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1.模式定义:
2.模式特点:
聚集和JAVA聚集
3.使用场景:
4.模式实现:
白箱聚集与外禀迭代
(1)抽象迭代(Iterator)角色:
public interface Iterator { /** * 迭代方法:移动到第一个元素 */ public void first(); /** * 迭代方法:移动到下一个元素 */ public void next(); /** * 迭代方法:是否为最后一个元素 */ public boolean isDone(); /** * 迭代方法:返还当前元素 */ public Object currentItem(); }
(2)具体迭代(ConcreteIterator)角色:
public class ConcreteIterator implements Iterator { //持有被迭代的具体的聚合对象 private ConcreteAggregate agg; //内部索引,记录当前迭代到的索引位置 private int index = 0; //记录当前聚集对象的大小 private int size = 0; public ConcreteIterator(ConcreteAggregate agg){ this.agg = agg; this.size = agg.size(); index = 0; } /** * 迭代方法:返还当前元素 */ @Override public Object currentItem() { return agg.getElement(index); } /** * 迭代方法:移动到第一个元素 */ @Override public void first() { index = 0; } /** * 迭代方法:是否为最后一个元素 */ @Override public boolean isDone() { return (index >= size); } /** * 迭代方法:移动到下一个元素 */ @Override public void next() { if(index < size) { index ++; } } }
(3)聚集(Aggregate)角色:
public abstract class Aggregate { /** * 工厂方法,创建相应迭代对象的接口 */ public abstract Iterator createIterator(); }
(4)具体聚集(ConcreteAggregate)角色:
public class ConcreteAggregate extends Aggregate { private Object[] objArray = null; /** * 构造方法,传入聚合对象的具体内容 */ public ConcreteAggregate(Object[] objArray){ this.objArray = objArray; } @Override public Iterator createIterator() { return new ConcreteIterator(this); } /** * 取值方法:向外界提供聚集元素 */ public Object getElement(int index){ if(index < objArray.length){ return objArray[index]; }else{ return null; } } /** * 取值方法:向外界提供聚集的大小 */ public int size(){ return objArray.length; } }
(5)客户端(Client)角色:
public class Client { public void operation(){ Object[] objArray = {"One","Two","Three","Four","Five","Six"}; //创建聚合对象 Aggregate agg = new ConcreteAggregate(objArray); //循环输出聚合对象中的值 Iterator it = agg.createIterator(); while(!it.isDone()){ System.out.println(it.currentItem()); it.next(); } } public static void main(String[] args) { Client client = new Client(); client.operation(); } }
外禀迭代的意义
[1]抽象聚集角色类
public abstract class Aggregate { /** * 工厂方法,创建相应迭代对象的接口 */ public abstract Iterator createIterator(); }
[2]抽象迭代角色类
public interface Iterator { /** * 迭代方法:移动到第一个元素 */ public void first(); /** * 迭代方法:移动到下一个元素 */ public void next(); /** * 迭代方法:是否为最后一个元素 */ public boolean isDone(); /** * 迭代方法:返还当前元素 */ public Object currentItem(); }
[3]具体聚集角色类
public class ConcreteAggregate extends Aggregate { private Object[] objArray = null; /** * 构造方法,传入聚合对象的具体内容 */ public ConcreteAggregate(Object[] objArray){ this.objArray = objArray; } @Override public Iterator createIterator() { return new ConcreteIterator(); } /** * 内部成员类,具体迭代类 */ private class ConcreteIterator implements Iterator { //内部索引,记录当前迭代到的索引位置 private int index = 0; //记录当前聚集对象的大小 private int size = 0; /** * 构造函数 */ public ConcreteIterator(){ this.size = objArray.length; index = 0; } /** * 迭代方法:返还当前元素 */ @Override public Object currentItem() { return objArray[index]; } /** * 迭代方法:移动到第一个元素 */ @Override public void first() { index = 0; } /** * 迭代方法:是否为最后一个元素 */ @Override public boolean isDone() { return (index >= size); } /** * 迭代方法:移动到下一个元素 */ @Override public void next() { if(index < size) { index ++; } } } }
[4]客户端类
public class Client { public void operation(){ Object[] objArray = {"One","Two","Three","Four","Five","Six"}; //创建聚合对象 Aggregate agg = new ConcreteAggregate(objArray); //循环输出聚合对象中的值 Iterator it = agg.createIterator(); while(!it.isDone()){ System.out.println(it.currentItem()); it.next(); } } public static void main(String[] args) { Client client = new Client(); client.operation(); } }
主动迭代和被动迭代
Fail Fast
Fail Fast在JAVA聚集中的使用
private class Itr implements Iterator<E> { /** * Index of element to be returned by subsequent call to next. */ int cursor = 0; /** * Index of element returned by most recent call to next or * previous. Reset to -1 if this element is deleted by a call * to remove. */ int lastRet = -1; /** * The modCount value that the iterator believes that the backing * List should have. If this expectation is violated, the iterator * has detected concurrent modification. */ int expectedModCount = modCount; public boolean hasNext() { return cursor != size(); } public E next() { checkForComodification(); try { E next = get(cursor); lastRet = cursor++; return next; } catch (IndexOutOfBoundsException e) { checkForComodification(); throw new NoSuchElementException(); } } public void remove() { if (lastRet == -1) throw new IllegalStateException(); checkForComodification(); try { AbstractList.this.remove(lastRet); if (lastRet < cursor) cursor--; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException e) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }