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<!DOCTYPE html>
<html lang="en">
<head>
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    <title>Functional Interface - Quick Revision Guide</title>
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</head>
<body>
<div class="container">
    <header>
        <h1>Java Functional Interfaces - Real-World Revision Guide</h1>
        <p>SAM, lambdas, built-in interfaces, and custom domain interfaces.</p>
    </header>

    <main>
        <section>
            <h2><span class="icon">🧠</span>Core Concepts</h2>
            <ul>
                <li><strong>Functional Interface:</strong> Exactly <span class="highlight">one abstract method</span> (SAM).</li>
                <li><strong>@FunctionalInterface:</strong> Enforces SAM at compile-time and improves readability.</li>
                <li><strong>Can still have:</strong> Multiple <code>default</code> and <code>static</code> methods.</li>
                <li><strong>Main benefit:</strong> Enables passing behavior as lambda expressions.</li>
            </ul>
        </section>

        <section>
            <h2><span class="icon">❓</span>Why Not Classes?</h2>
            <div class="solution-box">
                <p><code>@FunctionalInterface</code> applies only to interfaces. Classes represent structure/state, while functional interfaces define behavior contracts that lambdas implement dynamically.</p>
            </div>
        </section>

        <section>
            <h2><span class="icon">📦</span>Example 1: Built-in Interface (<code>Function&lt;T, R&gt;</code>)</h2>
            <p><strong>Use case:</strong> Convert <code>UserEntity</code> (DB model) into <code>UserDTO</code> (API-safe model).</p>
            <p><strong>Key method:</strong> <code>R apply(T t)</code> — called directly or by <code>Stream.map()</code> internally.</p>

            <pre><code>import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

class UserEntity {
    private final Long id;
    private final String firstName;
    private final String lastName;
    private final String email;
    private final String passwordHash;

    public UserEntity(Long id, String firstName, String lastName, String email, String passwordHash) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        this.email = email;
        this.passwordHash = passwordHash;
    }

    public Long getId() { return id; }
    public String getFirstName() { return firstName; }
    public String getLastName() { return lastName; }
    public String getEmail() { return email; }
}

class UserDTO {
    private final Long id;
    private final String fullName;
    private final String email;

    public UserDTO(Long id, String fullName, String email) {
        this.id = id;
        this.fullName = fullName;
        this.email = email;
    }

    @Override
    public String toString() {
        return "UserDTO{id=" + id + ", fullName='" + fullName + "', email='" + email + "'}";
    }
}

public class DtoMappingExample {
    private static final Function&lt;UserEntity, UserDTO&gt; convertToDTO = entity ->
        new UserDTO(
            entity.getId(),
            entity.getFirstName() + " " + entity.getLastName(),
            entity.getEmail().toLowerCase()
        );

    public static void main(String[] args) {
        UserEntity singleDbUser = new UserEntity(101L, "Alice", "Wonderland", "ALICE@example.com", "$2a$12$pPlk8..");
        UserDTO singleUserDTO = convertToDTO.apply(singleDbUser);
        System.out.println(singleUserDTO);

        List&lt;UserEntity&gt; databaseUsers = Arrays.asList(
            new UserEntity(1L, "John", "Doe", "John.Doe@example.com", "$2a$12$eImiTx.."),
            new UserEntity(2L, "Jane", "Smith", "JANE.SMITH@example.com", "$2a$12$Kj9xY..")
        );

        List&lt;UserDTO&gt; apiResponse = databaseUsers.stream()
                .map(convertToDTO)
                .collect(Collectors.toList());

        apiResponse.forEach(System.out::println);
    }
}</code></pre>
        </section>

        <section>
            <h2><span class="icon">🧩</span>Example 2: Custom Functional Interface (Domain Logic)</h2>
            <p><strong>Use case:</strong> Credential validation with explicit business intent.</p>

            <pre><code>@FunctionalInterface
public interface IdentityValidator {
    boolean validate(String username, String password);
}

public class AuthenticationRegistry {
    public void executeValidation() {
        IdentityValidator prodCheck = (user, pass) ->
            user.startsWith("admin_") && pass.length() > 12;

        boolean result = prodCheck.validate("admin_root", "SecurePassword123!");
        System.out.println("Authentication validation passing status: " + result);
    }

    public static void main(String[] args) {
        new AuthenticationRegistry().executeValidation();
    }
}</code></pre>

            <div class="analogy-box">
                <strong>Key takeaway:</strong> <code>IdentityValidator</code> is clearer and more domain-driven than generic <code>BiPredicate&lt;String, String&gt;</code>.
            </div>
        </section>

        <section>
            <h2><span class="icon">✅</span>Quick Revision Checklist</h2>
            <ul>
                <li>SAM = one abstract method only.</li>
                <li>Prefer <code>@FunctionalInterface</code> for compiler checks.</li>
                <li>Use built-ins for common transformations/filtering.</li>
                <li>Create custom interfaces when business meaning matters.</li>
                <li>Lambdas make code concise, composable, and test-friendly.</li>
            </ul>
        </section>
    </main>

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        Functional Interface Revision Page • Java
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