System Design (LLD)Access Modifiers and Visibility

Access Modifiers and Visibility

LevelBeginner

Duration60 mins

TopicAccess Modifiers and Visibility

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Public — Accessible Everywhere

The Most Permissive Door

In object-oriented programming, access modifiers are the gatekeepers of your code. They determine who can see, use, or modify the members (attributes and methods) of a class. Among all access modifiers, public is the most permissive—it opens the door wide, allowing access from anywhere in your codebase.

Understanding when and how to use public is fundamental to designing well-structured software. While it offers maximum accessibility, this unrestricted access comes with significant responsibility. Misusing public can lead to tightly coupled, fragile systems where changes in one part unexpectedly break others.

What You Will Learn

By the end of this page, you will understand: • What the public access modifier means and how it works • The scope and reach of public members across your codebase • When public access is appropriate and when it's dangerous • Best practices for designing public interfaces • Real-world examples of public access in production systems

Understanding Public Access

The public access modifier is the simplest to understand: a public member is accessible from any code, in any class, in any package, from anywhere in the application. There are no restrictions whatsoever.

When you declare a member as public, you're making a contract with the outside world. You're saying: "This member is part of my official interface. Other code can depend on it, call it, and use it freely."

The Universal Visibility Principle:

Public access means:

What Public Access Allows

•Any class in the same package can access the member directly
•Any class in any other package can access the member (with proper imports)
•Subclasses inherit and can access the member regardless of their location
•Client code — any code that uses your class — can call public methods and read public fields
•Reflection and testing frameworks can access public members without any special permissions

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// File: com/payments/PaymentProcessor.java
package com.payments;
 
public class PaymentProcessor {
    // Public constant - accessible everywhere
    public static final String API_VERSION = "2.1.0";
    
    // Public method - accessible from any code
    public boolean processPayment(String customerId, double amount) {
        // Validate and process
        if (!validateAmount(amount)) {
            return false;
        }
        return executeTransaction(customerId, amount);
    }
    
    // Public getter - part of the class's public API
    public String getProcessorStatus() {
        return this.currentStatus;
    }
    
    // Private helper (not public - internal use only)
    private boolean validateAmount(double amount) {
        return amount > 0 && amount < 100000;
    }
    
    private boolean executeTransaction(String id, double amount) {
        // Internal implementation
        return true;
    }
    
    private String currentStatus = "ACTIVE";
}
 
// File: com/ecommerce/CheckoutService.java
package com.ecommerce;
 
import com.payments.PaymentProcessor;
 
public class CheckoutService {
    public void checkout(String customerId, double total) {
        PaymentProcessor processor = new PaymentProcessor();
        
        // Accessing public static field from another package
        System.out.println("Using API: " + PaymentProcessor.API_VERSION);
        
        // Calling public method from another package
        boolean success = processor.processPayment(customerId, total);
        
        // Accessing public getter from another package
        String status = processor.getProcessorStatus();
        
        // These would cause compile errors - private members:
        // processor.validateAmount(total);  // ERROR: cannot access
        // processor.currentStatus;          // ERROR: cannot access
    }
}

Language Variations

While the concept of public access is universal across OOP languages, the syntax varies: • Java: public void method() — explicit keyword • C++: public: section in class declaration • Python: No keyword needed (naming convention: no underscore prefix means public) • C#: public void Method() — similar to Java • TypeScript: public keyword (default for class members)

The Scope of Public Access

To truly understand public access, we need to visualize its scope. Unlike more restrictive modifiers that limit visibility to certain boundaries, public access transcends all boundaries.

Conceptual Model: Access Boundaries

Think of your codebase as a series of nested circles, each representing a scope of visibility:

Scope Boundaries and Public Access
Boundary	Description	Public Access?
Class Scope	Code within the same class definition	✅ Yes — always accessible
Subclass Scope	Code in classes that inherit from the class	✅ Yes — inherited and accessible
Package/Module Scope	Code in the same package or module	✅ Yes — directly accessible
External Package Scope	Code in different packages entirely	✅ Yes — accessible with import
Different Applications	Code in separate compiled applications	✅ Yes — if exposed as library/API

The Ripple Effect of Public Members:

When you make a member public, you're not just making it accessible — you're creating a dependency surface. Every piece of code that uses that public member now depends on it. This has profound implications:

The Cost of Public Exposure

•Breaking Changes Amplify — Changing a public method's signature affects every caller. With hundreds of callers, even minor changes become migration projects.
•Versioning Complexity — Public APIs must be versioned carefully. You can't simply remove or rename public members without breaking clients.
•Documentation Burden — Every public member requires documentation because external code depends on understanding its contract.
•Testing Surface Expands — Public methods need thorough testing as they're the guarantees you make to the world.
•Security Exposure — Public members are attack surfaces. Exposing internals publicly can leak implementation details that attackers exploit.

