Data Hiding in OOP

public class Temperature { private double celsius; // Hidden data

public Temperature(double celsius) { setCelsius(celsius); // Use setter for validation }

public double getCelsius() { return celsius; }

public void setCelsius(double celsius) { if (celsius < -273.15) { // Absolute zero check throw new IllegalArgumentException( "Temperature cannot be below absolute zero (-273.15°C)"); } this.celsius = celsius; }

public double getFahrenheit() { return celsius * 9.0 / 5.0 + 32; }

public double getKelvin() { return celsius + 273.15; } }

class EncryptedMessage { private: std::string content; // Hidden std::string key; // Hidden bool encrypted; // Hidden

// Private helper methods std::string xorCipher(const std::string& text, const std::string& key) { std::string result = text; for (size_t i = 0; i < text.size(); i++) { result[i] = text[i] ^ key[i % key.size()]; } return result; }

public: EncryptedMessage(const std::string& msg, const std::string& key) : content(msg), key(key), encrypted(false) {}

void encrypt() { if (!encrypted) { content = xorCipher(content, key); encrypted = true; } }

std::string decrypt(const std::string& providedKey) { if (providedKey != key) { throw std::runtime_error("Invalid key"); } if (encrypted) { return xorCipher(content, key); } return content; }

bool isEncrypted() const { return encrypted; } };

class CreditCard: def __init__(self, number, holder, cvv): self.__number = number # Name-mangled (strongly hidden) self.__holder = holder self.__cvv = cvv self.__transactions = []

@property def masked_number(self): """Only reveals last 4 digits""" return "**--**-" + self.__number[-4:]

@property def holder(self): return self.__holder

def charge(self, amount, merchant): if amount <= 0: raise ValueError("Amount must be positive") self.__transactions.append({ "amount": amount, "merchant": merchant }) return True

def get_statement(self): """Read-only view of transactions""" return [t.copy() for t in self.__transactions] # Return copies!

card = CreditCard("4532015112830366", "John Doe", "123") print(card.masked_number) # **--**-0366

print(card.__cvv) # AttributeError!

public class AudioPlayer { // Level 1: Completely hidden implementation detail private byte[] rawAudioBuffer; private int bufferPosition;

// Level 2: Hidden from world, available to subclasses protected String currentFormat; protected int sampleRate;

// Level 3: Public interface public void play(String filename) { /* ... */ } public void pause() { /* ... */ } public void stop() { /* ... */ } public int getVolume() { /* ... */ } public void setVolume(int level) { /* ... */ } }

public class Schedule { private List<String> appointments;

// BAD: Exposes internal list — caller can modify it! public List<String> getAppointments() { return appointments; // Returns the actual internal list }

// GOOD: Returns a defensive copy public List<String> getAppointments() { return new ArrayList<>(appointments); // Returns a copy }

// BETTER: Returns unmodifiable view public List<String> getAppointments() { return Collections.unmodifiableList(appointments); } }