设计模式之迭代器模式

作用

提供一种遍历集合元素的统一接口，用一致的方法遍历集合元素，不需要知道集合对象的底层表示，不暴露其内部的结构。

原理

Iterator：迭代器接口，hasNext、next、remove。

ConcreteIterator：具体迭代器类。

Aggregate：统一聚合接口，讲客户端与具体聚合解耦。

ConcreteAggregate：具体的聚合持有对象集合，并提供一个方法，返回一个迭代器，该迭代器可以正确遍历集合。

Client：客户端，通过Iterator和Aggregate依赖子类。

案例

Iterator迭代器接口

java.util.Iterator

ConcreteIterator具体迭代器类

public class ComputerCollegeIterator implements Iterator<Department> {

    private Department[] departments;
    private int position = 0;

    public ComputerCollegeIterator(Department[] departments) {
        this.departments = departments;
    }

    @Override
    public boolean hasNext() {
        if (position >= departments.length || departments[position] == null) {
            return false;
        }
        return true;
    }

    @Override
    public Department next() {
        Department department = departments[position];
        position += 1;
        return department;
    }
}

public class InfoCollegeIterator implements Iterator<Department> {

    private List<Department> departments;
    private int index = -1;

    public InfoCollegeIterator(List<Department> departments) {
        this.departments = departments;
    }

    @Override
    public boolean hasNext() {
        if (index >= departments.size() - 1) {
            return false;
        } else {
            index += 1;
            return true;
        }
    }

    @Override
    public Department next() {
        return departments.get(index);
    }
}

Aggregate统一聚合接口

public interface College {

    String getName();

    void addDepartment(String name, String desc);

    Iterator createIterator();

}

ConcreteAggregate具体聚合持有对象集合

public class ComputerCollege implements College {

    private Department[] departments;
    private int numOfDepartment = 0;

    public ComputerCollege() {
        departments = new Department[5];
        addDepartment("Java专业", "Java专业");
        addDepartment("PHP专业", "PHP专业");
        addDepartment("大数据专业", "大数据专业");
    }

    @Override
    public String getName() {
        return "计算机学院";
    }

    @Override
    public void addDepartment(String name, String desc) {
        Department department = new Department();
        department.setName(name);
        department.setDesc(desc);
        departments[numOfDepartment] = department;
        numOfDepartment += 1;
    }

    @Override
    public Iterator createIterator() {
        return new ComputerCollegeIterator(departments);
    }
}

public class InfoCollege implements College {

    private List<Department> departments;

    public InfoCollege() {
        departments = new ArrayList<>();
        addDepartment("信息安全专业", "信息安全专业");
        addDepartment("网络安全专业", "网络安全专业");
        addDepartment("服务器安全专业", "服务器安全专业");
    }

    @Override
    public String getName() {
        return "信息学院";
    }

    @Override
    public void addDepartment(String name, String desc) {
        Department department = new Department();
        department.setName(name);
        department.setDesc(desc);
        departments.add(department);
    }

    @Override
    public Iterator createIterator() {
        return new InfoCollegeIterator(departments);
    }
}

Client通过Iterator和Aggregate依赖子类

public class Client {

    public static void main(String[] args) {
        List<College> colleges = new ArrayList<>();

        ComputerCollege computerCollege = new ComputerCollege();
        InfoCollege infoCollege = new InfoCollege();

        colleges.add(computerCollege);
        colleges.add(infoCollege);

        Iterator<College> iterator = colleges.iterator();
        while (iterator.hasNext()) {
            College college = iterator.next();
            System.out.println("=== " + college.getName() + "=====");
            Iterator collegeIterator = college.createIterator();
            while (collegeIterator.hasNext()) {
                Department d = (Department) collegeIterator.next();
                System.out.println(d.getName());
            }
        }
    }

}

源码

java.util.ArrayList

具体迭代器：java.util.ArrayList.Itr

聚合接口，返回迭代器：java.util.List