Builder Pattern

• Separate the construction of a complex object from its representation so that the same construction process can create different representations.
• ref: https://java-design-patterns.com/patterns/builder/#explanation
• Helps in avoiding long constructors (telescoping constructors) with lots of params
• Object created at the end is immutable
• Class & Builder

public final class Hero {
  private final String name;
  private final HairColor hairColor;
  private final Weapon weapon;
 
  private Hero(Builder builder) {
    this.name = builder.name;
    this.hairColor = builder.hairColor;
    this.weapon = builder.weapon;
  }
 
  public static class Builder {
    private final String name;
    private HairColor hairColor;
    private Weapon weapon;
 
    public Builder(String name) {
      if (name == null) {
        throw new IllegalArgumentException("name can not be null");
      }
      this.name = name;
    }
 
    public Builder withHairColor(HairColor hairColor) {
      this.hairColor = hairColor;
      return this;
    }
 
    public Builder withWeapon(Weapon weapon) {
      this.weapon = weapon;
      return this;
    }
 
    public Hero build() {
      return new Hero(this);
    }
  }
}

• Driver

var mage = new Hero.Builder("Riobard")
	.withHairColor(HairColor.BLACK)
	.withWeapon(Weapon.DAGGER)
	.build();

StringBuilder

• String does not allow appending because String is immutable
• Each method you invoke on String creates a new object and returns it
• StringBuilder is mutable and can concatenate strings
• toString() finally generate an immutable String

StringBuilder sb = new StringBuilder();
sb.append("hello");
sb.append(" world");
sb.toString();

Fluent Builder Pattern

• This pattern forces the user to set the attributes in a particular order
• example: Stream API, Mockito