Checked vs unchecked exceptions in Java - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of code with checked exception handling

What is the output of this Java code snippet?

Java

import java.io.*;

public class Test {
    public static void main(String[] args) {
        try {
            throw new IOException("IO error");
        } catch (IOException e) {
            System.out.println("Caught checked exception");
        }
    }
}

A Compilation error: unhandled exception

B Caught unchecked exception

C Caught checked exception

D No output

💡 Hint

Think about how checked exceptions must be caught or declared.

IOException is a checked exception. The code catches it and prints the message.

❓ Predict Output

intermediate

Output of code with unchecked exception

What happens when this Java code runs?

Java

public class Test {
    public static void main(String[] args) {
        throw new NullPointerException("Null pointer");
    }
}

A Program throws NullPointerException and terminates

B Program prints "Null pointer"

C Compilation error: unhandled exception

D Program runs without output

💡 Hint

Unchecked exceptions do not need to be caught or declared.

NullPointerException is unchecked and not caught, so the program terminates with the exception.

🧠 Conceptual

advanced

Difference between checked and unchecked exceptions

Which statement correctly describes the difference between checked and unchecked exceptions in Java?

A Neither checked nor unchecked exceptions require catching or declaring.

B Unchecked exceptions must be caught or declared; checked exceptions do not require this.

C Both checked and unchecked exceptions must always be caught or declared.

D Checked exceptions must be caught or declared; unchecked exceptions do not require this.

💡 Hint

Think about compiler enforcement for exception handling.

Checked exceptions are checked at compile time and must be handled or declared. Unchecked exceptions are not checked at compile time.

❓ Predict Output

advanced

Compilation error due to unchecked exception declaration

What error occurs when compiling this Java code?

Java

public class Test {
    public static void main(String[] args) throws NullPointerException {
        throw new NullPointerException("Error");
    }
}

A Compilation error: cannot declare unchecked exception in throws clause

B Compiles and runs, throws NullPointerException

C No output, program runs normally

D Compilation error: unhandled exception

💡 Hint

Consider if declaring unchecked exceptions in throws clause is allowed.

Declaring unchecked exceptions in throws clause is allowed but not required. The code compiles and throws the exception at runtime.

❓ Predict Output

expert

Output and exception behavior with mixed checked and unchecked exceptions

What is the output when running this Java program?

Java

import java.io.IOException;

public class Test {
    public static void riskyMethod() throws IOException {
        if (false) {
            throw new IOException("Checked IO error");
        } else {
            throw new IllegalArgumentException("Unchecked argument error");
        }
    }

    public static void main(String[] args) {
        try {
            riskyMethod();
        } catch (IOException e) {
            System.out.println("Caught checked exception");
        }
    }
}

A Program terminates with IllegalArgumentException

B Caught checked exception

C Compilation error: unhandled exception

D No output

💡 Hint

Consider which exceptions are caught and which are not.

The method may throw IOException (checked) or IllegalArgumentException (unchecked). Only IOException is caught. If unchecked exception occurs, program terminates.

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