The Principle of Minimal Exposure

Make members as private as possible, and only as public as necessary. Every public member is a commitment. Once code depends on it, changing it becomes expensive. Start private, promote to public only when there's a clear need.

When Public Access is Appropriate

Despite the risks, public access is essential. Without public members, classes would be islands—unable to communicate or collaborate. The key is knowing when public access serves your design goals.

Legitimate Uses of Public Access:

When to Use Public

•Defining the Class's API — Public methods form the contract your class offers to the world. Methods like processPayment(), save(), or sendNotification() are naturally public.
•Constants and Configuration — Immutable public static fields (like Math.PI or HttpStatus.OK) provide named values that clients need.
•Factory Methods — Static methods that create instances (LocalDate.of(), List.of()) must be public for clients to call them.
•Interface Implementation — Methods declared in public interfaces must be public in implementing classes.
•Entry Points — The main() method must be public for the JVM to invoke it. Similarly, controller endpoints in web frameworks must be public.
•Framework Requirements — Many frameworks (Spring, JPA, serialization libraries) require public getters/setters for property access.

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public class UserAccount {
    // Public constants - immutable, useful for callers
    public static final int MAX_USERNAME_LENGTH = 50;
    public static final int MIN_PASSWORD_LENGTH = 8;
    
    // Private internal state - hidden from the world
    private String username;
    private String passwordHash;
    private LocalDateTime createdAt;
    private boolean isActive;
    
    // Public factory method - controlled construction
    public static UserAccount create(String username, String plainPassword) {
        if (!isValidUsername(username)) {
            throw new IllegalArgumentException("Invalid username");
        }
        if (!isValidPassword(plainPassword)) {
            throw new IllegalArgumentException("Password too weak");
        }
        return new UserAccount(username, hashPassword(plainPassword));
    }
    
    // Private constructor - forces use of factory
    private UserAccount(String username, String passwordHash) {
        this.username = username;
        this.passwordHash = passwordHash;
        this.createdAt = LocalDateTime.now();
        this.isActive = true;
    }
    
    // Public behavior methods - the class's real API
    public boolean authenticate(String plainPassword) {
        return verifyPassword(plainPassword, this.passwordHash);
    }
    
    public void deactivate() {
        this.isActive = false;
    }
    
    public boolean isActive() {
        return this.isActive;
    }
    
    // Public getter - read-only access to username
    public String getUsername() {
        return this.username;
    }
    
    // Private helpers - internal implementation
    private static boolean isValidUsername(String username) {
        return username != null && 
               username.length() <= MAX_USERNAME_LENGTH &&
               username.matches("[a-zA-Z0-9_]+");
    }
    
    private static boolean isValidPassword(String password) {
        return password != null && password.length() >= MIN_PASSWORD_LENGTH;
    }
    
    private static String hashPassword(String plain) {
        // BCrypt hashing in real code
        return "hashed:" + plain.hashCode();
    }
    
    private static boolean verifyPassword(String plain, String hash) {
        return hash.equals(hashPassword(plain));
    }
}

Analyzing the Design:

Notice how little is actually public in the UserAccount class:

Public: 2 constants, 1 factory method, 4 behavior methods, 1 getter
Private: 4 fields, 1 constructor, 4 helper methods

This ratio reveals a well-encapsulated design. The public surface is minimal and meaningful—each public member represents a deliberate choice about what the class exposes.

Public Fields — The Anti-Pattern

While public methods are essential, public fields are almost always a design mistake. They violate encapsulation by exposing internal state directly, removing the class's ability to control access or validate changes.

The Problem with Public Fields:

Public Field (Anti-Pattern)

•No validation — Anyone can set any value, even invalid ones
•No side effects — Can't trigger actions on change
•No lazy loading — Can't defer expensive computations
•No debugging hooks — Can't track who changed what
•Breaks encapsulation — Internal representation is exposed
•Locks the implementation — Can't change the field's type or storage

Public Accessor (Best Practice)

•Validation — Setter can reject invalid inputs
•Side effects — Can trigger events, logging, caching
•Lazy loading — Getter can compute or fetch on demand
•Debugging — Can log access, add breakpoints
•Encapsulation — Internal structure is hidden
•Flexibility — Can change storage without affecting callers

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// ANTI-PATTERN: Public fields
public class Rectangle {
    public double width;   // Anyone can modify
    public double height;  // No validation possible
}
 
// What can go wrong:
Rectangle rect = new Rectangle();
rect.width = -50;      // Negative width?! No error thrown.
rect.height = 0;       // Zero height?! Creates invalid rectangle.
 
