工厂模式案例与理解

工厂模式适用的场景：

1.用户需要一个类的子类的实例，但不希望该类与子类形成耦合。

2.用户需要一个类的子类的实例，单用户不知道该类有哪些子类可用。

设计的核心思想是把类的实例化延迟到子类。

案例1 ：java.util中的Iterator类的设计。

java中Collection接口继承了Iterable接口，Iterable接口中定义了iterator()方法，所有实现了Collection接口的类都可以继承该方法。

我们来看一下代码中具体是如何设计的。

我们一般这样来获得Iterator：

List<String> testArrayList = new ArrayList<String>();
Iterator arrayListIterator = testArrayList.iterator();

ArrayList类实现了Iterator()方法：

/**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * An optimized version of AbstractList.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();
            }
        }

ArrayList的iterator返回一个实现了Iterator接口的内部类，该内部类实现了具体的ArrayList的Iterator的方法细节。

再看下LinkedList是如何实现Iterator的：

/**
     * Returns an iterator over the elements in this list (in proper
     * sequence).<p>
     *
     * This implementation merely returns a list iterator over the list.
     *
     * @return an iterator over the elements in this list (in proper sequence)
     */
    public Iterator<E> iterator() {
        return listIterator();
    }

/**
     * {@inheritDoc}
     *
     * <p>This implementation returns {@code listIterator(0)}.
     *
     * @see #listIterator(int)
     */
    public ListIterator<E> listIterator() {
        return listIterator(0);
    }

/**
     * {@inheritDoc}
     *
     * <p>This implementation returns a straightforward implementation of the
     * {@code ListIterator} interface that extends the implementation of the
     * {@code Iterator} interface returned by the {@code iterator()} method.
     * The {@code ListIterator} implementation relies on the backing list's
     * {@code get(int)}, {@code set(int, E)}, {@code add(int, E)}
     * and {@code remove(int)} methods.
     *
     * <p>Note that the list iterator returned by this implementation will
     * throw an {@link UnsupportedOperationException} in response to its
     * {@code remove}, {@code set} and {@code add} methods unless the
     * list's {@code remove(int)}, {@code set(int, E)}, and
     * {@code add(int, E)} methods are overridden.
     *
     * <p>This implementation can be made to throw runtime exceptions in the
     * face of concurrent modification, as described in the specification for
     * the (protected) {@link #modCount} field.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public ListIterator<E> listIterator(final int index) {
        rangeCheckForAdd(index);

        return new ListItr(index);
    }

private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public E previous() {
            checkForComodification();
            try {
                int i = cursor - 1;
                E previous = get(i);
                lastRet = cursor = i;
                return previous;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.set(lastRet, e);
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                AbstractList.this.add(i, e);
                lastRet = -1;
                cursor = i + 1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

由于ArrayList和LinkedList的底层实现一个使用动态数组、一个使用链表，所以Iterator的实现细节必然不同。
但是这种不同并没有反映在我们的使当中，对于这两种不同我们并没有实用类似

//以下代码是我瞎编的，并不存在这两个类
ArrayLiArrayListIterator arrayListIterator  = new ArrayLiArrayListIterator ();
LinkedListIterator linkedListIterator = new LinkedListIterator();

的方法获得实例，而是直接用List类就能获得实例。