What is the purpose of the throw and throws keywords?

Table of Contents

• Introduction
• What is the throw Keyword?
  • Syntax of throw
  • Example: Using the throw Keyword
  • Key Points About throw:
• What is the throws Keyword?
  • Syntax of throws
  • Example: Using the throws Keyword
  • Key Points About throws:
• Difference Between throw and throws
  • Summary of Differences:
• Practical Example Combining throw and throws
• Conclusion

Introduction

In Java, exception handling is a powerful mechanism to handle errors or unexpected situations that occur during the execution of a program. The throw and throws keywords are integral parts of this mechanism, allowing developers to raise and propagate exceptions in a controlled manner. While they both relate to exceptions, they serve different purposes in exception handling.

In this guide, we will explore the purpose of the **throw** and **throws** keywords, how they are used, and the differences between them.

What is the throw Keyword?

The throw keyword is used to explicitly throw an exception in Java. It is used within a method or block of code to trigger an exception. When you use the throw keyword, you are indicating that something exceptional has occurred, and you want to interrupt the normal flow of the program to handle that condition.

Syntax of throw

• **ExceptionType**: The type of exception you are throwing (e.g., RuntimeException, IOException).
• **"Error message"**: An optional message that provides more details about the exception.

Example: Using the throw Keyword

In the example above:

• The throw keyword is used in the withdraw() method to throw an IllegalArgumentException if the withdrawal amount exceeds the available balance.
• The exception is caught in the main() method, and the error message is printed.

Key Points About throw:

• Manual exception throwing: You use throw to raise an exception manually, which allows you to create custom exception handling logic.
• Exception can be any subclass of **Throwable**: You can throw any object that is an instance of Throwable (typically Exception or Error).

What is the throws Keyword?

The throws keyword is used to declare exceptions that a method might throw. It is used in the method signature to indicate that the method is capable of throwing certain exceptions. The throws keyword does not throw exceptions directly; rather, it indicates that the calling code should handle or propagate those exceptions.

Syntax of throws

• **ExceptionType1, ExceptionType2**: A comma-separated list of exception types that the method may throw.

Example: Using the throws Keyword

In this example:

• The readFile() method declares that it can throw an IOException by using the throws keyword.
• The calling code in the main() method is required to handle or declare the exception (in this case, using a try-catch block).

Key Points About throws:

• Declaration, not throwing: The throws keyword is used to declare the exceptions that might be thrown by the method. It does not throw exceptions; it just indicates that the method is capable of throwing them.
• Used with checked exceptions: The throws keyword is used to declare checked exceptions, which the method must either catch or declare.

Difference Between throw and throws

While both throw and throws are related to exceptions, they serve distinct purposes in Java exception handling.

Keyword	Purpose	Usage
throw	Used to explicitly throw an exception from a method or block of code.	You use throw within a method to actually raise an exception.
throws	Used to declare exceptions that a method might throw.	You use throws in the method signature to indicate that the method can throw certain exceptions.

Summary of Differences:

• **throw** is used inside a method to actually throw an exception.
• **throws** is used in the method signature to declare that a method may throw certain exceptions.

Practical Example Combining throw and throws

In this example:

• The processNumber() method declares that it can throw an IllegalArgumentException using the throws keyword.
• The method throws the exception manually using the throw keyword if the number is negative.
• The exception is caught in the main() method and the error message is printed.

Conclusion

The throw and throws keywords are crucial for proper exception handling in Java.

• Use the throw keyword to explicitly throw an exception when an error condition is encountered.
• Use the throws keyword in the method signature to declare the exceptions that a method may throw, which helps in propagating exceptions up the call stack.

By understanding and using these keywords correctly, you can write cleaner and more robust Java code, ensuring proper exception handling and improving error reporting in your applications.