Introduction to Appearance Pattern and Decorator Pattern in Java Design Patterns (Code Example)

This article brings you an introduction to the appearance mode and decorator mode in Java design patterns (code examples). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you. help.

Preface

In Previous article we learned about the adapter mode and bridge mode of the structural mode. In this article, we will learn about the appearance mode and decorator mode of structural mode.

Facade Mode

Introduction

Facade mode hides the complexity of the system and provides the client with an interface through which the client can access the system. This type of design pattern is a structural pattern that adds an interface to an existing system to hide the complexity of the system.

To put it simply, it provides a simple interface to the outside world and hides the implementation logic. For example, with the power button of a commonly used computer, we only need to press the power button to start or shut down it. There is no need to know how it starts (starting the CPU, starting the memory, starting the hard disk) or how to shut it down (turning off the hard disk, turning off the memory, Turn off the CPU);

Here we can still use the example of playing games on a computer to Appearance mode for a simple explanation.
There are some online games on the computer, including DNF, LOL and WOW. We only need to double-click the icon on the computer to start and play the game. We do not need to care about how the game starts and runs.

The steps that need to be implemented are as follows:

1. Establish the interface of the game;

2. Establish the classes of LOL, DNF and WOW and Implement the game interface;

3. Define an appearance class and provide it to the client for calling.

4. Call the appearance class.

Code example:

interface Game{
    void play();
}

class DNF implements Game{

    @Override
    public void play() {
        System.out.println("正在玩DNF...");
    }
}

class LOL implements Game{
    @Override
    public void play() {
        System.out.println("正在玩LOL...");
    }
}

class WOW implements Game{
    @Override
    public void play() {
        System.out.println("正在玩WOW...");
    }
}

class Computer{
    
    private Game dnf;
    private Game lol;
    private Game wow;
    
    public Computer() {
        dnf=new DNF();
        lol=new LOL();
        wow=new WOW();
    }
    
    public void playDNF(){
        dnf.play();
    }
    
    public void playLOL(){
        lol.play();
    }
    
    public void playWOW(){
        wow.play();
    }   
}

public static void main(String[] args) {
        Computer computer=new Computer();
        computer.playDNF();
        computer.playLOL();
        computer.playWOW();
    }

Running result:

    正在玩DNF...
    正在玩LOL...
    正在玩WOW...

In the above code example, When we want to play a game, we only need to instantiate Appearance Class and call the game method in it. We don't need to care about how the game starts and runs. Moreover, each game is independent of each other and does not affect each other. Just because one game cannot be played, other games will not be able to run either. In fact, I feel that the appearance mode is very similar to the interface we usually use. They all provide interfaces to the outside world, and there is no need to care about how they are implemented.

Advantages of appearance mode:

Reduces coupling, which in some ways also improves security.

Disadvantages of appearance mode:

Does not comply with the opening and closing principle and is not easy to change.

Usage scenarios

When there are multiple complex modules or subsystems in the system.

Decorator Pattern

Introduction

The Decorator pattern allows adding new functionality to an existing object without changing its structure. This type of design pattern is a structural pattern, which acts as a wrapper around an existing class.

The decorator mode, as the name suggests, is to decorate something so that it can provide some additional functions. For example, decorating people and wearing different costumes when doing different things. For example, putting on a jersey means getting ready to play ball, putting on a swimsuit means getting ready to go swimming, etc.

The decorator pattern can dynamically add some additional responsibilities to an object.

Here we still use an example to illustrate.
Among the current toy models, there are two very popular models, GUNDAM model and MrGu model. When we are piecing together the models, Generally, the model is assembled first, and then some additional accessories, such as weapons, are added. Here we have assembled the GUNDAM model and the MrGu model and then equipped them with their respective weapons.

The specific implementation steps are as follows:

1. Create a model interface of abstract components, with the assembly method;

2. Create Specific component classes (GUNDAM class and MrGu class), and implement the above model interface;

3. Define a decorator to accept the client's request and respond according to the client's request Call;

4. Define a class that specifically implements decoration, which is used to add corresponding functions to the object.

Code example:

interface Model{
    void  assemble();
}

class GUNDAM implements Model{
    @Override
    public void  assemble() {
        System.out.println("组装一个高达模型");
    }
}

class MrGu implements Model{
    @Override
    public void  assemble() {
        System.out.println("组装一个扎古模型");
    }
}

abstract class  AddExtra implements Model{
    protected  Model model;
    
    public AddExtra(Model model){
        this.model=model;
    }
    public  void assemble(){
        model.assemble();
    }
}

class LightSaber extends AddExtra{

    public LightSaber(Model model) {
        super(model);
    }
    
    public  void assemble(){
        model.assemble();
        addLightSaber();
    }
    public void addLightSaber(){
        System.out.println("添加光剑");
    }
}


class RocketLauncher extends AddExtra{

    public RocketLauncher(Model model) {
        super(model);
    }
    
    public  void assemble(){
        model.assemble();
        addRocketLauncher();
    }
    public void addRocketLauncher(){
        System.out.println("添加火箭筒");
    }
}

public static void main(String[] args) {
    
        Model gundam=new GUNDAM();
        Model mrgu=new MrGu();
        gundam.assemble();
        mrgu.assemble();
            
        Model gModel=new LightSaber(new GUNDAM());
        gModel.assemble();
        Model mModel=new RocketLauncher(new MrGu());
        mModel.assemble();
}

Running result:

    组装一个高达模型
    组装一个扎古模型
    
    组装一个高达模型
    添加光剑
    组装一个扎古模型
    添加火箭筒

In the above code, if we only want to assemble Gundam Or this Zaku model, you can directly instantiate the model class and call the methods in it. If you need to add a weapon when assembling the model, you only need to add the corresponding function through the decorator class.
Through this example, we found that when trying to use the decorator mode, some classes can be extended without affecting the previous functions, improving flexibility.

Advantages of the decorator pattern:

The decorating class and the decorated class can develop independently, with low coupling, easy expansion, flexibility and convenience.

Disadvantages of the decorator pattern:

Decorating a class too much will increase complexity.

Usage scenarios
When the prototype remains unchanged and some functions are dynamically added.

Related recommendations:

Introduction to adapter pattern and bridge pattern in Java design patterns (code examples)

Introduction to builder pattern and prototype pattern in Java design patterns Introduction (Code Example)

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