// Even worse - accidentally assigned in a typo:
if (rect.width = 0) {  // Assignment, not comparison - silent bug!
    // This always executes and sets width to 0
}
 
// BEST PRACTICE: Encapsulated with accessors
public class Rectangle {
    private double width;
    private double height;
    
    public Rectangle(double width, double height) {
        setWidth(width);
        setHeight(height);
    }
    
    public void setWidth(double width) {
        if (width <= 0) {
            throw new IllegalArgumentException("Width must be positive");
        }
        this.width = width;
    }
    
    public void setHeight(double height) {
        if (height <= 0) {
            throw new IllegalArgumentException("Height must be positive");
        }
        this.height = height;
    }
    
    public double getWidth()  { return width; }
    public double getHeight() { return height; }
    public double getArea()   { return width * height; }
}
 
// Now invalid states are impossible:
Rectangle rect = new Rectangle(10, 20);  // Valid
rect.setWidth(-50);  // Throws IllegalArgumentException - caught immediately!

The Exception: Immutable Data Classes

Public fields can be acceptable in immutable data transfer objects where fields are final. In Java 16+, records provide this pattern safely:

// Record: public final fields are safe because they're immutable
public record Point(double x, double y) {}

// Usage: can't modify, so public exposure is harmless
Point p = new Point(3, 4);
System.out.println(p.x());  // 3 - can read, can't modify

Modern languages like Kotlin (data classes) and Scala (case classes) have similar constructs.

Public Access in Class Hierarchies

When classes form inheritance hierarchies, public access takes on additional dimensions. Public members are inherited by subclasses and remain public — they become part of the subclass's interface too.

Key Principles for Public Members in Hierarchies:

•Public methods are inherited — A public method in the parent is automatically public in all subclasses. Subclasses can override it but cannot reduce its visibility.
•Public static members belong to the class — They're not truly inherited; you access them via the parent class name, though syntax may allow child class access.
•Overriding cannot reduce visibility — If a parent method is public, the overriding method must also be public. You can't make it protected or private.
•Interface methods are always public — When implementing an interface, all interface methods must be public in the implementation.

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// Base class with public methods
public abstract class Animal {
    // Public method - part of Animal's contract
    public abstract void speak();
    
    // Public method with implementation
    public void sleep() {
        System.out.println("Zzz...");
    }
}
 
// Subclass inherits public methods
public class Dog extends Animal {
    @Override
    public void speak() {
        System.out.println("Woof!");
    }
    
    // sleep() is automatically inherited and public
    
    // COMPILE ERROR: Cannot reduce visibility
    // @Override
    // protected void sleep() { }  // Error: Cannot reduce visibility
}
 
// Client code sees all public methods
public class AnimalShelter {
    public void nighttime(Animal animal) {
        // Can call public methods on any Animal
        animal.speak();   // Works for any subclass
        animal.sleep();   // Inherited public method works too
    }
}
 
// Interface example - all methods are implicitly public
public interface Flyable {
    void fly();  // Implicitly public abstract
    
    default void land() {  // Implicitly public
        System.out.println("Landing...");
    }
}
 
public class Eagle extends Animal implements Flyable {
    @Override
    public void speak() {
        System.out.println("Screech!");
    }
    
    @Override
    public void fly() {  // MUST be public
        System.out.println("Soaring high!");
    }
    // land() is inherited as public from interface
}

The Liskov Substitution Implication

The rule that "you cannot reduce visibility when overriding" connects to the Liskov Substitution Principle (LSP). If client code can call parent.method(), it must also work on any child that substitutes for the parent. Reducing visibility would break this substitutability — code that worked with the parent would suddenly fail with the child.

Real-World Public API Design

Designing public APIs for libraries and frameworks requires thinking at a larger scale. Your public members will be used by thousands of developers, making backward compatibility critical.

Principles of Effective Public API Design:

Public API Design Principles
Principle	Description	Example
Minimal Surface	Expose only what clients truly need	Java's Collections.unmodifiableList() returns List, not ArrayList
Consistent Naming	Use predictable, conventional names	get/set prefixes, plural for collections, -able for capabilities
Fail Fast	Validate inputs at the public boundary	Throw IllegalArgumentException on invalid args immediately
Defensive Copies	Don't expose mutable internals	Return copies of internal collections, not the collections themselves
Document Contracts	Every public member needs clear documentation	Javadoc for methods, exceptions thrown, thread-safety guarantees
Version and Deprecate	Signal when public members will change	@Deprecated with migration path before removal

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/**
 * A thread-safe repository for storing and retrieving user data.
 * 
 * <p>All public methods in this class are thread-safe and may be called
 * from multiple threads concurrently without external synchronization.
 * 
 * @since 1.0
 * @see User
 * @see UserNotFoundException
 */
public class UserRepository {
    
    private final ConcurrentMap<String, User> users = new ConcurrentHashMap<>();
    
