Throw keyword in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to throw an exception when the number is negative.

Java

if (number < 0) {
    [1] new IllegalArgumentException("Negative number not allowed");
}

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

Atry

Bthrows

Ccatch

Dthrow

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throws' instead of 'throw' to throw an exception.

Trying to use 'catch' or 'try' to throw an exception.

The throw keyword is used to actually throw an exception object in Java.

2fill in blank

medium

Complete the method to throw a NullPointerException when input is null.

Java

public void checkInput(String input) {
    if (input == null) {
        [1] new NullPointerException("Input cannot be null");
    }
}

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

Athrows

Bthrow

Ctry

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throws' inside the method body instead of 'throw'.

Trying to catch the exception instead of throwing it.

The throw keyword is used to throw an exception instance inside the method.

3fill in blank

hard

Fix the error in throwing a checked exception inside the method.

Java

public void readFile(String path) [1] {
    if (path == null) {
        throw new IOException("File path is null");
    }
}

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

Athrows IOException

Bthrow IOException

Cthrows IOException()

Dthrow IOException()

Attempts:

3 left

💡 Hint

Common Mistakes

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

Adding parentheses after IOException in the signature.

Checked exceptions like IOException must be declared in the method signature using throws.

4fill in blank

hard

Fill both blanks to throw an ArithmeticException when dividing by zero.

Java

public int divide(int a, int b) [1] {
    if (b == 0) {
        [2] new ArithmeticException("Cannot divide by zero");
    }
    return a / b;
}

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

Athrows ArithmeticException

Bthrow

Cthrows IOException

Dthrow new

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw' in the method signature instead of 'throws'.

Using 'throw new' as a single option instead of splitting keywords.

The method declares throws ArithmeticException and inside the method, use throw to throw the exception.

5fill in blank

hard

Fill all three blanks to throw a custom exception when age is invalid.

Java

public void setAge(int age) [1] {
    if (age < 0 || age > 150) {
        [2] new [3]("Invalid age: " + age);
    }
}

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

Athrows InvalidAgeException

Bthrow

CInvalidAgeException

Dthrows Exception

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throws Exception' instead of the specific custom exception.

Not using 'throw' keyword inside the method body.

The method declares it throws InvalidAgeException, inside it uses throw to throw a new InvalidAgeException object.

Practice

1.

What does the throw keyword do in Java?

easy

A. It catches an exception and handles it.

B. It sends an exception to stop normal program flow when an error occurs.

C. It declares a method can throw exceptions.

D. It creates a new thread for parallel execution.

Solution

1. Step 1: Understand the role of throw

   The throw keyword is used to send an exception object explicitly when an error happens.

2. Step 2: Differentiate from other keywords

   throw does not catch exceptions (that's catch), nor declare exceptions (that's throws), nor create threads.

3. Final Answer:

   It sends an exception to stop normal program flow when an error occurs. -> Option B

4. Quick Check:

   throw sends exception = B [OK]

Hint: Remember: throw sends, catch handles exceptions [OK]

Common Mistakes:

• Confusing throw with throws keyword
• Thinking throw catches exceptions
• Mixing throw with thread creation

2.

Which of the following is the correct way to throw a new IllegalArgumentException in Java?

?

easy

A. throws new IllegalArgumentException("Invalid argument");

B. throw IllegalArgumentException("Invalid argument");

C. throw new IllegalArgumentException("Invalid argument");

D. throw new IllegalArgumentException;

Solution

1. Step 1: Check syntax for throwing exceptions

   To throw an exception, use throw new ExceptionType("message") with parentheses and semicolon.

2. Step 2: Identify correct option

   throw new IllegalArgumentException("Invalid argument"); uses correct syntax with new, parentheses, and semicolon. Options B and D miss parentheses or new. throws new IllegalArgumentException("Invalid argument"); uses throws which is for method declarations, not throwing.

3. Final Answer:

   throw new IllegalArgumentException("Invalid argument"); -> Option C

4. Quick Check:

   throw + new + parentheses = A [OK]

Hint: Throw exceptions with 'throw new ExceptionType()' syntax [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using 'throws' instead of 'throw'
• Missing parentheses after exception class

3.

What will be the output of the following Java code?

public class TestThrow {
    public static void main(String[] args) {
        try {
            throw new RuntimeException("Error happened");
        } catch (RuntimeException e) {
            System.out.println(e.getMessage());
        }
    }
}

medium

A. Error happened

B. RuntimeException

C. Compilation error

D. No output

Solution

1. Step 1: Analyze the try block

   The code throws a new RuntimeException with message "Error happened".

2. Step 2: Analyze the catch block

   The catch block catches the exception and prints its message using e.getMessage(), which is "Error happened".

3. Final Answer:

   Error happened -> Option A

4. Quick Check:

   Exception message printed = C [OK]

Hint: Catch prints exception message with getMessage() [OK]

Common Mistakes:

• Expecting exception type name instead of message
• Thinking code causes compilation error
• Assuming no output without catch

4.

Identify the error in the following code snippet:

public class Example {
    public static void main(String[] args) {
        throw new Exception("Problem");
    }
}

medium

A. Missing try-catch block or throws declaration for checked exception.

B. Incorrect exception message format.

C. Cannot throw exceptions in main method.

D. Exception class does not exist.

Solution

1. Step 1: Identify exception type

   The code throws Exception, which is a checked exception in Java.

2. Step 2: Check handling of checked exceptions

   Checked exceptions must be either caught in a try-catch block or declared with throws in the method signature. This code does neither, causing a compile error.

3. Final Answer:

   Missing try-catch block or throws declaration for checked exception. -> Option A

4. Quick Check:

   Checked exceptions need handling = D [OK]

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

Common Mistakes:

• Ignoring checked exception rules
• Thinking main cannot throw exceptions
• Confusing checked and unchecked exceptions

5.

Consider this method that throws an exception if the input is negative:

public void checkNumber(int num) {
    if (num < 0) {
        throw new IllegalArgumentException("Negative number not allowed");
    }
    System.out.println("Number is " + num);
}

How should you call this method safely in your code?

hard

A. Use throw keyword again when calling checkNumber.

B. Call checkNumber without any try-catch because IllegalArgumentException is checked.

C. Declare throws IllegalArgumentException in the calling method and do not catch.

D. Call checkNumber inside a try-catch block catching IllegalArgumentException.

Solution

1. Step 1: Identify exception type thrown

   The method throws IllegalArgumentException, which is an unchecked exception.

2. Step 2: Decide safe calling practice

   Although unchecked exceptions do not require declaration, to handle errors safely, call the method inside a try-catch block catching IllegalArgumentException.

3. Final Answer:

   Call checkNumber inside a try-catch block catching IllegalArgumentException. -> Option D

4. Quick Check:

   Catch unchecked exceptions to handle errors safely = A [OK]

Hint: Catch exceptions even if unchecked for safer code [OK]

Common Mistakes:

• Thinking unchecked exceptions must be declared
• Not catching exceptions leading to crashes
• Misusing throw keyword when calling methods