Java Custom Exceptions: Creating Robust User-Defined Exceptions

While the standard Java runtime library provides dozens of built-in exception types, enterprise applications require domain-specific custom exceptions (e.g. InsufficientFundsException, InvalidAadhaarNumberException) to communicate business rule violations clearly.

Step 1: Defining the Custom Exception Class

// Custom Checked Exception: Extends Exception
public class InsufficientFundsException extends Exception {
    private double attemptedAmount;
    private double currentBalance;

    // Standard constructor passing descriptive message to superclass
    public InsufficientFundsException(String message, double attempted, double current) {
        super(message);
        this.attemptedAmount = attempted;
        this.currentBalance = current;
    }

    // Custom getters for detailed diagnostic reporting
    public double getAttemptedAmount() { return attemptedAmount; }
    public double getCurrentBalance() { return currentBalance; }
}

Step 2: Throwing and Catching in Business Logic

public class BankAccount {
    private double balance = 5000.0;

    public void withdraw(double amount) throws InsufficientFundsException {
        if (amount > balance) {
            // Throw custom exception with domain diagnostics
            throw new InsufficientFundsException(
                "Withdrawal rejected: Overdraft limit exceeded!", 
                amount, 
                balance
            );
        }
        balance -= amount;
        System.out.println("Withdrawal approved. Remaining balance: $" + balance);
    }

    public static void main(String[] args) {
        BankAccount acc = new BankAccount();
        try {
            acc.withdraw(7500.0); // Fails!
        } catch (InsufficientFundsException e) {
            System.err.println("Transaction Failed: " + e.getMessage());
            System.err.println("Attempted: $" + e.getAttemptedAmount());
            System.err.println("Available: $" + e.getCurrentBalance());
        }
    }
}