A story about making a Builder that inherits the Builder

Introduction

It's natural if you think calmly, but I thought about it for a moment, so as a memo ...

Make a builder

I'll omit the Getter / Setter, but there is a Square class that has vertical and horizontal lengths in the fields.

Square.java

public class Square {
  private int width;
  private int height;
}

If you make it possible to set the value using Builder and create a class that returns the area at the end, it will be as follows.

SquareBuilder.java

public class SquareBuilder {

  public static class Builder {

    Square square = new Square();

    public Builder() {}

    public Builder width(int width) {
      square.setWidth(width);
      return this;
    }

    public Builder height(int height) {
      square.setHeight(height);
      return this;
    }

    public int build() {
      return square.getWidth() * square.getHeight();
    }
  }
}

Then, you can find the area like this

Main.java

public class Main {
  public static void main(String[] args) {
    SquareBuilder.Builder squareBuilder = new SquareBuilder.Builder();
    System.out.println("Area is:" + squareBuilder.width(5).height(10).build());
  }
}

There is no particular problem so far. : slight_smile:

Create a Builder that inherits the Builder

Next, create a Cube class that inherits from Square. (Omit Getter / Setter.)

Cube.java

public class Cube extends Square {
  private int depth;
}

Aside from whether the height was depth ...: kissing: It also inherits Builder

CubeBuilder.java

public class CubeBuilder extends SquareBuilder {

  public static class Builder extends SquareBuilder.Builder {

    private Cube cube = new Cube();

    public Builder() {}

    public Builder depth(int depth) {
      cube.setDepth(depth);
      return this;
    }

    @Override
    public int build() {
      return super.build() * cube.getDepth();
    }
  }
}

Then, you can find the volume like this.

Main.java

public class Main {
  public static void main(String[] args) {
    CubeBuilder.Builder cubeBuilder = new CubeBuilder.Builder();
    System.out.println("Volume is:" + cubeBuilder.depth(2).width(5).height(10).build());
  }
}

This code goes through compilation and requires volume, but it's actually a problem. You have to call depth () first to find the volume. : worried:

The reason is that the return values of width () and height () are of type SquareBuilder.Builder, so you can't calldepth (), which is defined only in CubeBuilder.Builder. ..

We will use Generics here.

Reference: Subclassing a Java Builder class

SquareBuilder.java

public class SquareBuilder {

  public static class Builder<T extends Builder<T>> {

    Square square = new Square();

    public Builder() {}

    public T width(int width) {
      square.setWidth(width);
      return (T) this;
    }

    public T height(int height) {
      square.setHeight(height);
      return (T) this;
    }

    public int build() {
      return square.getWidth() * square.getHeight();
    }
  }
}

CubeBuilder.java

public class CubeBuilder extends SquareBuilder {

  public static class Builder extends SquareBuilder.Builder<Builder> {

    private Cube cube = new Cube();

    public Builder() {}

    public Builder depth(int depth) {
      cube.setDepth(depth);
      return this;
    }

    @Override
    public int build() {
      return super.build() * cube.getDepth();
    }
  }
}

As a result, the return type of width () and height () will be a class that inherits SquareBuilder.Builder instead of SquareBuilder.Builder, so you need to calldepth ()first. lose.

Main.java

public class Main {
  public static void main(String[] args) {
    SquareBuilder.Builder squareBuilder = new SquareBuilder.Builder();
    System.out.println("Area is:" + squareBuilder.width(5).height(10).build());

    CubeBuilder.Builder cubeBuilder = new CubeBuilder.Builder();
    System.out.println("Volume is:" + cubeBuilder.width(5).height(10).depth(2).build());
  }
}

However, I feel a little uncomfortable because there is a Warning. : mask:

Background of posting

It's a program that puts values in POJOs with a lot of fields and outputs them in JSON format, so it became troublesome to put values in Setter one by one, and I was addicted to using Builder.

bonus

In this example, the area cannot be calculated, but Warning can be eliminated by making SquareBuilder an abstract class.

SquareBuilder.java

public class SquareBuilder {

  public abstract static class Builder<T extends Builder<T>> {

    Square square = new Square();

    public Builder() {}

    public abstract T getThis();

    public T width(int width) {
      square.setWidth(width);
      return getThis();
    }

    public T height(int height) {
      square.setHeight(height);
      return getThis();
    }

    public int build() {
      return square.getWidth() * square.getHeight();
    }
  }
}

CubeBuilder.java

public class CubeBuilder extends SquareBuilder {

  public static class Builder extends SquareBuilder.Builder<Builder> {

    private Cube cube = new Cube();

    public Builder() {}

    @Override
    public Builder getThis() {
      return this;
    }

    public Builder depth(int depth) {
      cube.setDepth(depth);
      return this;
    }

    @Override
    public int build() {
      return super.build() * cube.getDepth();
    }
  }
}