工厂方法

定义

定义一个用于创建对象的接口，让子类决定实例化哪个类，工厂方法让类的实例化推迟到了子类

类型

创建型

适用场景

• 创建对象需要大量重复的代码
• 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指定创建哪个对象

优点

• 用户只需要关心所需产品对应的工厂，无需关心创建细节
• 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，增加了复杂度
• 增加了系统的抽象性和理解难度

示例代码

public abstract class Video {
    public abstract void produce();

}

public class FEVideo extends Video{
    @Override
    public void produce() {
        System.out.println("录制FE课程视频");
    }
}

public class JavaVideo extends Video {
    @Override
    public void produce() {
        System.out.println("录制Java课程视频");
    }
}

public class PythonVideo extends Video {
    @Override
    public void produce() {
        System.out.println("录制Python课程视频");
    }
}

public abstract class VideoFactory {

    public abstract Video getVideo();

//    public Video getVideo(Class c){
//        Video video = null;
//        try {
//            video = (Video) Class.forName(c.getName()).newInstance();
//        } catch (InstantiationException e) {
//            e.printStackTrace();
//        } catch (IllegalAccessException e) {
//            e.printStackTrace();
//        } catch (ClassNotFoundException e) {
//            e.printStackTrace();
//        }
//        return video;
//    }
//
//
//    public Video getVideo(String type){
//        if("java".equalsIgnoreCase(type)){
//            return new JavaVideo();
//        }else if("python".equalsIgnoreCase(type)){
//            return new PythonVideo();
//        }
//        return null;
//    }

}

public class FEVideoFactory extends VideoFactory{
    @Override
    public Video getVideo() {
        return new FEVideo();
    }
}

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

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

public class Test {
    public static void main(String[] args) {
        VideoFactory videoFactory = new PythonVideoFactory();
        VideoFactory videoFactory2 = new JavaVideoFactory();
        VideoFactory videoFactory3 = new FEVideoFactory();
        Video video = videoFactory.getVideo();
        video.produce();

    }

}

UML

应用场景

Collection和ArrayList

Collection

//Collection对应抽象工厂VideoFactroy
public interface Collection<E> extends Iterable<E> {
  Iterator<E> iterator();
}

ArrayList

//ArrayList对应具体的实现工厂JavaVideoFactory，Itr对应具体的产品JavaVideo,Iterator对应抽象的产品Video
 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;

        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();
        }
    }

URLStream

//对应抽象工厂VideoFactory
public interface URLStreamHandlerFactory {
    /**
     * Creates a new {@code URLStreamHandler} instance with the specified
     * protocol.
     *
     * @param   protocol   the protocol ("{@code ftp}",
     *                     "{@code http}", "{@code nntp}", etc.).
     * @return  a {@code URLStreamHandler} for the specific protocol.
     * @see     java.net.URLStreamHandler
     */
    URLStreamHandler createURLStreamHandler(String protocol);
}

//Factory对应具体实现的工厂JavaVideoFactory
private static class Factory implements URLStreamHandlerFactory {
        private static String PREFIX = "sun.net.www.protocol";

        private Factory() {
        }
		
        public URLStreamHandler createURLStreamHandler(String var1) {
            String var2 = PREFIX + "." + var1 + ".Handler";

            try {
                Class var3 = Class.forName(var2);
                return (URLStreamHandler)var3.newInstance();
            } catch (ReflectiveOperationException var4) {
                throw new InternalError("could not load " + var1 + "system protocol handler", var4);
            }
        }
    }

//对应抽象产品Video
public abstract class URLStreamHandler {
    /**
     * Opens a connection to the object referenced by the
     * {@code URL} argument.
     * This method should be overridden by a subclass.
     *
     * <p>If for the handler's protocol (such as HTTP or JAR), there
     * exists a public, specialized URLConnection subclass belonging
     * to one of the following packages or one of their subpackages:
     * java.lang, java.io, java.util, java.net, the connection
     * returned will be of that subclass. For example, for HTTP an
     * HttpURLConnection will be returned, and for JAR a
     * JarURLConnection will be returned.
     *
     * @param      u   the URL that this connects to.
     * @return     a {@code URLConnection} object for the {@code URL}.
     * @exception  IOException  if an I/O error occurs while opening the
     *               connection.
     */
    abstract protected URLConnection openConnection(URL u) throws IOException;
}

//对应具体的产品实现类JavaVideo
public class Handler extends URLStreamHandler {

}