设计模式（八）过滤器模式

过滤器也是一种结构型模式，它允许我们使用不同的规则过滤某一组对象。

首先我们来编写一个实体类，用作被过滤的对象。

public class Person {    private int age;    private String name;    private Gender gender;    @Override    public String toString() {        final StringBuilder sb = new StringBuilder("Person{");        sb.append("age=").append(age);        sb.append(", name='").append(name).append('\'');        sb.append(", gender=").append(gender);        sb.append('}');        return sb.toString();    }    public int getAge() {        return age;    }    public void setAge(int age) {        this.age = age;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Gender getGender() {        return gender;    }    public void setGender(Gender gender) {        this.gender = gender;    }    public Person(int age, String name, Gender gender) {        this.age = age;        this.name = name;        this.gender = gender;    }}enum Gender {    Male, Female}

然后来编写过滤器。这里编写了两个过滤器，过滤年龄和性别的。还编写了一个与过滤器，用于同时应用两个过滤器。

public interface Filter {    List<Person> filter(List<Person> people);}class AgeGreaterThan20Filter implements Filter {    @Override    public List<Person> filter(List<Person> people) {        List<Person> result = new ArrayList<>();        for (Person p : people) {            if (p.getAge() > 20) {                result.add(p);            }        }        return result;    }}class FemaleFilter implements Filter {    @Override    public List<Person> filter(List<Person> people) {        List<Person> result = new ArrayList<>();        for (Person p : people) {            if (p.getGender() == Gender.Female) {                result.add(p);            }        }        return result;    }}class AndFilter implements Filter {    private Filter one;    private Filter other;    public AndFilter(Filter one, Filter other) {        this.one = one;        this.other = other;    }    @Override    public List<Person> filter(List<Person> people) {        List<Person> tmp = one.filter(people);        return other.filter(tmp);    }}

之后来测试一下。我们可以看到，由于过滤器和实体类解耦，所以我们可以随便使用过滤器来处理数据。

List<Person> people = new ArrayList<>();people.add(new Person(22, "yitian", Gender.Male));people.add(new Person(23, "zhang3", Gender.Female));people.add(new Person(24, "li4", Gender.Male));people.add(new Person(25, "wang5", Gender.Female));people.add(new Person(13, "zhao6", Gender.Male));people.add(new Person(10, "fuck", Gender.Female));Filter femaleFilter = new FemaleFilter();List<Person> women = femaleFilter.filter(people);System.out.println("女人们:" + women);Filter ageGreaterThan20Filter = new AgeGreaterThan20Filter();List<Person> gt20 = ageGreaterThan20Filter.filter(people);System.out.println("大于20的人:" + gt20);Filter andFilter = new AndFilter(femaleFilter, ageGreaterThan20Filter);System.out.println("既是女性有大于20的人:" + andFilter.filter(people));

如果使用过Java 8的话会发现lambda表达式和流类库会帮我们非常方便的实现过滤器模式的功能。

//lambda表达式List<Person> men = people.stream()        .filter(o -> o.getGender() == Gender.Male)        .filter(o -> o.getAge() >= 23)        .collect(Collectors.toList());System.out.println("大于23的男性:" + men);

通过新的语言特性，我们可以简化设计模式的使用，更快速的开发程序。当然，了解设计模式还是很有必要的。领会设计模式的思想，可以让我们更深刻的理解程序设计的奥秘，对我们学习编程还是很有帮助的。