定义与类型

定义：定义一个创建对象的接口，但让实现这个接口的类来决定实例化哪个类。工厂方法让类的实例化推迟到子类中进行。
类型：创建型

适用场景

• 创建对象需要大量重复代码
• 客户端（应用层）不依赖于产品类实例如何被创建、实现等细节
• 一个类通过其子类来制定创建哪个对象

优点

• 用户只需要关心所需产品对应的工厂，无须关心创建细节
• 加入新产品符合开闭原则，提高可扩展性

缺点

• 类的个数容易过多，增加复杂度
• 增加了系统的抽象性和理解难度

代码示例

以之前的简单工厂，不同的课程视频为例

1. 创建VideoFactory类

/**
 * @author lijiayin
 */
public abstract class VideoFactory {
    public abstract Video getVideo();
}

2. 创建Video类

/**
 * @author lijiayin
 */
public abstract class Video {
    
    public abstract void produce();
}

3. 创建JavaVideo类

/**
 * @author lijiayin
 */
public class JavaVideo extends Video {
    @Override
    public void produce() {
        System.out.println("Java");
    }
}

4. 创建PythonVideo类

/**
 * @author lijiayin
 */
public class PythonVideo extends Video {
    @Override
    public void produce() {
        System.out.println("Python");
    }
}

5. 创建JavaVideoFactory类

/**
 * @author lijiayin
 */
public class JavaVideoFactory extends VideoFactory {
    @Override
    public Video getVideo() {
        return new JavaVideo();
    }
}

6. 创建PythonVideoFactory类

/**
 * @author lijiayin
 */
public class PythonVideoFactory extends VideoFactory {
    @Override
    public Video getVideo() {
        return new PythonVideo();
    }
}

7. 创建测试类

/**
 * @author lijiayin
 */
public class Test {
    public static void main(String[] args) {
        VideoFactory videoFactory = new JavaVideoFactory();
        Video video = videoFactory.getVideo();
        video.produce();
        VideoFactory videoFactory2 = new PythonVideoFactory();
        Video video2 = videoFactory2.getVideo();
        video2.produce();
    }
}

8. 测试结果

   
   测试结果.png

9. UML图

   
   UML.png

JDK示例

• Collection接口，相当于VideoFactory抽象类，定义了Iterator<E> iterator()方法，相当于Video getVideo()方法；

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

• ArrayList类，相当于JavaVideoFactory类，实现了具体的iterator()方法；

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

• Iterator接口，相当于Video抽象类，定义了需要实现的产品；

package java.util;

import java.util.function.Consumer;

/**
 * An iterator over a collection.  {@code Iterator} takes the place of
 * {@link Enumeration} in the Java Collections Framework.  Iterators
 * differ from enumerations in two ways:
 *
 * <ul>
 *      <li> Iterators allow the caller to remove elements from the
 *           underlying collection during the iteration with well-defined
 *           semantics.
 *      <li> Method names have been improved.
 * </ul>
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements returned by this iterator
 *
 * @author  Josh Bloch
 * @see Collection
 * @see ListIterator
 * @see Iterable
 * @since 1.2
 */
public interface Iterator<E> {
    /**
     * Returns {@code true} if the iteration has more elements.
     * (In other words, returns {@code true} if {@link #next} would
     * return an element rather than throwing an exception.)
     *
     * @return {@code true} if the iteration has more elements
     */
    boolean hasNext();

    /**
     * Returns the next element in the iteration.
     *
     * @return the next element in the iteration
     * @throws NoSuchElementException if the iteration has no more elements
     */
    E next();

    /**
     * Removes from the underlying collection the last element returned
     * by this iterator (optional operation).  This method can be called
     * only once per call to {@link #next}.  The behavior of an iterator
     * is unspecified if the underlying collection is modified while the
     * iteration is in progress in any way other than by calling this
     * method.
     *
     * @implSpec
     * The default implementation throws an instance of
     * {@link UnsupportedOperationException} and performs no other action.
     *
     * @throws UnsupportedOperationException if the {@code remove}
     *         operation is not supported by this iterator
     *
     * @throws IllegalStateException if the {@code next} method has not
     *         yet been called, or the {@code remove} method has already
     *         been called after the last call to the {@code next}
     *         method
     */
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }

    /**
     * Performs the given action for each remaining element until all elements
     * have been processed or the action throws an exception.  Actions are
     * performed in the order of iteration, if that order is specified.
     * Exceptions thrown by the action are relayed to the caller.
     *
     * @implSpec
     * <p>The default implementation behaves as if:
     * <pre>{@code
     *     while (hasNext())
     *         action.accept(next());
     * }</pre>
     *
     * @param action The action to be performed for each element
     * @throws NullPointerException if the specified action is null
     * @since 1.8
     */
    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

• ArrayList的内部类Itr，相当于JavaVideo类，具体实现了产品；

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    ......
    /**
     * 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;

        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();
        }
    }
    ......
}