Abstract Classes vs Concrete Classes

Intermediate13 min read•Updated: 2026-10-22

📐 Abstract Classes vs Concrete Classes

Introduction

An abstract class (declared with abstract) represents a partial blueprint. It can contain both implemented methods with code bodies and abstract methods (methods without a body) that subclasses are forced to implement.

💻 Abstract Class Architecture

1abstract class BankAccount {
2    protected String accountNumber;
3    protected double balance;
4 
5    public BankAccount(String accNum, double balance) {
6        this.accountNumber = accNum;
7        this.balance = balance;
8    }
9 
10    // Concrete method: Reusable shared behavior
11    public void deposit(double amount) {
12        if (amount > 0) this.balance += amount;
13    }
14 
15    // Abstract method: Every subclass MUST define its own interest formula!
16    public abstract void applyMonthlyInterest();
17 
18    public double getBalance() { return balance; }
19}
20 
21class SavingsAccount extends BankAccount {
22    public SavingsAccount(String accNum, double balance) {
23        super(accNum, balance);
24    }
25 
26    @Override
27    public void applyMonthlyInterest() {
28        double interest = balance * (0.045 / 12.0); // 4.5% annual interest
29        balance += interest;
30        System.out.printf("Credited Savings Interest: ₹%.2f%n", interest);
31    }
32}
33 
34public class AbstractDemo {
35    public static void main(String[] args) {
36        // BankAccount acc = new BankAccount(); // 💥 Compiler Error: Cannot instantiate abstract class!
37        SavingsAccount sa = new SavingsAccount("MSK-1001", 50000.0);
38        sa.applyMonthlyInterest();
39    }
40}

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Interfaces: Declaration, Multiple Interface Implementation, and default/static Methods

Abstract Classes vs Concrete Classes

Intermediate13 min read•Updated: 2026-10-22

Study Materials

📐 Abstract Classes vs Concrete Classes

Introduction

💻 Abstract Class Architecture

1abstract class BankAccount {
2    protected String accountNumber;
3    protected double balance;
4 
5    public BankAccount(String accNum, double balance) {
6        this.accountNumber = accNum;
7        this.balance = balance;
8    }
9 
10    // Concrete method: Reusable shared behavior
11    public void deposit(double amount) {
12        if (amount > 0) this.balance += amount;
13    }
14 
15    // Abstract method: Every subclass MUST define its own interest formula!
16    public abstract void applyMonthlyInterest();
17 
18    public double getBalance() { return balance; }
19}
20 
21class SavingsAccount extends BankAccount {
22    public SavingsAccount(String accNum, double balance) {
23        super(accNum, balance);
24    }
25 
26    @Override
27    public void applyMonthlyInterest() {
28        double interest = balance * (0.045 / 12.0); // 4.5% annual interest
29        balance += interest;
30        System.out.printf("Credited Savings Interest: ₹%.2f%n", interest);
31    }
32}
33 
34public class AbstractDemo {
35    public static void main(String[] args) {
36        // BankAccount acc = new BankAccount(); // 💥 Compiler Error: Cannot instantiate abstract class!
37        SavingsAccount sa = new SavingsAccount("MSK-1001", 50000.0);
38        sa.applyMonthlyInterest();
39    }
40}

Next Lesson

Interfaces: Declaration, Multiple Interface Implementation, and default/static Methods

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
The final Keyword: Final Variables, Methods, and Classes	Interfaces: Declaration, Multiple Interface Implementation, and default/static Methods

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next