Checked vs unchecked exceptions in Java - Hands-On Comparison

Checked vs Unchecked Exceptions in Java

📖 Scenario: Imagine you are writing a simple Java program that reads a file and processes its content. Sometimes, the file might not be found, or there might be an error in the program logic. Java uses two types of exceptions to handle these situations: checked and unchecked exceptions.

🎯 Goal: You will create a Java program that demonstrates the difference between checked and unchecked exceptions by handling a file reading operation and a division operation that might cause an error.

📋 What You'll Learn

Create a method that throws a checked exception (FileNotFoundException).

Create a method that throws an unchecked exception (ArithmeticException).

Use try-catch blocks to handle the checked exception.

Do not handle the unchecked exception explicitly.

Print messages to show when exceptions are caught or when the program continues.

💡 Why This Matters

🌍 Real World

Handling checked and unchecked exceptions is essential when working with files, databases, and user input in real-world Java applications.

💼 Career

Understanding exception types helps developers write robust Java code that gracefully handles errors and improves software quality.

1

Create a method that throws a checked exception

Create a method called readFile that throws FileNotFoundException. Inside the method, write throw new FileNotFoundException("File not found").

Java

import java.io.FileNotFoundException;

public class ExceptionDemo {
    // Create method readFile that throws FileNotFoundException
    // Your code here
}

Hint

Remember to declare the exception in the method signature using throws FileNotFoundException.

2

Create a method that throws an unchecked exception

Add a method called divide that takes two integers a and b and returns the result of a / b. This method will throw an unchecked ArithmeticException if b is zero.

Java

import java.io.FileNotFoundException;

public class ExceptionDemo {
    public static void readFile() throws FileNotFoundException {
        throw new FileNotFoundException("File not found");
    }

    // Create method divide that returns a / b
    // Your code here
}

Hint

Unchecked exceptions like ArithmeticException do not need to be declared in the method signature.

3

Handle the checked exception with try-catch

In the main method, call readFile() inside a try-catch block. Catch the FileNotFoundException and print "Caught checked exception: " followed by the exception message.

Java

import java.io.FileNotFoundException;

public class ExceptionDemo {
    public static void readFile() throws FileNotFoundException {
        throw new FileNotFoundException("File not found");
    }

    public static int divide(int a, int b) {
        return a / b;
    }

    public static void main(String[] args) {
        // Call readFile() inside try-catch
        // Your code here
    }
}

Hint

Use a try block to call readFile() and catch the exception with catch (FileNotFoundException e).

4

Call the unchecked exception method and print result

In the main method, after the try-catch block, call divide(10, 0) and print the result with System.out.println("Result: " + result). Do not use try-catch for this call.

Java

import java.io.FileNotFoundException;

public class ExceptionDemo {
    public static void readFile() throws FileNotFoundException {
        throw new FileNotFoundException("File not found");
    }

    public static int divide(int a, int b) {
        return a / b;
    }

    public static void main(String[] args) {
        try {
            readFile();
        } catch (FileNotFoundException e) {
            System.out.println("Caught checked exception: " + e.getMessage());
        }

        // Call divide(10, 0) and print result
        // Your code here
    }
}

Hint

Calling divide(10, 0) will cause an ArithmeticException and stop the program. This shows unchecked exceptions do not require try-catch.

Practice

1. Which of the following statements correctly describes checked exceptions in Java?

easy

A. They are runtime exceptions that do not require handling.

B. They are errors that cannot be handled by the program.

C. They must be either caught or declared in the method signature.

D. They are always caused by bugs in the code.

Solution

1. Step 1: Understand checked exceptions

   Checked exceptions are exceptions that the compiler forces you to handle or declare.

2. Step 2: Compare options with definition

   They must be either caught or declared in the method signature. states they must be caught or declared, which matches the definition.

