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<!DOCTYPE html>
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    <title>Builder Pattern - Quick Revision Guide</title>
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<body>
    <div class="container">
        <header>
            <h1>🏗️ Builder Pattern</h1>
            <p>Quick Revision Guide</p>
        </header>

        <main>
            <!-- Core Concept Section -->
            <section>
                <h2><span class="icon">🧠</span> Core Concept</h2>
                <h3>The "Checklist" Analogy</h3>
                <div class="analogy-box">
                    Instead of forcing every possible combination of an object into separate constructors (like a massive, confusing restaurant menu), the Builder Pattern acts as a checklist. You build your object step-by-step and "bake" it at the end.
                </div>
            </section>

            <!-- Problem Section -->
            <section>
                <h2><span class="icon">🚨</span> The Problem</h2>
                <h3>Telescoping Constructors</h3>
                <p>Creating objects with many optional fields traditionally requires massive, unreadable constructors.</p>
                <div class="problem-box">
                    <pre><code><span class="comment">// Hard to read, easy to mix up arguments, forces passing `null` or `false`</span>
Car myCar = <span class="keyword">new</span> <span class="class-name">Car</span>(<span class="string">"V8"</span>, <span class="keyword">true</span>, <span class="keyword">false</span>, <span class="keyword">true</span>, 4, <span class="string">"Red"</span>, <span class="keyword">false</span>);</code></pre>
                </div>
            </section>

            <!-- Solution Section -->
            <section>
                <h2><span class="icon">🛠️</span> The Solution & Implementation Rules</h2>
                <p>You build the object incrementally and call a <code>build()</code> method to lock it in.</p>

                <h3>Key Implementation Rules in Java:</h3>
                <div class="rules-list">
                    <ul style="margin: 0; padding: 0;">
                        <li>
                            <strong>Private & Final Fields:</strong> The target class (e.g., Car) fields are private and final to ensure immutability once built.
                        </li>
                        <li>
                            <strong>Private Constructor:</strong> The target class constructor must be private so external code cannot bypass the builder. It takes the Builder as its only parameter.
                        </li>
                        <li>
                            <strong>Getters Only:</strong> The target class provides getters but no setters (read-only).
                        </li>
                        <li>
                            <strong>Return this:</strong> Builder configuration methods must return <code>this</code> to allow method chaining (e.g., <code>.setEngine("V8").addSunroof()</code>).
                        </li>
                        <li>
                            <strong>The build() Method:</strong> The final step inside the Builder that returns <code>new Car(this)</code>.
                        </li>
                    </ul>
                </div>
            </section>

            <!-- Why Static Nested Class Section -->
            <section>
                <h2><span class="icon">🏗️</span> Why a Static Nested Class?</h2>
                <p>The standard in Java is to make the Builder a static inner class (e.g., <code>public static class CarBuilder</code>) for three main reasons:</p>
                <div class="solution-box">
                    <ul>
                        <li>
                            <strong>Access:</strong> A nested class has full access to the private constructor of the outer Car class.
                        </li>
                        <li>
                            <strong>Independence:</strong> Because it is static, you can instantiate CarBuilder without needing a pre-existing instance of Car (<code>new Car.CarBuilder()</code>).
                        </li>
                        <li>
                            <strong>Organization:</strong> It logically groups the builder with its target class, avoiding namespace clutter in your project folders.
                        </li>
                    </ul>
                </div>
            </section>

            <!-- Code Skeleton Section -->
            <section>
                <h2><span class="icon">💻</span> Quick Code Skeleton</h2>
                <pre><code><span class="keyword">public class</span> <span class="class-name">Car</span> {
    <span class="keyword">private final</span> <span class="class-name">String</span> engine;

    <span class="comment">// 1. Private constructor accessed only by Builder</span>
    <span class="keyword">private</span> <span class="class-name">Car</span>(<span class="class-name">CarBuilder</span> builder) {
        <span class="keyword">this</span>.engine = builder.engine;
    }

    <span class="keyword">public</span> <span class="class-name">String</span> getEngine() { <span class="keyword">return</span> engine; }

    <span class="comment">// 2. Static Nested Class</span>
    <span class="keyword">public static class</span> <span class="class-name">CarBuilder</span> {
        <span class="keyword">private</span> <span class="class-name">String</span> engine;

        <span class="comment">// 3. Returns 'this' for method chaining</span>
        <span class="keyword">public</span> <span class="class-name">CarBuilder</span> setEngine(<span class="class-name">String</span> engine) {
            <span class="keyword">this</span>.engine = engine;
            <span class="keyword">return this</span>;
        }

        <span class="comment">// 4. Build method returns the final object</span>
        <span class="keyword">public</span> <span class="class-name">Car</span> build() {
            <span class="keyword">return new</span> <span class="class-name">Car</span>(<span class="keyword">this</span>);
        }
    }
}
</code></pre>