    /**
     * Retrieves a user by their unique identifier.
     * 
     * @param userId the unique identifier of the user, must not be null
     * @return the user with the given ID, never null
     * @throws IllegalArgumentException if userId is null or blank
     * @throws UserNotFoundException if no user exists with the given ID
     */
    public User findById(String userId) {
        // Fail fast: validate at the public boundary
        requireNonBlank(userId, "userId");
        
        User user = users.get(userId);
        if (user == null) {
            throw new UserNotFoundException("No user found with ID: " + userId);
        }
        
        // Defensive copy: don't expose the internal User object if mutable
        return user.copy();
    }
    
    /**
     * Returns all users in the repository.
     * 
     * <p>The returned list is an unmodifiable snapshot; subsequent modifications
     * to the repository will not affect the returned list.
     * 
     * @return an unmodifiable list of all users, never null but may be empty
     */
    public List<User> findAll() {
        // Defensive copy: return immutable snapshot
        return List.copyOf(users.values());
    }
    
    /**
     * Saves a user to the repository.
     * 
     * <p>If a user with the same ID already exists, it will be replaced.
     * 
     * @param user the user to save, must not be null
     * @return the saved user
     * @throws IllegalArgumentException if user is null or has invalid data
     */
    public User save(User user) {
        Objects.requireNonNull(user, "user must not be null");
        user.validate();  // User validates its own invariants
        
        // Defensive copy on input
        User toStore = user.copy();
        users.put(toStore.getId(), toStore);
        
        return toStore.copy();  // Return a copy, not the stored instance
    }
    
    /**
     * @deprecated Use {@link #findById(String)} instead. 
     *             This method will be removed in version 2.0.
     */
    @Deprecated(since = "1.5", forRemoval = true)
    public User getUser(String id) {
        return findById(id);
    }
    
    // Private helper - not part of public API
    private static void requireNonBlank(String value, String paramName) {
        if (value == null || value.isBlank()) {
            throw new IllegalArgumentException(paramName + " must not be blank");
        }
    }
}

Public API Code Review Checklist

Before making any member public, ask:

✓ Is this truly needed by external code? ✓ Can this change without breaking clients? ✓ Is the contract clearly documented? ✓ Are inputs validated at the boundary? ✓ Am I exposing mutable internals? ✓ Is, there a simpler or smaller interface that works?

If any answer is uncertain, start with package-private or protected access instead.

Common Mistakes with Public Access

Even experienced developers make mistakes with public access. Recognizing these patterns helps you avoid them in your own code.

Anti-Patterns to Avoid:

Public Access Anti-Patterns

•"Just make it public" — Developers under time pressure often make everything public to avoid access errors. This creates massive coupling and technical debt.
•Exposing internal collections — Returning this.items instead of a copy lets callers modify your internal state, causing subtle bugs.
•Public helper methods — Methods that are only meaningful for internal implementation should be private. Public calculateInternalHash() clutters the API.
•Getters and setters for everything — Blindly generating getters/setters exposes all fields, defeating encapsulation. Only expose what clients need.
•Public constructors with complex setup — If object creation requires validation or setup logic, use factory methods instead of throwing in constructors.
•Leaking implementation types — Returning ArrayList instead of List couples clients to a specific implementation. Use interface types in public signatures.

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// MISTAKE 1: Exposing internal collection
public class ShoppingCart {
    private List<Item> items = new ArrayList<>();
    
    // BAD: Returns the internal list - callers can modify it!
    public List<Item> getItems() {
        return items;  // Danger: cart.getItems().clear() empties the cart!
    }
    
    // GOOD: Return an unmodifiable view or copy
    public List<Item> getItems() {
        return Collections.unmodifiableList(items);
    }
}
 
// MISTAKE 2: Leaking implementation types
public class UserService {
    // BAD: Exposes implementation type
    public ArrayList<User> findActiveUsers() { ... }
    
    // GOOD: Use interface types
    public List<User> findActiveUsers() { ... }
}
 
// MISTAKE 3: Public constructor with complex validation
public class EmailAddress {
    private final String value;
    
    // BAD: Public constructor that throws
    public EmailAddress(String email) {
        if (!isValid(email)) {
            throw new IllegalArgumentException("Invalid email");
        }
        this.value = email;
    }
    
    // GOOD: Private constructor + public factory
    private EmailAddress(String email) {
        this.value = email;
    }
    
    public static EmailAddress of(String email) {
        if (!isValid(email)) {
            throw new IllegalArgumentException("Invalid email: " + email);
        }
        return new EmailAddress(email);
    }
    
    public static Optional<EmailAddress> tryParse(String email) {
        return isValid(email) 
            ? Optional.of(new EmailAddress(email))
            : Optional.empty();
    }
}

