设计模式--迭代器模式

设计模式--迭代器模式

1 概述

由于迭代器模式实在太常见了，所以自己就简单写一下就好。

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1.1 定义
迭代器模式(Iterator Design)提供一种方法顺序访问容器中的各个元素，而又不暴露其内部具体的实现。

1.2 应用
JDK中的java.util包所有Collection集合都实现了Iterable接口，该接口有一个方法返回Iterator对象。
JDK中foreach语法内部就是迭代器原理。

2 详解

JDK中Iterable接口，该接口会返回一个Iterator对象。

public interface Iterable<T> {
    Iterator<T> iterator();
}

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());
    }
}

JDK中Collection接口

public interface Collection<E> extends Iterable<E> {
    Iterator<E> iterator();
}

JDK中ArrayList类

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    
    @Override 
    public Iterator<E> iterator() {
        return new Itr();
    }
    
    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;

        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();
        }
    }

}