                <h3>Usage Example:</h3>
                <div class="usage-example">
                    Car myCar = <span class="keyword">new</span> Car.CarBuilder()
                        .setEngine("V8")
                        .build();
                </div>
            </section>

            <!-- Enterprise Example Section -->
            <section>
                <h2><span class="icon">🌐</span> Enterprise Example: SCIM User Object</h2>
                <p>In real enterprise applications (like identity management, directory synchronization, and security automation), objects can become incredibly complex.</p>
                <p>Below is a robust Java implementation of a <strong>SCIM User Object</strong>. It demonstrates three advanced, real-world patterns:</p>
                <div class="solution-box">
                    <ol>
                        <li><strong>Mandatory Fields:</strong> Enforced via the Builder's constructor (you can't start building a SCIM User without a <code>userName</code>).</li>
                        <li><strong>Sensible Defaults:</strong> Setting <code>active = true</code> implicitly unless overridden.</li>
                        <li><strong>Collection Management & Immutability:</strong> Dynamically appending to lists (<code>emails</code>, <code>roles</code>) and returning unmodifiable collections to guarantee safety.</li>
                    </ol>
                </div>

                <h3>Complete SCIM User Implementation:</h3>
                <pre><code><span class="keyword">import</span> java.util.ArrayList;
<span class="keyword">import</span> java.util.Collections;
<span class="keyword">import</span> java.util.List;

<span class="keyword">public class</span> <span class="class-name">ScimUser</span> {
    <span class="comment">// Immutable properties</span>
    <span class="keyword">private final</span> <span class="class-name">String</span> id;            <span class="comment">// Optional (assigned by identity provider)</span>
    <span class="keyword">private final</span> <span class="class-name">String</span> userName;      <span class="comment">// MANDATORY</span>
    <span class="keyword">private final</span> <span class="class-name">String</span> externalId;    <span class="comment">// Optional</span>
    <span class="keyword">private final boolean</span> active;       <span class="comment">// Optional (Defaults to true)</span>
    <span class="keyword">private final</span> <span class="class-name">List</span>&lt;<span class="class-name">String</span>&gt; emails;  <span class="comment">// Multi-valued</span>
    <span class="keyword">private final</span> <span class="class-name">List</span>&lt;<span class="class-name">String</span>&gt; roles;   <span class="comment">// Multi-valued</span>

    <span class="comment">// 1. Private constructor maps Builder values to final fields</span>
    <span class="keyword">private</span> <span class="class-name">ScimUser</span>(<span class="class-name">Builder</span> builder) {
        <span class="keyword">this</span>.id = builder.id;
        <span class="keyword">this</span>.userName = builder.userName;
        <span class="keyword">this</span>.externalId = builder.externalId;
        <span class="keyword">this</span>.active = builder.active;

        <span class="comment">// Defensive copying to prevent external mutation of the lists</span>
        <span class="keyword">this</span>.emails = builder.emails != <span class="keyword">null</span> ?
                <span class="class-name">Collections</span>.unmodifiableList(<span class="keyword">new</span> <span class="class-name">ArrayList</span>&lt;&gt;(builder.emails)) : <span class="class-name">Collections</span>.emptyList();
        <span class="keyword">this</span>.roles = builder.roles != <span class="keyword">null</span> ?
                <span class="class-name">Collections</span>.unmodifiableList(<span class="keyword">new</span> <span class="class-name">ArrayList</span>&lt;&gt;(builder.roles)) : <span class="class-name">Collections</span>.emptyList();
    }

    <span class="comment">// Getters only (No setters to ensure thread safety and immutability)</span>
    <span class="keyword">public</span> <span class="class-name">String</span> getId() { <span class="keyword">return</span> id; }
    <span class="keyword">public</span> <span class="class-name">String</span> getUserName() { <span class="keyword">return</span> userName; }
    <span class="keyword">public</span> <span class="class-name">String</span> getExternalId() { <span class="keyword">return</span> externalId; }
    <span class="keyword">public boolean</span> isActive() { <span class="keyword">return</span> active; }
    <span class="keyword">public</span> <span class="class-name">List</span>&lt;<span class="class-name">String</span>&gt; getEmails() { <span class="keyword">return</span> emails; }
    <span class="keyword">public</span> <span class="class-name">List</span>&lt;<span class="class-name">String</span>&gt; getRoles() { <span class="keyword">return</span> roles; }

    @Override
    <span class="keyword">public</span> <span class="class-name">String</span> toString() {
        <span class="keyword">return</span> <span class="string">"ScimUser [id="</span> + id + <span class="string">", userName="</span> + userName + <span class="string">", externalId="</span> + externalId +
               <span class="string">", active="</span> + active + <span class="string">", emails="</span> + emails + <span class="string">", roles="</span> + roles + <span class="string">"]"</span>;
    }

