Phase 12: The 4 Pillars Implemented in Code

See all 4 OOP Pillars side-by-side in executable Java code snippet form.

Pillar 1: Encapsulation in Code

public class BankAccount {
    private double balance; // Private field protects state

    public BankAccount(double initialBalance) {
        if (initialBalance >= 0) this.balance = initialBalance;
    }

    public double getBalance() { return this.balance; }

    public void deposit(double amount) {
        if (amount > 0) this.balance += amount; // Protected invariant
    }
}
        

Pillar 2: Abstraction in Code

// Interface defines abstract contract (what to do, hiding how)
public interface PaymentGateway {
    boolean processPayment(double amount);
}

// Concrete class implements internal payment details
public class StripePayment implements PaymentGateway {
    @Override
    public boolean processPayment(double amount) {
        // Complex HTTP API call abstracted away!
        System.out.println("Processing $" + amount + " via Stripe.");
        return true;
    }
}
        

Pillar 3: Inheritance in Code

// Superclass (Parent)
public class Employee {
    protected String name;
    protected double salary;

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }
}

// Subclass (Child) inherits name and salary from Employee (IS-A relationship)
public class SoftwareEngineer extends Employee {
    private String programmingLanguage;

    public SoftwareEngineer(String name, double salary, String lang) {
        super(name, salary); // Reuses parent constructor!
        this.programmingLanguage = lang;
    }
}
        

Pillar 4: Polymorphism in Code

public class Main {
    public static void main(String[] args) {
        // Polymorphic reference array (Parent reference -> Child objects)
        PaymentGateway[] gateways = new PaymentGateway[] {
            new StripePayment(),
            new PayPalPayment()
        };

        // Same interface method call produces multiple different behaviors!
        for (PaymentGateway pg : gateways) {
            pg.processPayment(150.0);
        }
    }
}
        