Summary: Mastering Public Access

We've comprehensively explored the public access modifier — the most permissive but also the most consequential visibility level in object-oriented programming. Let's consolidate the essential knowledge:

Key Takeaways

•Public means accessible everywhere — Any code, in any package, can access public members. This creates maximum flexibility but also maximum exposure.
•Public members are commitments — Once code depends on a public member, changing it affects all clients. Think carefully before making anything public.
•Prefer minimal public surfaces — Expose only what clients truly need. Start with private, promote to public only when there's a clear requirement.
•Public fields are almost always wrong — Use getters/setters or immutable records instead. Direct field access removes validation and flexibility.
•Public APIs require extra care — Validate inputs at boundaries, document contracts thoroughly, use defensive copies, and version changes carefully.
•Inheritance amplifies public access — Public methods are inherited completely, and overriding cannot reduce visibility.

What's Next:

Now that we understand public access — the wide-open door — we'll explore its polar opposite: private access. Private members are invisible to the outside world, providing the foundation for encapsulation. Understanding when to use private versus public is one of the most fundamental skills in object-oriented design.

Page Complete

You now have a comprehensive understanding of the public access modifier — its power, its costs, and the principles for using it wisely. Public access forms the visible surface of your classes, and designing that surface well is a key skill of professional software engineering.

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Loading learning content...

System Design (LLD)Access Modifiers and Visibility

Access Modifiers and Visibility

LevelBeginner

Duration60 mins

TopicAccess Modifiers and Visibility

1 / 4

Public — Accessible Everywhere

The Most Permissive Door

What You Will Learn

Understanding Public Access

The Universal Visibility Principle:

Public access means:

What Public Access Allows

•Any class in the same package can access the member directly
•Any class in any other package can access the member (with proper imports)
•Subclasses inherit and can access the member regardless of their location
•Client code — any code that uses your class — can call public methods and read public fields
•Reflection and testing frameworks can access public members without any special permissions

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// File: com/payments/PaymentProcessor.java
package com.payments;
 
public class PaymentProcessor {
    // Public constant - accessible everywhere
    public static final String API_VERSION = "2.1.0";
    
    // Public method - accessible from any code
    public boolean processPayment(String customerId, double amount) {
        // Validate and process
        if (!validateAmount(amount)) {
            return false;
        }
        return executeTransaction(customerId, amount);
    }
    
    // Public getter - part of the class's public API
    public String getProcessorStatus() {
        return this.currentStatus;
    }
    
    // Private helper (not public - internal use only)
    private boolean validateAmount(double amount) {
        return amount > 0 && amount < 100000;
    }
    
    private boolean executeTransaction(String id, double amount) {
        // Internal implementation
        return true;
    }
    
    private String currentStatus = "ACTIVE";
}
 
// File: com/ecommerce/CheckoutService.java
package com.ecommerce;
 
import com.payments.PaymentProcessor;
 
public class CheckoutService {
    public void checkout(String customerId, double total) {
        PaymentProcessor processor = new PaymentProcessor();
        
        // Accessing public static field from another package
        System.out.println("Using API: " + PaymentProcessor.API_VERSION);
        
        // Calling public method from another package
        boolean success = processor.processPayment(customerId, total);
        
        // Accessing public getter from another package
        String status = processor.getProcessorStatus();
        
        // These would cause compile errors - private members:
        // processor.validateAmount(total);  // ERROR: cannot access
        // processor.currentStatus;          // ERROR: cannot access
    }
}

Language Variations

The Scope of Public Access

To truly understand public access, we need to visualize its scope. Unlike more restrictive modifiers that limit visibility to certain boundaries, public access transcends all boundaries.

Conceptual Model: Access Boundaries

Think of your codebase as a series of nested circles, each representing a scope of visibility:

Scope Boundaries and Public Access
Boundary	Description	Public Access?
Class Scope	Code within the same class definition	✅ Yes — always accessible
Subclass Scope	Code in classes that inherit from the class	✅ Yes — inherited and accessible
Package/Module Scope	Code in the same package or module	✅ Yes — directly accessible
External Package Scope	Code in different packages entirely	✅ Yes — accessible with import
Different Applications	Code in separate compiled applications	✅ Yes — if exposed as library/API

The Ripple Effect of Public Members:

The Cost of Public Exposure

•Breaking Changes Amplify — Changing a public method's signature affects every caller. With hundreds of callers, even minor changes become migration projects.
•Versioning Complexity — Public APIs must be versioned carefully. You can't simply remove or rename public members without breaking clients.
•Documentation Burden — Every public member requires documentation because external code depends on understanding its contract.
•Testing Surface Expands — Public methods need thorough testing as they're the guarantees you make to the world.
•Security Exposure — Public members are attack surfaces. Exposing internals publicly can leak implementation details that attackers exploit.