    <span class="comment">// 2. Static Inner Builder</span>
    <span class="keyword">public static class</span> <span class="class-name">Builder</span> {
        <span class="keyword">private</span> <span class="class-name">String</span> id;
        <span class="keyword">private final</span> <span class="class-name">String</span> userName; <span class="comment">// Final inside builder enforces it's set at creation</span>
        <span class="keyword">private</span> <span class="class-name">String</span> externalId;
        <span class="keyword">private boolean</span> active = <span class="keyword">true</span>; <span class="comment">// Sensible default state</span>
        <span class="keyword">private final</span> <span class="class-name">List</span>&lt;<span class="class-name">String</span>&gt; emails = <span class="keyword">new</span> <span class="class-name">ArrayList</span>&lt;&gt;();
        <span class="keyword">private final</span> <span class="class-name">List</span>&lt;<span class="class-name">String</span>&gt; roles = <span class="keyword">new</span> <span class="class-name">ArrayList</span>&lt;&gt;();

        <span class="comment">// Enforce mandatory parameters through the Builder's constructor</span>
        <span class="keyword">public</span> <span class="class-name">Builder</span>(<span class="class-name">String</span> userName) {
            <span class="keyword">if</span> (userName == <span class="keyword">null</span> || userName.isBlank()) {
                <span class="keyword">throw new</span> <span class="class-name">IllegalArgumentException</span>(<span class="string">"Mandatory field 'userName' cannot be null or empty."</span>);
            }
            <span class="keyword">this</span>.userName = userName;
        }

        <span class="keyword">public</span> <span class="class-name">Builder</span> id(<span class="class-name">String</span> id) {
            <span class="keyword">this</span>.id = id;
            <span class="keyword">return this</span>;
        }

        <span class="keyword">public</span> <span class="class-name">Builder</span> externalId(<span class="class-name">String</span> externalId) {
            <span class="keyword">this</span>.externalId = externalId;
            <span class="keyword">return this</span>;
        }

        <span class="keyword">public</span> <span class="class-name">Builder</span> active(<span class="keyword">boolean</span> active) {
            <span class="keyword">this</span>.active = active;
            <span class="keyword">return this</span>;
        }

        <span class="comment">// Methods to handle multi-valued SCIM fields cleanly</span>
        <span class="keyword">public</span> <span class="class-name">Builder</span> addEmail(<span class="class-name">String</span> email) {
            <span class="keyword">if</span> (email != <span class="keyword">null</span> && !email.isBlank()) {
                <span class="keyword">this</span>.emails.add(email);
            }
            <span class="keyword">return this</span>;
        }

        <span class="keyword">public</span> <span class="class-name">Builder</span> addRole(<span class="class-name">String</span> role) {
            <span class="keyword">if</span> (role != <span class="keyword">null</span> && !role.isBlank()) {
                <span class="keyword">this</span>.roles.add(role);
            }
            <span class="keyword">return this</span>;
        }

        <span class="comment">// Finalize construction</span>
        <span class="keyword">public</span> <span class="class-name">ScimUser</span> build() {
            <span class="keyword">return new</span> <span class="class-name">ScimUser</span>(<span class="keyword">this</span>);
        }
    }
}</code></pre>

                <h3>🎮 How to Use the SCIM Builder:</h3>
                <pre><code><span class="keyword">public class</span> <span class="class-name">ScimDemo</span> {
    <span class="keyword">public static void</span> main(<span class="class-name">String</span>[] args) {
        <span class="comment">// Build a comprehensive, read-only Enterprise SCIM User</span>
        <span class="class-name">ScimUser</span> user = <span class="keyword">new</span> <span class="class-name">ScimUser</span>.<span class="class-name">Builder</span>(<span class="string">"john.doe@company.com"</span>) <span class="comment">// Enforces mandatory username</span>
                .id(<span class="string">"usr_9f830a1b"</span>)
                .externalId(<span class="string">"emp_12345"</span>)
                .active(<span class="keyword">true</span>)
                .addEmail(<span class="string">"john.doe@company.com"</span>)
                .addEmail(<span class="string">"j.doe@personal.com"</span>)
                .addRole(<span class="string">"User"</span>)
                .addRole(<span class="string">"Administrator"</span>)
                .build(); <span class="comment">// Locks the object in place</span>

        <span class="class-name">System</span>.out.println(user);
        <span class="comment">// Output: ScimUser [id=usr_9f830a1b, userName=john.doe@company.com, ...</span>
    }
}</code></pre>

                <div class="analogy-box">
                    <strong>Key Takeaways:</strong> This enterprise example showcases how the Builder Pattern scales to handle mandatory fields, defaults, collection management, and true immutability—making it the gold standard for creating safe, maintainable objects in production systems.
                </div>
            </section>
        </main>

        <footer>
            <p>Builder Pattern - Low Level Design Revision Guide</p>
        </footer>
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