Throwing custom exceptions in Java - Mini Project: Build & Apply

Throwing custom exceptions

📖 Scenario: Imagine you are building a simple banking application. You want to make sure that users cannot withdraw more money than they have in their account. To do this, you will create a custom exception called InsufficientFundsException and throw it when a withdrawal amount is too large.

🎯 Goal: Build a Java program that defines a custom exception InsufficientFundsException, uses it in a BankAccount class, and throws this exception when a withdrawal is attempted with insufficient balance.

📋 What You'll Learn

Create a custom exception class called InsufficientFundsException that extends Exception.

Create a BankAccount class with a balance variable.

Add a method withdraw that throws InsufficientFundsException if withdrawal amount is greater than balance.

Write code to catch the InsufficientFundsException and print a friendly message.

💡 Why This Matters

🌍 Real World

Custom exceptions are used in real-world applications to handle specific error cases clearly, such as banking errors, file handling problems, or invalid user input.

💼 Career

Knowing how to create and throw custom exceptions is important for writing robust Java applications and is a common requirement in software development jobs.

1

Create the custom exception class

Create a public class called InsufficientFundsException that extends Exception. Add a constructor that takes a String message and passes it to the superclass constructor using super(message).

Java

// Your code here

Need a hint?

Remember, to create a custom exception, extend the Exception class and call super(message) in the constructor.

2

Create the BankAccount class with balance

Create a public class called BankAccount. Inside it, declare a private double variable called balance. Add a constructor that takes a double parameter initialBalance and sets balance to it.

Java

public class InsufficientFundsException extends Exception {
    public InsufficientFundsException(String message) {
        super(message);
    }
}

// Your code here

Need a hint?

Define the balance variable as private and initialize it in the constructor.

3

Add withdraw method that throws the custom exception

Inside the BankAccount class, add a public method called withdraw that takes a double parameter amount and throws InsufficientFundsException. If amount is greater than balance, throw a new InsufficientFundsException with the message "Insufficient funds for withdrawal". Otherwise, subtract amount from balance.

Java

public class InsufficientFundsException extends Exception {
    public InsufficientFundsException(String message) {
        super(message);
    }
}

public class BankAccount {
    private double balance;

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

    // Your code here
}

Need a hint?

Use throw new InsufficientFundsException("Insufficient funds for withdrawal") when the amount is too large.

4

Test withdrawal and catch the exception

Create a public class called Main with a main method. Inside main, create a BankAccount object with initial balance 100.0. Use a try-catch block to call withdraw(150.0) on the account. Catch InsufficientFundsException and print the exception message using System.out.println(e.getMessage()).

Java

public class InsufficientFundsException extends Exception {
    public InsufficientFundsException(String message) {
        super(message);
    }
}

public class BankAccount {
    private double balance;

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

    public void withdraw(double amount) throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException("Insufficient funds for withdrawal");
        }
        balance -= amount;
    }
}

// Your code here

Need a hint?

Use a try-catch block to catch InsufficientFundsException and print the message.