Learning: java design pattern—factory pattern

1. The factory pattern mainly provides a transition interface for creating objects, so as to shield and isolate the specific process of creating objects to achieve the purpose of improving flexibility.

Factory pattern is divided into three categories in "Java and Patterns":
1) Simple Factory pattern (Simple Factory): not conducive to producing a series of products;

2) Factory Method pattern (Factory Method): and Called polymorphic factories; ）3) abstract factory model (ABSTRACT Factory): also known as a toolbox, produce product family, but it is not conducive to new products; More general.模 GOF divided the factory model into two categories in the book "Design Model": Factory Method and abstract factory mode. Think of the Simple Factory pattern as a special case of the Factory Method pattern, and the two are classified into the same category.


2. Simple Factory Pattern

The simple factory pattern is also called the static factory method pattern. It can be seen from the renaming that this mode must be very simple. Its purpose is simple: to define an interface for creating objects.


In the simple factory pattern, a factory class is at the center of the instantiation call of a product class. It determines which product class should be instantiated, just like a traffic policeman standing in the flow of passing vehicles and deciding to let them go. Vehicles in one direction flow in that direction.成 Let's take a look at its composition first:

2) Abstract product role: It is generally the parent class inherited by a specific product or the interface implemented. It is implemented in java by interface or abstract class.

3) Specific product role: The object created by the factory class is an instance of this role. Implemented by a concrete class in java.

3. Factory Method Pattern


Factory Method Pattern is a further abstraction and promotion of the Simple Factory Pattern. In the Factory Method Pattern, it is no longer just a factory class that determines which product class should be instantiated. This decision is handed over to A subclass of the abstract factory does it.
Look at its composition:

1) Abstract factory role: This is the core of the factory method pattern, and it has nothing to do with the application. It is the interface that a specific factory role must implement or the parent class that must be inherited. In java it is implemented by abstract classes or interfaces.

2) Specific factory role: It contains code related to specific business logic. Called by an application to create an object corresponding to a specific product.

3) Abstract product role: It is the parent class inherited by a specific product or the interface implemented. In Java, there are generally abstract classes or interfaces to implement.

4) Specific product role: The object created by the specific factory role is an instance of this role. It is implemented by specific classes in java.
The Factory Method Pattern uses multiple subclasses inherited from the Abstract Factory role to replace the "God class" in the Simple Factory Pattern. As mentioned above, this shares the pressure on the object; and this makes the structure flexible - when a new product (i.e., the upstart's car) is produced, as long as it is provided according to the abstract product role and abstract factory role Contracts are generated, which can then be used by customers without having to modify any existing code. It can be seen that the structure of the factory role also conforms to the opening and closing principle!

Code:

//抽象产品角色
public interface Moveable {
    void run();
}
//具体产品角色
public class Plane implements Moveable {
    @Override
    public void run() {
        System.out.println("plane....");
    }
}

public class Broom implements Moveable {
    @Override
    public void run() {
        System.out.println("broom.....");
    }
}


//抽象工厂
public abstract class VehicleFactory {
    abstract Moveable create();
}

//具体工厂
public class PlaneFactory extends VehicleFactory{
    public Moveable create() {
        return new Plane();
    }
}

public class BroomFactory extends VehicleFactory{
    public Moveable create() {
        return new Broom();
    }
}

//测试类
public class Test {
    public static void main(String[] args) {
        VehicleFactory factory = new BroomFactory();
        Moveable m = factory.create();
        m.run();
    }
}

It can be seen that the addition of factory methods has doubled the number of objects. When there are many types of products, a large number of corresponding factory objects will appear, which is not what we want. Because if this situation cannot be avoided, you can consider using a combination of the simple factory pattern and the factory method pattern to reduce factory classes: that is, use a simple factory for similar types on the product tree (usually brothers among the leaves of the tree) mode to achieve.

4. Comparison between simple factory and factory method patterns

The difference in definition between factory method pattern and simple factory pattern is obvious. The core of the factory method pattern is an abstract factory class, unlike the simple factory pattern, which puts the core on a real class. The factory method pattern allows many real factory classes to inherit from the abstract factory class, which can actually become a synthesis of multiple simple factory patterns, thus popularizing the simple factory pattern.
On the other hand, the simple factory pattern is degenerated from the factory method pattern. Imagine that if we are very sure that a system only needs a real factory class, then we might as well merge the abstract factory class into the real factory class. And in this way, we have degraded to the simple factory model.

5. Abstract Factory Pattern

Code:

//抽象工厂类

public abstract class AbstractFactory {
    public abstract Vehicle createVehicle();
    public abstract Weapon createWeapon();
    public abstract Food createFood();
}

//具体工厂类，其中Food,Vehicle，Weapon是抽象类，

public class DefaultFactory extends AbstractFactory{
    @Override
    public Food createFood() {
        return new Apple();
    }
    @Override
    public Vehicle createVehicle() {
        return new Car();
    }
    @Override
    public Weapon createWeapon() {
        return new AK47();
    }
}

//测试类

public class Test {
    public static void main(String[] args) {
        AbstractFactory f = new DefaultFactory();
        Vehicle v = f.createVehicle();
        v.run();
        Weapon w = f.createWeapon();
        w.shoot();
        Food a = f.createFood();
        a.printName();
    }
}

In the Abstract Factory pattern, there may be one or more abstract products (AbstractProduct), thus forming one or more product families (Product Family). In the case of only one product family, the abstract factory pattern actually degenerates into the factory method pattern.

6. Summary.

(1) The simple factory pattern uses a specific class to create instances of other classes. The parent class is the same and the parent class is specific.
(2) The factory method pattern has an abstract parent class defining a public interface, and the subclass is responsible for generating specific objects. The purpose of this is to delay the instantiation of the class to the subclass.
(3) The abstract factory pattern provides an interface for creating a series of related or interdependent objects without specifying their specific classes. It targets hierarchical structures with multiple products. The factory method pattern is aimed at the hierarchical structure of a product.

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