[Design Patterns] Creational Design Patterns - Builder

dnjscksdn98·2020년 9월 16일

Design Patterns

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🚀 Builder Patterns

  • We have a complex process to construct an object involving multiple steps, then builder design pattern can help us.

  • In Builder we remove the logic related to object contruction from code & abstract it in separate classes.

✓ How to implement a Builder

  • We start by creating a builder
    • Identify the parts of the product & provide methods to create those parts.
    • It should provide a method to assemble or build the object.
    • It must provide a way/method to get fully built object out. Optionally builder can keep reference to an object it has built so the same can be returned again in future.
  • A Director can be a separate class or client that can play the role of director.

✓ Compare & Contrast with Prototype Patterns


  • When we have complex constructor and builder allows us to work with that.
  • We can create a builder as a separate class and so it can work with legacy code.


  • Prototype allows to altogether skip using contructor.
  • In Java this pattern works using clone method, and needs to modify existing code so may not work with legacy code.

🚫 Pitfalls

  • Possibility of partially initialized objects
    • An object can be set with only a few or none of the properties using withXXX methods and call build(). If required properties are missing, build method should provide suitable defaults or throw an exception.

🎯 Example Code

public class UserDto {

  private String name;
  private String address;
  private String age;

  public String getName() {
    return name;

  private void setName(String name) {
    this.name = name;

  public String getAddress() {
    return address;

  private void setAddress(String address) {
    this.address = address;

  public String getAge() {
    return age;

  private void setAge(String age) {
    this.age = age;

  public String toString() {
    return String.format("Name: %s\nAge: %s\nAddress: %s", name, age, address);

  public static UserDtoBuilder getBuilder() {
    return new UserDtoBuilder();

  public static class UserDtoBuilder {
    private String firstName;
    private String lastName;
    private String age;
    private String address;
    private UserDto userDto;

    public UserDtoBuilder withFirstName(String fname) {
      firstName = fname;
      return this;

    public UserDtoBuilder withLastName(String lname) {
      lastName = lname;
      return this;

    public UserDtoBuilder withBirth(LocalDate birth) {
      Period ageInYears = Period.between(birth, LocalDate.now());
      age = Integer.toString(ageInYears.getYears());
      return this;

    public UserDtoBuilder withAddress(Address address) {
      this.address = address.getHouseNumber() + ", " + address.getStreet() + ", " + address.getCity() + ", " + address.getState() + ", " + address.getZipCode();
      return this;

    public UserDto build() {
      this.userDto = new UserDto();
      userDto.setName(firstName + " " + lastName);
      return this.userDto;

    public UserDto getUserDto() {
      return this.userDto;

✓ Summary

  • Think of builder pattern when you have a complex constructor or an object is built in multiple steps.
Alex's Develog 🤔

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