迭代器模式在开源代码中的应用

逃离我推掉我的手 2022-12-05 10:25 150阅读 0赞

迭代器模式的作用：提供一种方法来顺序访问聚合对象中的一系列数据，而不暴露聚合对象的内部表示。

### **案例** ###

迭代器模式最经典的应用就是 JDK 中工具类 Iterator 接口的设计，定义了对集合的访问方法。

public interface Iterator<E> {
    
    	boolean hasNext();
    
    	E next();
    
    	default void remove() {
    		throw new UnsupportedOperationException("remove");
    	}
    
    	default void forEachRemaining(Consumer<? super E> action) {
    		Objects.requireNonNull(action);
    		while (hasNext())
    			action.accept(next());
    	}
    }

每种集合中都有 iterator() 方法，返回各自的迭代器，迭代器按照各自算法实现了 Iterator 接口，满足遍历需求。

如 ArrayLis 中

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    
    	public Iterator<E> iterator() {
    		return new Itr();
    	}
    
    	//ArrayList 迭代器的内部实现类
    	private class Itr implements Iterator<E> {
    		int cursor;	   // index of next element to return
    		int lastRet = -1; // index of last element returned; -1 if no such
    		int expectedModCount = modCount;
    
    		Itr() {}
    
    		public boolean hasNext() {
    			return cursor != size;
    		}
    
    		@SuppressWarnings("unchecked")
    		public E next() {
    			checkForComodification();
    			int i = cursor;
    			if (i >= size)
    				throw new NoSuchElementException();
    			Object[] elementData = ArrayList.this.elementData;
    			if (i >= elementData.length)
    				throw new ConcurrentModificationException();
    			cursor = i + 1;
    			return (E) elementData[lastRet = i];
    		}
    
    		public void remove() {
    			if (lastRet < 0)
    				throw new IllegalStateException();
    			checkForComodification();
    
    			try {
    				ArrayList.this.remove(lastRet);
    				cursor = lastRet;
    				lastRet = -1;
    				expectedModCount = modCount;
    			} catch (IndexOutOfBoundsException ex) {
    				throw new ConcurrentModificationException();
    			}
    		}
    
    		@Override
    		@SuppressWarnings("unchecked")
    		public void forEachRemaining(Consumer<? super E> consumer) {
    			Objects.requireNonNull(consumer);
    			final int size = ArrayList.this.size;
    			int i = cursor;
    			if (i >= size) {
    				return;
    			}
    			final Object[] elementData = ArrayList.this.elementData;
    			if (i >= elementData.length) {
    				throw new ConcurrentModificationException();
    			}
    			while (i != size && modCount == expectedModCount) {
    				consumer.accept((E) elementData[i++]);
    			}
    			// update once at end of iteration to reduce heap write traffic
    			cursor = i;
    			lastRet = i - 1;
    			checkForComodification();
    		}
    
    		final void checkForComodification() {
    			if (modCount != expectedModCount)
    				throw new ConcurrentModificationException();
    		}
    	}
    
    }

HashSet 中

public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable {
    	
    	// HashSet 是基于 HashMap 实现，直接返回 key 的 Set 的 Iterator
    	public Iterator<E> iterator() {
    		return map.keySet().iterator();
    	}
    	
    }

队列 PriorityQueue 中

public class PriorityQueue<E> extends AbstractQueue<E> implements java.io.Serializable {
    	public Iterator<E> iterator() {
    		return new Itr();
    	}
    	
    	//内部对 Iterator 接口进行实现，迭代 PriorityQueue
    	private final class Itr implements Iterator<E> {
    		private int cursor = 0;
    
    		private int lastRet = -1;
    
    		private ArrayDeque<E> forgetMeNot = null;
    
    		private E lastRetElt = null;
    
    		private int expectedModCount = modCount;
    
    		public boolean hasNext() {
    			return cursor < size ||
    				(forgetMeNot != null && !forgetMeNot.isEmpty());
    		}
    
    		@SuppressWarnings("unchecked")
    		public E next() {
    			if (expectedModCount != modCount)
    				throw new ConcurrentModificationException();
    			if (cursor < size)
    				return (E) queue[lastRet = cursor++];
    			if (forgetMeNot != null) {
    				lastRet = -1;
    				lastRetElt = forgetMeNot.poll();
    				if (lastRetElt != null)
    					return lastRetElt;
    			}
    			throw new NoSuchElementException();
    		}
    
    		public void remove() {
    			if (expectedModCount != modCount)
    				throw new ConcurrentModificationException();
    			if (lastRet != -1) {
    				E moved = PriorityQueue.this.removeAt(lastRet);
    				lastRet = -1;
    				if (moved == null)
    					cursor--;
    				else {
    					if (forgetMeNot == null)
    						forgetMeNot = new ArrayDeque<>();
    					forgetMeNot.add(moved);
    				}
    			} else if (lastRetElt != null) {
    				PriorityQueue.this.removeEq(lastRetElt);
    				lastRetElt = null;
    			} else {
    				throw new IllegalStateException();
    			}
    			expectedModCount = modCount;
    		}
    	}
    }

--------------------

### 【Java学习资源】整理推荐 ###

*  [迭代器模式在开源代码中的应用][Link 1]
 *  [中介者模式的实际应用][Link 2]
 *  [观察者模式在开源代码中的应用][Link 3]
 *  [职责链模式在开源代码中的应用][Link 4]
 *  [命令模式在开源代码中的应用][Link 5]
 *  [策略模式在开源代码中应用][Link 6]
 *  [模板方法模式在开源代码中应用][Link 7]
 *  [组合模式在开源代码中的应用][Link 8]
 *  [享元模式在开源代码中的应用][Link 9]
 *  [外观模式在开源代码中的应用][Link 10]
 *  [装饰器模式在开源代码中的应用][Link 11]
 *  [桥接模式在开源代码中的应用][Link 12]
 *  [适配器模式在开源代码中的应用][Link 13]
 *  [代理模式在开源代码中的应用][Link 14]
 *  [原型模式在开源代码中的应用][Link 15]
 *  [建造者模式在开源代码中的应用][Link 16]
 *  [工厂模式在开源代码中的应用][Link 17]
 *  [单例模式在开源代码中的应用][Link 18]
 *  [编码规范][Link 19]
 *  [设计模式][Link 20]
 *  [重构][Link 21]
 *  [设计原则][Link 22]
 *  [面向对象到底是什么][Link 23]
 *  [代码质量有哪些评判标准？][Link 24]

--------------------

### 【Java面试题与答案】整理推荐 ###

*  [基础与语法][Link 25]
 *  [集合][Link 26]
 *  [网络编程][Link 27]
 *  [并发编程][Link 28]
 *  [Web][]
 *  [安全][Link 29]
 *  [设计模式][Link 30]
 *  [框架][Link 31]
 *  [算法与数据结构][Link 32]
 *  [异常][Link 33]
 *  [文件解析与生成][Link 34]
 *  [Linux][]
 *  [MySQL][]
 *  [Oracle][]
 *  [Redis][]
 *  [Dubbo][]

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