Creational Design Patterns: Builder Pattern

Previously we had a look at the factory and the abstract factory pattern. Those patterns serve their purpose and can be really useful however there are use cases where we have to create a very complex object and creating it requires different steps each one requiring different actions. In such cases the builder pattern can be really useful.
The builder design pattern is a creational design pattern and can be used in order to create complex objects step by step.

Supposing we have an object with many dependencies and in order to acquire each one these dependencies some certain actions have to be issued.
In such cases we can use the builder pattern in order to

  • Encapsulate creating and assembling the parts of a complex object in a separate Builder object.
  • Delegate the object creation to a Builder object instead of creating the objects directly.

Imagine the scenario of a backend system that has to compose and sent emails.
Creating an email might be a complex procedure. You have to specify the title, set the recipients, add a greeting add a closing statement. You might also want to use mustache instead. There is a wide range of options.

Having one class for all the actions needed to create an email might make our class bloated and loose its original purpose.

So we will start with the class responsible for sending the email.

package com.gkatzioura.design.creational.builder;

public class Email {

    private final String title;
    private final String recipients;
    private final String message;

    public Email(String title, String recipients, String message) {
        this.title = title;
        this.recipients = recipients;
        this.message = message;
    }

    public String getTitle() {
        return title;
    }

    public String getRecipients() {
        return recipients;
    }

    public String getMessage() {
        return message;
    }

    public void send() {

    }
}

As you can see the class contains only three string fields and there is no extra processing on them. So we shall create a builder class which will handle the message formatting, the recipient representation and the creation of the Email class.

package com.gkatzioura.design.creational.builder;

import java.util.HashSet;
import java.util.Set;

public class EmailBuilder {

    private Set recipients = new HashSet();
    private String title;
    private String greeting;
    private String mainText;
    private String closing;

    public EmailBuilder addRecipient(String recipient) {
        this.recipients.add(recipient);
        return this;
    }

    public EmailBuilder removeRecipient(String recipient) {
        this.recipients.remove(recipient);
        return this;
    }

    public EmailBuilder setTitle(String title) {
        this.title = title;
        return this;
    }

    public EmailBuilder setGreeting(String greeting) {
        this.greeting = greeting;
        return this;
    }

    public EmailBuilder setMainText(String mainText) {
        this.mainText = mainText;
        return this;
    }

    public EmailBuilder setClosing(String closing) {
        this.closing = closing;
        return this;
    }

    public Email create() {

        String message = greeting+"\n"+mainText+"\n"+closing;
        String recipientSection = commaSeparatedRecipients();

        return new Email(title,recipientSection,message);
    }

    private String commaSeparatedRecipients() {

        StringBuilder sb = new StringBuilder();
        for(String recipient:recipients) {
            sb.append(",").append(recipient);
        }

        return sb.toString().replaceFirst(",","");
    }

}

The next is step to make the email creation more strict so that creating an Email would only be possible through the EmailBuilder.

package com.gkatzioura.design.creational.builder;

import java.util.HashSet;
import java.util.Set;

public class Email {

    private final String title;
    private final String recipients;
    private final String message;

    private Email(String title, String recipients, String message) {
        this.title = title;
        this.recipients = recipients;
        this.message = message;
    }

    public String getTitle() {
        return title;
    }

    public String getRecipients() {
        return recipients;
    }

    public String getMessage() {
        return message;
    }

    public void send() {

    }

    public static class EmailBuilder {

        private Set recipients = new HashSet();
        private String title;
        private String greeting;
        private String mainText;
        private String closing;

        public EmailBuilder addRecipient(String recipient) {
            this.recipients.add(recipient);
            return this;
        }

        public EmailBuilder removeRecipient(String recipient) {
            this.recipients.remove(recipient);
            return this;
        }

        public EmailBuilder setTitle(String title) {
            this.title = title;
            return this;
        }

        public EmailBuilder setGreeting(String greeting) {
            this.greeting = greeting;
            return this;
        }

        public EmailBuilder setMainText(String mainText) {
            this.mainText = mainText;
            return this;
        }

        public EmailBuilder setClosing(String closing) {
            this.closing = closing;
            return this;
        }

        public Email build() {

            String message = greeting+"\n"+mainText+"\n"+closing;
            String recipientSection = commaSeparatedRecipients();

            return new Email(title,recipientSection,message);
        }

        private String commaSeparatedRecipients() {

            StringBuilder sb = new StringBuilder();
            for(String recipient:recipients) {
                sb.append(",").append(recipient);
            }

            return sb.toString().replaceFirst(",","");
        }

    }
}

The end result of using the builder pattern for creating an email will be like this.

Email email = new Email.EmailBuilder()
    .addRecipient("john@Doe.com")
    .setMainText("Check the builder pattern")
    .setGreeting("Hi John!")
    .setClosing("Regards")
    .setTitle("Builder pattern resources")
    .build();

To summarize by using the builder pattern we were able to create a complex object and its complex parts.

You can find the sourcecode on github.

On the next blog we will talk about the singleton pattern.

Also I have compiled a cheat sheet containing a summary of the Creational Design Patterns.
Sign up in the link to receive it.

3 thoughts on “Creational Design Patterns: Builder Pattern

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