Throw keyword in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the throw keyword do in Java?

The throw keyword is used to explicitly throw an exception from a method or block of code. It signals that an error or unusual condition has occurred.

beginner

How is throw different from throws in Java?

throw is used to actually throw an exception object, while throws is used in a method signature to declare that the method might throw certain exceptions.

beginner

Can you throw any object using throw in Java?

No, you can only throw objects that are instances of Throwable or its subclasses, such as Exception or Error.

beginner

What happens after an exception is thrown using throw?

The normal flow of the program stops, and Java looks for a matching catch block to handle the exception. If none is found, the program terminates.

beginner

Show a simple example of using throw to throw an exception.

Example:

if (age < 18) {
  throw new IllegalArgumentException("Age must be 18 or older");
}

What type of object can be thrown using the throw keyword?

A Only primitive types

B Any object

C Only String objects

D Only objects of type Throwable or its subclasses

Only objects that inherit from Throwable, such as Exception or Error, can be thrown.

Which keyword is used to declare exceptions a method might throw?

A throw

B try

C throws

D catch

throws declares exceptions in method signatures; throw actually throws an exception.

What happens immediately after an exception is thrown with throw?

A The program looks for a matching catch block

B The exception is ignored

C The program continues normally

D The exception is automatically handled

Java searches for a catch block that matches the thrown exception to handle it.

Can you throw multiple exceptions at once using a single throw statement?

A Yes

B No

C Only if they are of the same type

D Only in Java 17+

A single throw statement throws only one exception object.

Which of the following is a correct way to throw an exception?

A throw new Exception("Error occurred");

B throws new Exception("Error occurred");

C throw Exception("Error occurred");

D catch new Exception("Error occurred");

The correct syntax to throw an exception is using throw followed by a new exception object.

Explain how the throw keyword works in Java and when you would use it.

Think about signaling errors manually in your code.

Describe the difference between throw and throws in Java exception handling.

One is for declaring, the other is for actually throwing.

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