The Principle of Minimal Exposure

When Public Access is Appropriate

Legitimate Uses of Public Access:

When to Use Public

•Defining the Class's API — Public methods form the contract your class offers to the world. Methods like processPayment(), save(), or sendNotification() are naturally public.
•Constants and Configuration — Immutable public static fields (like Math.PI or HttpStatus.OK) provide named values that clients need.
•Factory Methods — Static methods that create instances (LocalDate.of(), List.of()) must be public for clients to call them.
•Interface Implementation — Methods declared in public interfaces must be public in implementing classes.
•Entry Points — The main() method must be public for the JVM to invoke it. Similarly, controller endpoints in web frameworks must be public.
•Framework Requirements — Many frameworks (Spring, JPA, serialization libraries) require public getters/setters for property access.

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public class UserAccount {
    // Public constants - immutable, useful for callers
    public static final int MAX_USERNAME_LENGTH = 50;
    public static final int MIN_PASSWORD_LENGTH = 8;
    
    // Private internal state - hidden from the world
    private String username;
    private String passwordHash;
    private LocalDateTime createdAt;
    private boolean isActive;
    
    // Public factory method - controlled construction
    public static UserAccount create(String username, String plainPassword) {
        if (!isValidUsername(username)) {
            throw new IllegalArgumentException("Invalid username");
        }
        if (!isValidPassword(plainPassword)) {
            throw new IllegalArgumentException("Password too weak");
        }
        return new UserAccount(username, hashPassword(plainPassword));
    }
    
    // Private constructor - forces use of factory
    private UserAccount(String username, String passwordHash) {
        this.username = username;
        this.passwordHash = passwordHash;
        this.createdAt = LocalDateTime.now();
        this.isActive = true;
    }
    
    // Public behavior methods - the class's real API
    public boolean authenticate(String plainPassword) {
        return verifyPassword(plainPassword, this.passwordHash);
    }
    
    public void deactivate() {
        this.isActive = false;
    }
    
    public boolean isActive() {
        return this.isActive;
    }
    
    // Public getter - read-only access to username
    public String getUsername() {
        return this.username;
    }
    
    // Private helpers - internal implementation
    private static boolean isValidUsername(String username) {
        return username != null && 
               username.length() <= MAX_USERNAME_LENGTH &&
               username.matches("[a-zA-Z0-9_]+");
    }
    
    private static boolean isValidPassword(String password) {
        return password != null && password.length() >= MIN_PASSWORD_LENGTH;
    }
    
    private static String hashPassword(String plain) {
        // BCrypt hashing in real code
        return "hashed:" + plain.hashCode();
    }
    
    private static boolean verifyPassword(String plain, String hash) {
        return hash.equals(hashPassword(plain));
    }
}

Analyzing the Design:

Notice how little is actually public in the UserAccount class:

Public: 2 constants, 1 factory method, 4 behavior methods, 1 getter
Private: 4 fields, 1 constructor, 4 helper methods

This ratio reveals a well-encapsulated design. The public surface is minimal and meaningful—each public member represents a deliberate choice about what the class exposes.

Public Fields — The Anti-Pattern

The Problem with Public Fields:

Public Field (Anti-Pattern)

•No validation — Anyone can set any value, even invalid ones
•No side effects — Can't trigger actions on change
•No lazy loading — Can't defer expensive computations
•No debugging hooks — Can't track who changed what
•Breaks encapsulation — Internal representation is exposed
•Locks the implementation — Can't change the field's type or storage

Public Accessor (Best Practice)

•Validation — Setter can reject invalid inputs
•Side effects — Can trigger events, logging, caching
•Lazy loading — Getter can compute or fetch on demand
•Debugging — Can log access, add breakpoints
•Encapsulation — Internal structure is hidden
•Flexibility — Can change storage without affecting callers

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// ANTI-PATTERN: Public fields
public class Rectangle {
    public double width;   // Anyone can modify
    public double height;  // No validation possible
}
 
// What can go wrong:
Rectangle rect = new Rectangle();
rect.width = -50;      // Negative width?! No error thrown.
rect.height = 0;       // Zero height?! Creates invalid rectangle.
 
