Java design patterns-Builder pattern

This article mainly introduces the usage of Builder. Share it with everyone for your reference. The details are as follows:

I was looking at the source code of Mybatis recently, and in the process of reading and parsing the XML configuration file, I found that the builder mode was used. Therefore, I plan to review this design pattern.

Origin

Suppose we need to draw a little person, we may have the following constructor definition:

public Person(HeadType headType, HairType hairType, HairColor hairColor, FaceType faceType, BodyType bodyType, ArmType amrType, LegType legTyype) {
}

Seeing such a constructor, we estimate that we will come back later I was confused when I saw it. There are so many parameters, which makes subsequent maintenance very troublesome for us.

The construction pattern can solve such problems.

Use

The goal is to draw a little person

1. Define the abstract Builder

First define the abstract PersonBuilder . This class defines the steps required to draw a villain, so that each object generated through PersonBuilder is essentially the same, but can have different personalities.

abstract class PersonBuilder {
    protected Graphics graphics;

    public PersonBuilder(Graphics graphics) {
        this.graphics = graphics;
    }

    public abstract void buildHead();
    public abstract void buildBody();
    public abstract void buildArmLeft();
    public abstract void buildArmRight();
    public abstract void buildLegLeft();
    public abstract void buildLegRight();
}

2. Define a specific Builder class

and define a specific implementation class PersonFatBuilder. This class inherits PersonBuilder and implements abstract methods.

<pre class="brush:js;toolbar:false;">public class PersonFatBuilder extends PersonBuilder { public PersonFatBuilder(Graphics graphics) { super(graphics); } @Override public void buildHead() { graphics.drawOval(50, 20, 30, 30); graphics.drawArc(50, 30, 10, 5, 45, 135); graphics.drawArc(70, 30, 10, 5, 45, 135); graphics.drawArc(60, 35, 10, 5, 200, 135); } @Override public void buildBody() { graphics.drawRect(55, 50, 20, 50); } @Override public void buildArmLeft() { graphics.drawLine(55, 50, 40, 100); } @Override public void buildArmRight() { graphics.drawLine(75, 50, 90, 100); } @Override public void buildLegLeft() { graphics.drawLine(55, 100, 45, 150); } @Override public void buildLegRight() { graphics.drawLine(75, 100, 85, 150); } }</pre>

3. Define the specific Director class

This class is responsible for the specific construction process and does not care about what is built.

public class PersonDirector {
    private PersonBuilder personBuilder;

    public PersonDirector(PersonBuilder personBuilder) {
        this.personBuilder = personBuilder;
    }

    public void drawPerson() {
        personBuilder.buildHead();
        personBuilder.buildBody();
        personBuilder.buildArmLeft();
        personBuilder.buildArmRight();
        personBuilder.buildLegLeft();
        personBuilder.buildLegRight();
    }
}

4. Test

Create a window and draw the villain.

 public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            // 创建窗口对象
            JFrame frame = new JFrame();
            frame.setVisible(true);
            frame.setTitle("画人");
            frame.setSize(250, 300);

            // 设置窗口关闭按钮的默认操作(点击关闭时退出进程)
            frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

            // 把窗口位置设置到屏幕的中心
            frame.setLocationRelativeTo(null);
            frame.setContentPane(new JPanel(){
                @Override
                protected void paintComponent(Graphics g) {
                    super.paintComponent(g);
                    PersonThinBuilder thinBuilder = new PersonThinBuilder(g);
                    PersonDirector director = new PersonDirector(thinBuilder);
                    director.drawPerson();


                }
            });
        }
    });
}

The results are as follows:

Definition

Literal definition

Compare the construction of complex objects with it Separation of representations allows the same construction process to create different representations.

In other words, it allows you to create different types of objects while avoiding pollution of the constructor. This pattern is useful when there are multiple types of objects. Or there are many steps involved in creating an object.

Structural diagram

Quoting a diagram from "Dahua Design Pattern"

##Abstract class

Builder: Create ProductObject and abstract interface.

Inherited class

ConcreateBuilder: Specific builder, constructs and assembles various components.

Specific product category

Product: The object we need to build.

Director: Used to create products, it has member variables of type Builder inside.

Advantages

1. ##Director

   No need to know the internal details of Product, it only provides the required information to the builder, It is processed by the specific builder ConcreateBuilder to complete the construction of the product.

The builder pattern disperses the complex product creation process into different objects, thereby achieving more precise control over the product creation process and making the creation process clearer.
Each specific builder can create a complete product object, and it is independent of each other. Therefore, the caller can get different objects through different concrete builders. When a new product appears, there is no need to change the original code, just add a builder.
Example

Now if we want to build a fat little man with facial features. Then we only need to add a

PersonFatBuilder

class, and there is no need to change the original code. <pre class="brush:js;toolbar:false;">public class PersonFatBuilder extends PersonBuilder { public PersonFatBuilder(Graphics graphics) { super(graphics); } @Override public void buildHead() { graphics.drawOval(50, 20, 30, 30); graphics.drawArc(50, 30, 10, 5, 45, 135); graphics.drawArc(70, 30, 10, 5, 45, 135); graphics.drawArc(60, 35, 10, 5, 200, 135); } @Override public void buildBody() { graphics.drawRect(55, 50, 20, 50); } @Override public void buildArmLeft() { graphics.drawLine(55, 50, 40, 100); } @Override public void buildArmRight() { graphics.drawLine(75, 50, 90, 100); } @Override public void buildLegLeft() { graphics.drawLine(55, 100, 45, 150); } @Override public void buildLegRight() { graphics.drawLine(75, 100, 85, 150); } }</pre>Result:

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