Exception propagation in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare that the method throws an exception.

Java

public void readFile() [1] IOException {
    // code to read file
}

Drag options to blanks, or click blank then click option'

Athrows

Bthrow

CthrowsException

DthrowsError

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw' instead of 'throws' in method declaration.

Misspelling the keyword.

The keyword throws declares that a method may throw an exception.

2fill in blank

medium

Complete the code to propagate the exception from methodB to methodA.

Java

public void methodA() [1] Exception {
    methodB();
}

public void methodB() [1] Exception {
    throw new Exception("Error");
}

Drag options to blanks, or click blank then click option'

AthrowsException

Bthrow

CthrowsError

Dthrows

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw' instead of 'throws' in method declarations.

Not declaring the exception in methodA.

Both methods must declare throws Exception to propagate the exception.

3fill in blank

hard

Fix the error in the code by completing the method signature to propagate the exception.

Java

public void process() [1] IOException {
    FileReader file = new FileReader("file.txt");
}

Drag options to blanks, or click blank then click option'

Athrow

Bthrows

CthrowsException

DthrowsError

Attempts:

3 left

💡 Hint

Common Mistakes

Not declaring the exception causes a compile error.

Using 'throw' instead of 'throws'.

The method must declare throws IOException because FileReader constructor throws IOException.

4fill in blank

hard

Fill both blanks to propagate the exception from methodC to methodD.

Java

public void methodD() [1] Exception {
    methodC();
}

public void methodC() [2] Exception {
    throw new Exception("Failure");
}

Drag options to blanks, or click blank then click option'

Athrows

Bthrow

CthrowsException

DthrowsError

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw' instead of 'throws'.

Not declaring the exception in methodD.

Both methods must declare throws Exception to propagate the exception properly.

5fill in blank

hard

Fill all three blanks to propagate exceptions correctly in nested method calls.

Java

public void start() [1] IOException {
    execute();
}

public void execute() [2] IOException {
    read();
}

public void read() [3] IOException {
    throw new IOException("Read error");
}

Drag options to blanks, or click blank then click option'

Athrow

Bthrows

CthrowsException

DthrowsError

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw' instead of 'throws'.

Not declaring the exception in all methods.

All methods must declare throws IOException to propagate the checked exception properly.

Practice

1. What does exception propagation mean in Java?

easy

A. An exception is passed up the call stack until caught or program ends

B. An exception is ignored and the program continues normally

C. An exception is automatically fixed by the JVM

D. An exception is converted into a warning message

Solution

1. Step 1: Understand exception propagation concept

   When an exception occurs, Java looks for a matching catch block in the current method. If none is found, it passes the exception to the caller method.

2. Step 2: Follow the exception up the call stack

   This passing continues up the call stack until a catch block handles it or the program terminates if uncaught.

3. Final Answer:

   An exception is passed up the call stack until caught or program ends -> Option A

4. Quick Check:

   Exception moves up call stack = A [OK]

Hint: Exception moves up call stack until caught [OK]

Common Mistakes:

• Thinking exceptions are ignored automatically
• Believing JVM fixes exceptions silently
• Confusing exceptions with warnings

2. Which of the following is the correct way to declare a method that might throw an exception?

easy

A. public void readFile() throws IOException {}

B. public void readFile() throw IOException {}

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

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

Solution

1. Step 1: Recall correct syntax for throws clause

   The keyword throws is used followed by the exception class name without parentheses.

2. Step 2: Check each option syntax

   public void readFile() throws IOException {} uses correct syntax: throws IOException. The other options have syntax errors.

3. Final Answer:

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

4. Quick Check:

   Correct throws syntax = B [OK]

Hint: Use 'throws' keyword followed by exception class name [OK]

Common Mistakes:

• Writing 'throw' instead of 'throws'
• Adding parentheses after exception name
• Using invalid symbols like '-'

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

public class Test {
  static void method() throws Exception {
    throw new Exception("Error occurred");
  }
  public static void main(String[] args) {
    try {
      method();
    } catch (Exception e) {
      System.out.println(e.getMessage());
    }
  }
}

medium

A. Compilation error

B. Exception in thread "main" java.lang.Exception

C. No output

D. Error occurred

Solution

1. Step 1: Analyze method throwing exception

   The method explicitly throws a new Exception with message "Error occurred".

2. Step 2: Check exception handling in main

   The main method calls method() inside try-catch. The catch block prints the exception message.

3. Final Answer:

   Error occurred -> Option D

4. Quick Check:

   Exception caught and message printed = C [OK]

Hint: Exception message prints if caught in try-catch [OK]

Common Mistakes:

• Assuming uncaught exception causes crash
• Confusing exception message with full stack trace
• Thinking code won't compile without throws in main

4. Identify the error in this code snippet:

public class Demo {
  static void risky() {
    throw new IOException("IO error");
  }
  public static void main(String[] args) {
    risky();
  }
}

medium

A. main method must catch IOException

B. IOException cannot be thrown directly

C. Method risky() must declare 'throws IOException'

D. No error, code is correct

Solution

1. Step 1: Check exception type thrown

   IOException is a checked exception and must be declared or caught.

2. Step 2: Verify method declaration

   Method risky() throws IOException but does not declare it with 'throws' keyword, causing a compile error.

3. Final Answer:

   Method risky() must declare 'throws IOException' -> Option C

4. Quick Check:

   Checked exceptions require throws declaration = A [OK]

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

Common Mistakes:

• Ignoring throws declaration for checked exceptions
• Thinking IOException is unchecked
• Assuming main must catch exception always

5. Consider this code:

class A {
  void process() throws Exception {
    throw new Exception("Error in A");
  }
}
class B extends A {
  @Override
  void process() throws Exception {
    super.process();
  }
}
public class Main {
  public static void main(String[] args) {
    A obj = new B();
    try {
      obj.process();
    } catch (Exception e) {
      System.out.println(e.getMessage());
    }
  }
}

What will be the output and why?

hard

A. Runtime error due to invalid override

B. Error in A, because exception propagates from superclass method

C. Compilation error due to throws mismatch

D. No output, exception is swallowed silently

Solution

1. Step 1: Understand method overriding with exceptions

   Subclass B overrides process() and calls super.process(), which throws Exception.

2. Step 2: Exception propagates to main and is caught

   Main calls obj.process() on B instance, exception thrown by A.process() propagates and is caught in main's try-catch, printing the message.

3. Final Answer:

   Error in A, because exception propagates from superclass method -> Option B

4. Quick Check:

   Exception propagates through override and caught = D [OK]

Hint: Overridden method can throw same exceptions up [OK]

Common Mistakes:

• Thinking override cannot throw exceptions declared in superclass
• Assuming exception is lost in subclass
• Confusing compile error with runtime behavior