C.1 The builder pattern

Chapter 7 contained a simplified implementation of the builder pattern. In reality engineers often make use of a number of techniques and language features when implementing the builder pattern. Listing C.1¹, demonstrates a more complete implementation of the builder pattern in Java. Some things to note in this implementation are as follows:

• The TextOptions class constructor is private to force other engineers to use the builder pattern.

• The TextOptions class constructor takes an instance of Builder as a parameter. This makes the code a little easier to read and maintain because it avoids very long lists of parameters and arguments.

• The TextOptions class provides a toBuilder() method that can be used to create a prepopulated instance of the Builder class from an instance of the TextOptions class.

• The Builder class is an inner class of the TextOptions class. This serves two purposes:

• • It makes the name spacing a little nicer, because the Builder can now be referred to using TextOptions.Builder.

  • In Java, this allows the TextOptions and Builder classes to have access to private member variables and methods on one another.

Listing C.1 Builder pattern implementation

public final class TextOptions {
  private final Font font;
  private final OptionalDouble fontSize;
  private final Optional<Color> color;
 
  private TextOptions(Builder builder) {      ❶
    font = builder.font;
    fontSize = builder.fontSize;
    color = builder.color;
  }
 
  public Font getFont() {
    return font;
  }
 
  public OptionalDouble getFontSize() {
    return fontSize;
  }
 
  public Optional<Color> getColor() {
    return color;
  }
 
  public Builder toBuilder() {                ❷
    return new Builder(this);
  }
 
  public static final class Builder {         ❸
    private Font font;
    private OptionalDouble fontSize = OptionalDouble.empty();
    private Optional<Color> color = Optional.empty();
 
    public Builder(Font font) {
      this.font = font;
    }
 
    private Builder(TextOptions options) {    ❹
      font = options.font;
      fontSize = options.fontSize;
      color = options.color;
    }
 
    public Builder setFont(Font font) {
      this.font = font;
      return this;
    }
 
    public Builder setFontSize(double fontSize) {
      this.fontSize = OptionalDouble.of(fontSize);
      return this;
    }
 
    public Builder clearFontSize() {
      fontSize = OptionalDouble.empty();
      return this;
    }
 
    public Builder setColor(Color color) {
      this.color = Optional.of(color);
      return this;
    }
 
    public Builder clearColor() {
      color = Optional.empty();
      return this;
    }
 
    public TextOptions build() {
      return new TextOptions(this);
    }
  }
}

❶ Constructor private and accepts a Builder as a parameter

❷ toBuilder() function allows creation of a pre-populated builder.

❸ The Builder class is an inner class of TextOptions.

❹ Private Builder constructor for copying from some TextOptions

Some examples of how this code might be used are as follows:

TextOptions options1 = new TextOptions.Builder(Font.ARIAL)
    .setFontSize(12.0)
    .build();
  
TextOptions options2 = options1.toBuilder()
    .setColor(Color.BLUE)
    .clearFontSize()
    .build();
    
TextOptions options3 = options2.toBuilder()
    .setFont(Font.VERDANA)
    .setColor(Color.RED)
    .build();