筛选，就比如过滤不同大小或者不同颜色的水果，需要颜色和大小过滤器，筛选条件独立为对象，可以通过灵活组合形成过滤链条。避免大量使用判断语句。

案例代码：筛选不同颜色的苹果

被过滤对象（苹果）

// 苹果对象
class Apple {private String color;private int weight;public Apple(String color, int weight) {this.color = color;this.weight = weight;}public String getColor() { return color; }public int getWeight() { return weight; }
}

过滤器接口

// 过滤器接口
interface Filter {List<Apple> filter(List<Apple> apples);
}

具体过滤器实现

// 颜色过滤器（筛选红色苹果）
class ColorFilter implements Filter {public List<Apple> filter(List<Apple> apples) {List<Apple> result = new ArrayList<>();for (Apple apple : apples) {if ("red".equalsIgnoreCase(apple.getColor())) {result.add(apple);}}return result;}
}// 重量过滤器（筛选重量大于150g的苹果）
class WeightFilter implements Filter {public List<Apple> filter(List<Apple> apples) {List<Apple> result = new ArrayList<>();for (Apple apple : apples) {if (apple.getWeight() > 150) {result.add(apple);}}return result;}
}

组合过滤器（多条件筛选）

// 组合过滤器（同时满足多个条件）
class AndFilter implements Filter {private Filter filter1;private Filter filter2;public AndFilter(Filter filter1, Filter filter2) {this.filter1 = filter1;this.filter2 = filter2;}public List<Apple> filter(List<Apple> apples) {List<Apple> temp = filter1.filter(apples);return filter2.filter(temp);}
}

使用过滤器模式

public class FilterDemo {public static void main(String[] args) {List<Apple> apples = Arrays.asList(new Apple("Red", 200),new Apple("Green", 160),new Apple("Red", 140));// 单条件筛选：红色苹果Filter colorFilter = new ColorFilter();List<Apple> redApples = colorFilter.filter(apples);System.out.println("红色苹果数量：" + redApples.size()); // 输出：2// 组合筛选：红色且重量>150gFilter weightFilter = new WeightFilter();Filter andFilter = new AndFilter(colorFilter, weightFilter);List<Apple> result = andFilter.filter(apples);System.out.println("符合条件的苹果数量：" + result.size()); // 输出：1}
}

应用场景案例：用户权限过滤
场景描述
筛选出同时满足以下条件的用户：

年龄在18岁以上
所在城市为“北京”
注册时间在2023年以后
代码实现

// 用户对象
class User {private String name;private int age;private String city;private LocalDate registerDate;public User(String name, int age, String city, LocalDate registerDate) {this.name = name;this.age = age;this.city = city;this.registerDate = registerDate;}// Getter方法省略...
}// 年龄过滤器
class AgeFilter implements Filter {private int minAge;public AgeFilter(int minAge) {this.minAge = minAge;}public List<User> filter(List<User> users) {List<User> result = new ArrayList<>();for (User user : users) {if (user.getAge() >= minAge) {result.add(user);}}return result;}
}// 城市过滤器
class CityFilter implements Filter {private String city;public CityFilter(String city) {this.city = city;}public List<User> filter(List<User> users) {List<User> result = new ArrayList<>();for (User user : users) {if (city.equalsIgnoreCase(user.getCity())) {result.add(user);}}return result;}
}// 注册时间过滤器
class DateFilter implements Filter {private LocalDate startDate;public DateFilter(LocalDate startDate) {this.startDate = startDate;}public List<User> filter(List<User> users) {List<User> result = new ArrayList<>();for (User user : users) {if (user.getRegisterDate().isAfter(startDate)) {result.add(user);}}return result;}
}// 使用示例
public class UserFilterDemo {public static void main(String[] args) {List<User> users = Arrays.asList(new User("张三", 25, "北京", LocalDate.of(2024, 1, 1)),new User("李四", 17, "上海", LocalDate.of(2024, 2, 1)),new User("王五", 30, "北京", LocalDate.of(2022, 5, 1)));// 创建过滤器Filter ageFilter = new AgeFilter(18);Filter cityFilter = new CityFilter("北京");Filter dateFilter = new DateFilter(LocalDate.of(2023, 1, 1));// 组合过滤：年龄>=18 && 城市=北京 && 注册时间>2023Filter combinedFilter = new AndFilter(ageFilter, new AndFilter(cityFilter, dateFilter));List<User> validUsers = combinedFilter.filter(users);System.out.println("有效用户数量：" + validUsers.size()); // 输出：1}
}