3. Final Answer:

   They must be either caught or declared in the method signature. -> Option C

4. Quick Check:

   Checked exceptions require handling = A [OK]

Hint: Checked exceptions need try-catch or throws declaration [OK]

Common Mistakes:

• Confusing checked with unchecked exceptions
• Thinking checked exceptions are always bugs
• Believing checked exceptions don't need handling

2. Which of the following is the correct way to declare a method that throws a checked exception IOException?

easy

A. public void readFile() throws IOException {}

B. public void readFile() throw IOException {}

C. public void readFile() throws IOException() {}

D. public void readFile() throws new IOException {}

Solution

1. Step 1: Recall correct syntax for throws

   The correct syntax uses the keyword throws followed by the exception class name without parentheses.

2. Step 2: Analyze each option

   public void readFile() throws IOException {} matches the correct syntax exactly. Options A, B, and C have syntax errors.

3. Final Answer:

   public void readFile() throws IOException {} -> Option A

4. Quick Check:

   Correct throws syntax = D [OK]

Hint: Use 'throws ExceptionName' without parentheses [OK]

Common Mistakes:

• Using 'throw' instead of 'throws' in method signature
• Adding parentheses after exception name
• Trying to instantiate exception in throws clause

3. What will be the output of the following Java code?

public class Test {
  public static void main(String[] args) {
    try {
      int a = 5 / 0;
    } catch (ArithmeticException e) {
      System.out.println("Caught unchecked exception");
    }
  }
}

medium

A. Compilation error due to unchecked exception

B. Caught checked exception

C. No output, program crashes silently

D. Caught unchecked exception

Solution

1. Step 1: Identify exception type

   Dividing by zero causes an ArithmeticException, which is an unchecked exception.

2. Step 2: Check catch block handling

   The catch block catches ArithmeticException and prints "Caught unchecked exception".

3. Final Answer:

   Caught unchecked exception -> Option D

4. Quick Check:

   Unchecked exceptions can be caught = B [OK]

Hint: ArithmeticException is unchecked and can be caught [OK]

Common Mistakes:

• Thinking unchecked exceptions cause compile errors
• Assuming program crashes without output
• Confusing checked and unchecked exceptions

4. Consider this code snippet:

public void readFile() {
  FileReader fr = new FileReader("file.txt");
}

What is the error and how to fix it?

medium

A. FileReader constructor syntax is wrong; remove parentheses.

B. FileReader constructor throws checked IOException; add try-catch or declare throws.

C. FileReader is an unchecked exception; no fix needed.

D. FileReader must be imported from java.util package.

Solution

1. Step 1: Identify exception type from FileReader

   The FileReader constructor throws a checked IOException.

2. Step 2: Fix by handling or declaring exception

   We must either surround with try-catch or declare throws IOException in method signature.

3. Final Answer:

   FileReader constructor throws checked IOException; add try-catch or declare throws. -> Option B

4. Quick Check:

   Checked exceptions require handling = C [OK]

Hint: Checked exceptions must be caught or declared [OK]

Common Mistakes:

• Ignoring checked exception requirement
• Confusing package for FileReader
• Thinking FileReader throws unchecked exceptions

5. You want to write a method that reads a file and returns its first line. The method should handle IOException internally but let runtime exceptions propagate. Which approach correctly applies checked vs unchecked exceptions?

hard

A. Use try-catch for IOException inside method; do not catch RuntimeExceptions.

B. Declare method throws IOException; catch RuntimeExceptions inside method.

C. Catch both IOException and RuntimeException inside method.

D. Do not catch any exceptions; let all propagate.

Solution

1. Step 1: Understand handling checked exceptions

   Checked exceptions like IOException should be handled or declared. Here, handling internally means try-catch inside method.

2. Step 2: Understand unchecked exceptions handling

   Unchecked exceptions (RuntimeExceptions) usually indicate bugs and should propagate to caller, so do not catch them here.

3. Final Answer:

   Use try-catch for IOException inside method; do not catch RuntimeExceptions. -> Option A

4. Quick Check:

   Handle checked, propagate unchecked = A [OK]

Hint: Catch checked exceptions; let unchecked exceptions propagate [OK]

Common Mistakes:

• Catching unchecked exceptions unnecessarily
• Declaring checked exceptions when handling internally
• Ignoring exception handling rules