Object Oriented Programming in Java emphasizes abstraction, encapsulation, inheritance, and polymorphism. Among these principles, abstraction plays a vital role in reducing complexity by hiding implementation details and exposing only essential features. In Java, abstraction can be achieved through abstract classes and interfaces. An abstract class allows partial abstraction by including both abstract and concrete methods, while an interface provides complete abstraction by defining method signatures without implementation. The following Java programs demonstrate the use of abstract class and interface with necessary assumptions. An abstract class is declared using the keyword abstract. It cannot be instantiated directly and may contain abstract methods (without body) as well as concrete methods (with body). Abstract classes are useful when different subclasses share common behavior but also require specific implementations of certain methods. Consider a scenario of a banking system in which different types of accounts calculate interest differently. We assume that all accounts have an account number and balance, and each type of account must implement its own method to calculate interest. The following program demonstrates the use of an abstract class: ```java abstract class BankAccount { protected int accountNumber; protected double balance; public BankAccount(int accountNumber, double balance) { this.accountNumber = accountNumber; this.balance = balance; } public void deposit(double amount) { balance += amount; System.out.println("Amount deposited: " + amount); } public void displayBalance() { System.out.println("Account Number: " + accountNumber); System.out.println("Current Balance: " + balance); } public abstract double calculateInterest(); } class SavingsAccount extends BankAccount { private double interestRate = 0.04; public SavingsAccount(int accountNumber, double balance) { super(accountNumber, balance); } public double calculateInterest() { return balance * interestRate; } } class CurrentAccount extends BankAccount { private double interestRate = 0.02; public CurrentAccount(int accountNumber, double balance) { super(accountNumber, balance); } public double calculateInterest() { return balance * interestRate; } } public class AbstractClassDemo { public static void main(String[] args) { BankAccount savings = new SavingsAccount(101, 10000); BankAccount current = new CurrentAccount(102, 15000); savings.deposit(2000);