// Even worse - accidentally assigned in a typo:
if (rect.width = 0) {  // Assignment, not comparison - silent bug!
    // This always executes and sets width to 0
}
 
// BEST PRACTICE: Encapsulated with accessors
public class Rectangle {
    private double width;
    private double height;
    
    public Rectangle(double width, double height) {
        setWidth(width);
        setHeight(height);
    }
    
    public void setWidth(double width) {
        if (width <= 0) {
            throw new IllegalArgumentException("Width must be positive");
        }
        this.width = width;
    }
    
    public void setHeight(double height) {
        if (height <= 0) {
            throw new IllegalArgumentException("Height must be positive");
        }
        this.height = height;
    }
    
    public double getWidth()  { return width; }
    public double getHeight() { return height; }
    public double getArea()   { return width * height; }
}
 
// Now invalid states are impossible:
Rectangle rect = new Rectangle(10, 20);  // Valid
rect.setWidth(-50);  // Throws IllegalArgumentException - caught immediately!

The Exception: Immutable Data Classes

Public fields can be acceptable in immutable data transfer objects where fields are final. In Java 16+, records provide this pattern safely:

// Record: public final fields are safe because they're immutable
public record Point(double x, double y) {}

// Usage: can't modify, so public exposure is harmless
Point p = new Point(3, 4);
System.out.println(p.x());  // 3 - can read, can't modify

Modern languages like Kotlin (data classes) and Scala (case classes) have similar constructs.

Public Access in Class Hierarchies

Key Principles for Public Members in Hierarchies:

•Public methods are inherited — A public method in the parent is automatically public in all subclasses. Subclasses can override it but cannot reduce its visibility.
•Public static members belong to the class — They're not truly inherited; you access them via the parent class name, though syntax may allow child class access.
•Overriding cannot reduce visibility — If a parent method is public, the overriding method must also be public. You can't make it protected or private.
•Interface methods are always public — When implementing an interface, all interface methods must be public in the implementation.

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// Base class with public methods
public abstract class Animal {
    // Public method - part of Animal's contract
    public abstract void speak();
    
    // Public method with implementation
    public void sleep() {
        System.out.println("Zzz...");
    }
}
 
// Subclass inherits public methods
public class Dog extends Animal {
    @Override
    public void speak() {
        System.out.println("Woof!");
    }
    
    // sleep() is automatically inherited and public
    
    // COMPILE ERROR: Cannot reduce visibility
    // @Override
    // protected void sleep() { }  // Error: Cannot reduce visibility
}
 
// Client code sees all public methods
public class AnimalShelter {
    public void nighttime(Animal animal) {
        // Can call public methods on any Animal
        animal.speak();   // Works for any subclass
        animal.sleep();   // Inherited public method works too
    }
}
 
// Interface example - all methods are implicitly public
public interface Flyable {
    void fly();  // Implicitly public abstract
    
    default void land() {  // Implicitly public
        System.out.println("Landing...");
    }
}
 
public class Eagle extends Animal implements Flyable {
    @Override
    public void speak() {
        System.out.println("Screech!");
    }
    
    @Override
    public void fly() {  // MUST be public
        System.out.println("Soaring high!");
    }
    // land() is inherited as public from interface
}

The Liskov Substitution Implication

Real-World Public API Design

Designing public APIs for libraries and frameworks requires thinking at a larger scale. Your public members will be used by thousands of developers, making backward compatibility critical.

Principles of Effective Public API Design:

Public API Design Principles
Principle	Description	Example
Minimal Surface	Expose only what clients truly need	Java's Collections.unmodifiableList() returns List, not ArrayList
Consistent Naming	Use predictable, conventional names	get/set prefixes, plural for collections, -able for capabilities
Fail Fast	Validate inputs at the public boundary	Throw IllegalArgumentException on invalid args immediately
Defensive Copies	Don't expose mutable internals	Return copies of internal collections, not the collections themselves
Document Contracts	Every public member needs clear documentation	Javadoc for methods, exceptions thrown, thread-safety guarantees
Version and Deprecate	Signal when public members will change	@Deprecated with migration path before removal

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/**
 * A thread-safe repository for storing and retrieving user data.
 * 
 * <p>All public methods in this class are thread-safe and may be called
 * from multiple threads concurrently without external synchronization.
 * 
 * @since 1.0
 * @see User
 * @see UserNotFoundException
 */
public class UserRepository {
    
    private final ConcurrentMap<String, User> users = new ConcurrentHashMap<>();
    
    /**
     * Retrieves a user by their unique identifier.
     * 
     * @param userId the unique identifier of the user, must not be null
     * @return the user with the given ID, never null
     * @throws IllegalArgumentException if userId is null or blank
     * @throws UserNotFoundException if no user exists with the given ID
     */
    public User findById(String userId) {
        // Fail fast: validate at the public boundary
        requireNonBlank(userId, "userId");
        
        User user = users.get(userId);
        if (user == null) {
            throw new UserNotFoundException("No user found with ID: " + userId);
        }
        
