Iterator pattern of behavioral pattern

Iterator pattern

Why the iterator pattern appears ？

In programming , We often need to access various elements in the aggregate object , For example, the traversal of linked lists , Our usual practice is to put the creation and traversal of the linked list in a class , But this approach is not conducive to expansion , Modifying the traversal algorithm requires modifying the source code , It violates the open close principle . But if you let users implement traversal by themselves , It will not only expose the internal representation of the aggregate object , It will also increase the burden on users , To solve the above problems , There is Iterator pattern . The iterator pattern refers to inserting an iterator between the client access class and the aggregate class , This separates the aggregate object from its traversal , At the same time, it will not expose the internal details of the aggregate object .

What is iterator mode ？

Definition ： Provides an object to sequentially access a series of data in an aggregate object , Without exposing the internal representation of the aggregate object . The iterator pattern is an object behavior pattern .
advantage ：

1. You can access the content of an aggregate object without exposing its internal representation .
2. Support different traversal methods , Subclasses of iterators can be customized to implement new traversal methods .
3. Observe the principle of single responsibility and the principle of opening and closing .
4. Good encapsulation , It provides a unified interface for traversing different aggregation structures .
5. There is little need for custom iterators in development , Open source framework provides API Generally enough .

The structure and implementation of patterns

The iterator pattern separates the traversal behavior of aggregate objects , Abstract to iterator class , The purpose is to... Without exposing the internal structure of the aggregate object , Let external code transparently access aggregated internal data .

structure

role

1. Abstract aggregation （Aggregate） role ： Define storage 、 add to 、 Delete aggregate objects and create interfaces for iterator objects .
2. Concrete aggregation （ConcreteAggregate） role ： Implement Abstract aggregation classes , Returns an instance of a concrete iterator .
3. Abstract iterator （Iterator） role ： Define interfaces to access and traverse aggregate elements , Usually contains hasNext()、first()、next() Other methods .
4. Specific iterators （Concretelterator） role ： Implement the methods defined in the abstract iterator interface , Complete the traversal of the aggregate object , Record the current position of the traversal .
   

Realization

Use the iterator to traverse the students in the classroom .

package com.pattern.Iterator;

/** * @author wlj * @Classname Aggregate * @Description  Aggregate interface  * @Date 7/25/2022 10:50 AM */
public interface Aggregate {
    
    //  Add aggregate objects 
    void addAggregate(Object obj);

    //  Delete aggregate object 
    void deleteAggregate(Object obj);

    //  Acquisition iterator 
    Iterator getIterator();
}
package com.pattern.Iterator;

import java.util.ArrayList;

/** * @author wlj * @Classname ConcreteAggregate * @Description  Concrete aggregate objects (ConcreteAggregate) * @Date 7/25/2022 10:58 AM */
public class ConcreteAggregate implements Aggregate{
    
    ArrayList<Object> arrayList = new ArrayList<Object>();

    @Override
    public void addAggregate(Object obj) {
    
        arrayList.add(obj);
    }

    @Override
    public void deleteAggregate(Object obj) {
    
        arrayList.remove(obj);
    }


    @Override
    public Iterator getIterator() {
    
        return new ConcreteIterator(arrayList);
    }
}
package com.pattern.Iterator;

/** * @author wlj * @Classname Iterator * @Description  Iterator interface  * @Date 7/25/2022 10:55 AM */
public interface Iterator {
    
    // first
    Object first();
    // next
    Object next();
    // hasNext
    boolean hasNext();
}
package com.pattern.Iterator;

import java.util.ArrayList;

/** * @author wlj * @Classname ClassroomIterator * @Description  Specific iterators  * @Date 7/25/2022 11:04 AM */
public class ConcreteIterator implements Iterator{
    
    private ArrayList<Object> list = null;
    private int index = -1;

    public ConcreteIterator(ArrayList<Object> list) {
    
        this.list = list;
    }

    @Override
    public Object first() {
    
        index = 0;
        Object object = list.get(index);
        return object;
    }

    @Override
    public Object next() {
    
        Object object = null;
        if(this.hasNext()){
    
            object  = list.get(++index);
        }
        return object;
    }

    @Override
    public boolean hasNext() {
    
        if (index < list.size() - 1) {
    
            return true;
        } else {
    
            return false;
        }
    }
}
package com.pattern.Iterator;

/** * @author wlj * @Classname Student * @Description  Aggregate target students  * @Date 7/25/2022 10:58 AM */
public class Student {
    
    // student name
    private String name;

    public Student(String name) {
    
        this.name = name;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    @Override
    public String toString() {
    
        return "Student{" +
                "name='" + name + '\'' +
                '}';
    }
}
package com.pattern.Iterator;

import java.util.ArrayList;

/** * @author wlj * @Classname Client * @Description  test  * @Date 7/25/2022 11:20 AM */
public class Client {
    
    public static void main(String[] args) {
    
        Aggregate classroom = new ConcreteAggregate();
        classroom.addAggregate(new Student(" Xiao Ming "));
        classroom.addAggregate(new Student(" Xiaohong "));
        classroom.addAggregate(new Student(" floret "));
        Iterator it = classroom.getIterator();
        while (it.hasNext()) {
    
            // index The default is -1, The first element is index=0
            Object obj = it.next();
            System.out.println(obj.toString());
        }
        Student first = (Student) it.first();
        System.out.println(first.toString());
    }
}
//  Output 
Student{
    name=' Xiao Ming '}
Student{
    name=' Xiaohong '}
Student{
    name=' floret '}
Student{
    name=' Xiao Ming '}

application

1. Java Medium Collection、List、Set、Map And so on all contain iterators .jdk Iterators have been provided , We generally don't need to realize .