Checked vs Unchecked Exception

1. Introduction

In Java, exceptions are broadly classified into two categories:

• Checked Exceptions
• Unchecked Exceptions

This classification is based on when the exception is checked by the compiler.

The difference between these two types affects how developers write code, handle errors, and design APIs. Understanding this distinction is important for writing reliable Java applications.

2. What Are Checked Exceptions

A checked exception is an exception that is checked at compile time.

The Java compiler ensures that these exceptions are either:

• handled using try-catch, or
• declared using the throws keyword

If neither is done, the program will not compile.

Checked exceptions typically represent recoverable situations such as file errors, database errors, or network failures.

Example:

import java.io.FileReader;

public class Demo {

    public static void main(String[] args) throws Exception {

        FileReader reader = new FileReader("data.txt");

        System.out.println("File opened");
    }
}

Here, FileReader may throw FileNotFoundException, so the compiler forces the developer to handle or declare it.

3. Common Checked Exceptions

Some commonly used checked exceptions include:

These exceptions usually occur due to external factors outside the program's control.

4. What Are Unchecked Exceptions

An unchecked exception is an exception that occurs during runtime and is not checked by the compiler.

The compiler does not require the developer to handle these exceptions explicitly.

Unchecked exceptions usually represent programming errors or invalid logic.

Example:

public class Demo {

    public static void main(String[] args) {

        int a = 10;
        int b = 0;

        int result = a / b;
        System.out.println(result);
    }
}

Output:

Exception in thread "main" java.lang.ArithmeticException: / by zero

The compiler allows the code, but the exception occurs during execution.

5. Common Unchecked Exceptions

Some commonly encountered unchecked exceptions include:

These exceptions usually occur because of programmer mistakes.

6. Exception Hierarchy Context

Checked and unchecked exceptions both come from the Exception class.

However, unchecked exceptions are subclasses of RuntimeException.

Hierarchy overview:

Throwable
   |
   |---- Error
   |
   |---- Exception
            |
            |---- RuntimeException  (Unchecked)
            |
            |---- Other Exceptions  (Checked)

Examples:

• IOException → checked
• NullPointerException → unchecked

7. Compiler Behavior

The main difference between checked and unchecked exceptions lies in how the compiler treats them.

Checked exceptions:

• must be handled or declared
• compilation fails if ignored

Unchecked exceptions:

• optional handling
• compiler does not enforce handling

Example of compile-time error:

import java.io.FileReader;

public class Demo {

    public static void main(String[] args) {

        FileReader reader = new FileReader("file.txt");
    }
}

This fails to compile because FileNotFoundException is not handled.

8. Handling Checked Exceptions

Checked exceptions must be handled using either of these approaches.

Using try-catch

import java.io.FileReader;

public class Demo {

    public static void main(String[] args) {

        try {
            FileReader reader = new FileReader("file.txt");
        } catch (Exception e) {
            System.out.println("File not found");
        }
    }
}

Using throws

import java.io.FileReader;

public class Demo {

    public static void main(String[] args) throws Exception {

        FileReader reader = new FileReader("file.txt");
    }
}

9. When to Use Checked Exceptions

Checked exceptions are useful when the caller can reasonably recover from the error.

Examples:

• file missing
• database unavailable
• network timeout
• invalid external data

In such cases, the caller can retry or provide alternative behavior.

10. When to Use Unchecked Exceptions

Unchecked exceptions are appropriate when the error is caused by programming mistakes.

Examples:

• invalid method arguments
• null references
• incorrect array indexing
• logical errors in code

These errors should typically be fixed in the code rather than handled at runtime.

11. Key Differences Between Checked and Unchecked Exceptions

12. Best Practices

When designing applications:

• Use checked exceptions for recoverable situations.
• Use unchecked exceptions for programming mistakes.
• Avoid catching very generic exceptions unnecessarily.
• Provide meaningful exception messages.
• Do not ignore exceptions silently.

Example of good practice:

if (age < 0) {
    throw new IllegalArgumentException("Age cannot be negative");
}

13. Summary

Java exceptions are divided into two main categories: checked and unchecked exceptions.

Checked exceptions are verified by the compiler and must be handled or declared. They typically represent recoverable problems caused by external factors.

Unchecked exceptions occur during runtime and are usually caused by programming mistakes. The compiler does not enforce their handling.

Understanding the difference between these two types helps developers design better error handling strategies and create more reliable Java applications.

Written By: Shiva Srivastava