        // Defensive copy: don't expose the internal User object if mutable
        return user.copy();
    }
    
    /**
     * Returns all users in the repository.
     * 
     * <p>The returned list is an unmodifiable snapshot; subsequent modifications
     * to the repository will not affect the returned list.
     * 
     * @return an unmodifiable list of all users, never null but may be empty
     */
    public List<User> findAll() {
        // Defensive copy: return immutable snapshot
        return List.copyOf(users.values());
    }
    
    /**
     * Saves a user to the repository.
     * 
     * <p>If a user with the same ID already exists, it will be replaced.
     * 
     * @param user the user to save, must not be null
     * @return the saved user
     * @throws IllegalArgumentException if user is null or has invalid data
     */
    public User save(User user) {
        Objects.requireNonNull(user, "user must not be null");
        user.validate();  // User validates its own invariants
        
        // Defensive copy on input
        User toStore = user.copy();
        users.put(toStore.getId(), toStore);
        
        return toStore.copy();  // Return a copy, not the stored instance
    }
    
    /**
     * @deprecated Use {@link #findById(String)} instead. 
     *             This method will be removed in version 2.0.
     */
    @Deprecated(since = "1.5", forRemoval = true)
    public User getUser(String id) {
        return findById(id);
    }
    
    // Private helper - not part of public API
    private static void requireNonBlank(String value, String paramName) {
        if (value == null || value.isBlank()) {
            throw new IllegalArgumentException(paramName + " must not be blank");
        }
    }
}

Public API Code Review Checklist

Before making any member public, ask:

If any answer is uncertain, start with package-private or protected access instead.

Common Mistakes with Public Access

Even experienced developers make mistakes with public access. Recognizing these patterns helps you avoid them in your own code.

Anti-Patterns to Avoid:

Public Access Anti-Patterns

•"Just make it public" — Developers under time pressure often make everything public to avoid access errors. This creates massive coupling and technical debt.
•Exposing internal collections — Returning this.items instead of a copy lets callers modify your internal state, causing subtle bugs.
•Public helper methods — Methods that are only meaningful for internal implementation should be private. Public calculateInternalHash() clutters the API.
•Getters and setters for everything — Blindly generating getters/setters exposes all fields, defeating encapsulation. Only expose what clients need.
•Public constructors with complex setup — If object creation requires validation or setup logic, use factory methods instead of throwing in constructors.
•Leaking implementation types — Returning ArrayList instead of List couples clients to a specific implementation. Use interface types in public signatures.

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// MISTAKE 1: Exposing internal collection
public class ShoppingCart {
    private List<Item> items = new ArrayList<>();
    
    // BAD: Returns the internal list - callers can modify it!
    public List<Item> getItems() {
        return items;  // Danger: cart.getItems().clear() empties the cart!
    }
    
    // GOOD: Return an unmodifiable view or copy
    public List<Item> getItems() {
        return Collections.unmodifiableList(items);
    }
}
 
// MISTAKE 2: Leaking implementation types
public class UserService {
    // BAD: Exposes implementation type
    public ArrayList<User> findActiveUsers() { ... }
    
    // GOOD: Use interface types
    public List<User> findActiveUsers() { ... }
}
 
// MISTAKE 3: Public constructor with complex validation
public class EmailAddress {
    private final String value;
    
    // BAD: Public constructor that throws
    public EmailAddress(String email) {
        if (!isValid(email)) {
            throw new IllegalArgumentException("Invalid email");
        }
        this.value = email;
    }
    
    // GOOD: Private constructor + public factory
    private EmailAddress(String email) {
        this.value = email;
    }
    
    public static EmailAddress of(String email) {
        if (!isValid(email)) {
            throw new IllegalArgumentException("Invalid email: " + email);
        }
        return new EmailAddress(email);
    }
    
    public static Optional<EmailAddress> tryParse(String email) {
        return isValid(email) 
            ? Optional.of(new EmailAddress(email))
            : Optional.empty();
    }
}

Summary: Mastering Public Access

Key Takeaways

•Public means accessible everywhere — Any code, in any package, can access public members. This creates maximum flexibility but also maximum exposure.
•Public members are commitments — Once code depends on a public member, changing it affects all clients. Think carefully before making anything public.
•Prefer minimal public surfaces — Expose only what clients truly need. Start with private, promote to public only when there's a clear requirement.
•Public fields are almost always wrong — Use getters/setters or immutable records instead. Direct field access removes validation and flexibility.
•Public APIs require extra care — Validate inputs at boundaries, document contracts thoroughly, use defensive copies, and version changes carefully.
•Inheritance amplifies public access — Public methods are inherited completely, and overriding cannot reduce visibility.

What's Next:

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