Java Custom Exceptions: Tailoring Error Handling for Your Application

Explore the concept of custom exceptions in Java, which are user-defined exceptions allowing you to manage errors according to your application's specific requirements. Derived from the Exception class, custom exceptions provide greater flexibility and control in error handling. Learn why you might need custom exceptions and how to implement them effectively to enhance your Java application's robustness and maintainability.

Java - Custom Exception

Java Custom Exception Overview

A custom exception is a user-defined exception that allows you to handle errors according to your application's specific needs. These exceptions are derived from the Exception class.

Need for Java Custom Exceptions

• To categorize exceptions based on different types of errors in your project.
• To enable application-level exception handling.

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Creating a Custom Exception in Java

To create a custom exception, define a class that extends the Exception class.

Syntax

class MyException extends Exception {
}

By extending the Exception class, you create your own checked exception.

Rules for Creating Custom Exceptions

• All exceptions must be a child of Throwable.
• For a checked exception, extend the Exception class.
• For a runtime exception, extend the RuntimeException class.

Java Custom Exception Example

The InsufficientFundsException class is a user-defined checked exception:

class InsufficientFundsException extends Exception {
private double amount;

public InsufficientFundsException(double amount) {
this.amount = amount;
}

public double getAmount() {
return amount;
}
}

To use this exception, the CheckingAccount class contains a withdraw() method that may throw an InsufficientFundsException:

class CheckingAccount {
private double balance;
private int number;

public CheckingAccount(int number) {
this.number = number;
}

public void deposit(double amount) {
balance += amount;
}

public void withdraw(double amount) throws InsufficientFundsException {
if(amount <= balance) {
    balance -= amount;
} else {
    double needs = amount - balance;
    throw new InsufficientFundsException(needs);
}
}

public double getBalance() {
return balance;
}

public int getNumber() {
return number;
}
}

The following BankDemo program demonstrates the usage of the deposit() and withdraw() methods:

package com.tutorialsarena;

public class BankDemo {

public static void main(String [] args) {
CheckingAccount c = new CheckingAccount(101);
System.out.println("Depositing $400...");
c.deposit(400.00);

try {
    System.out.println("\nWithdrawing $100...");
    c.withdraw(100.00);
    System.out.println("\nWithdrawing $350...");
    c.withdraw(350.00);
} catch (InsufficientFundsException e) {
    System.out.println("Sorry, but you are short $" + e.getAmount());
    e.printStackTrace();
}
}
}

Output

Depositing $400...

Withdrawing $100...

Withdrawing $350...
Sorry, but you are short $250.0
com.tutorialsarena.InsufficientFundsException
at com.tutorialsarena.CheckingAccount.withdraw(BankDemo.java:39)
at com.tutorialsarena.BankDemo.main(BankDemo.java:14)

Creating an Unchecked Exception

In the next example, we will create an unchecked exception by extending the RuntimeException class:

class InsufficientFundsException extends RuntimeException {
private double amount;

public InsufficientFundsException(double amount) {
this.amount = amount;
}

public double getAmount() {
return amount;
}
}

The following BankDemo program demonstrates the usage of the deposit and withdraw methods with unchecked exceptions:

package com.tutorialsarena;

public class BankDemo {

public static void main(String [] args) {
CheckingAccount c = new CheckingAccount(101);
System.out.println("Depositing $300...");
c.deposit(300.00);

System.out.println("\nWithdrawing $100...");
c.withdraw(100.00);
System.out.println("\nWithdrawing $250...");
c.withdraw(250.00);
}
}

Output

Depositing $300...

Withdrawing $100...

Withdrawing $250...
Exception in thread "main" 
com.tutorialsarena.InsufficientFundsException
at com.tutorialsarena.CheckingAccount.withdraw(BankDemo.java:35)
at com.tutorialsarena.BankDemo.main(BankDemo.java:13)