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Inheritance & Polymorphism

Side‑by‑side Java snippet · similarity & difference

🧬 Inheritance extends

// Parent class
class BankAccount {
  protected double balance;

  public void deposit(double amt) {
    balance += amt;
    System.out.println("Deposited: " + amt);
  }
}

// Child class inherits structure + behavior
class SavingsAccount extends BankAccount {
  private double interestRate;

  public void addInterest() {
    balance += balance * interestRate;
  }
}

// Usage – reuse via inheritance
SavingsAccount sa = new SavingsAccount();
sa.deposit(500);   // inherited method
sa.addInterest(); // own method

🎭 Polymorphism @Override

// Parent class defines common interface
class BankAccount {
  protected double balance;

  public void deposit(double amt) {
    balance += amt;
    System.out.println("Deposited: " + amt);
  }

  // overridden by subclasses
  public void accountType() {
    System.out.println("Generic account");
  }
}

class SavingsAccount extends BankAccount {
  @Override
  public void accountType() {
    System.out.println("Savings account");
  }
}

// Same call → different behavior
BankAccount acc = new SavingsAccount();
acc.accountType(); // "Savings account"

✅ Similarities

• Both use the extends keyword to build a parent–child relationship.
• Both rely on the same parent class (BankAccount) as a shared blueprint.
• Child class (SavingsAccount) automatically inherits fields like balance and methods like deposit().
• Both enable code reuse — common logic lives only in the superclass.
• Both are core pillars of OOP and appear together in real class diagrams.

⚠️ Differences

• Purpose: Inheritance = reuse structure; Polymorphism = vary behavior.
• Mechanism: Inheritance uses extends; polymorphism uses @Override (method overriding).
• Behavior: Inherited method deposit() runs identically in child; overridden accountType() runs differently.
• Reference type: Polymorphism allows BankAccount acc = new SavingsAccount() — inheritance alone does not.
• Diagram symbol: Inheritance = hollow triangle ▷; Polymorphism = often shown via override notes or dynamic dispatch.

🔑 Key insight: The two snippets share the same parent class and inheritance structure — but the right side adds overriding, turning a static “is‑a” relationship into a dynamic, interchangeable behavior. Inheritance gives you the blueprint; polymorphism lets you swap the behavior behind a common interface.