S07L32 – throw keyword in Java

Understanding the throw Keyword in Java

Table of Contents

  • Introduction
  • What is the throw Keyword?
  • Using the throw Keyword for Custom Exceptions
  • Exception Handling with throw
  • Example Code for Using the throw Keyword
  • Output of the Program and Explanation
  • Key Differences Between throw and throws
  • Conclusion

1. Introduction

The throw keyword in Java is used to explicitly throw exceptions from your code. It is commonly used to handle scenarios where a certain condition,
defined by the programmer, should result in an exception. In this article, we will delve into how and when to use the throw keyword, along with practical examples and explanations.

2. What is the throw Keyword?

In Java, the throw keyword is used to manually throw an exception. When used, it disrupts the normal flow of the program, forcing it to enter an error-handling state.
Unlike implicit exceptions, which are raised by the JVM (Java Virtual Machine), throw allows developers to create custom error scenarios, enhancing the flexibility of exception handling.

3. Using the throw Keyword for Custom Exceptions

Imagine a scenario where you want to throw an exception based on custom logic. The throw keyword enables you to throw exceptions explicitly.
For instance, if certain conditions in your program are not met (like invalid user input or invalid operations), you can throw an exception using the throw keyword.

For example, you may want to throw a RuntimeException when a certain condition is met, as demonstrated in the next section.

4. Exception Handling with throw

In a typical program, the throw keyword is used alongside conditional statements to throw an exception when certain conditions are met.
For instance, if a string value is equal to “0”, we can use the throw keyword to throw a runtime exception.

5. Example Code for Using the throw Keyword

Here is a practical example of how to use the throw keyword:

Explanation:

  • String Definition: We create a string x with a value of "0".
  • Conditional Statement: We check whether the string x is equal to "0". If it is, we manually throw a RuntimeException using the throw keyword.
  • Exception Thrown: Once the exception is thrown, the program stops its normal execution, and the message "Custom exception: String equals 0" is displayed.

6. Output of the Program and Explanation

Output:

Explanation of the Output:

  • The condition x.equals("0") evaluates to true, as the string x is indeed "0".
  • Since the condition is met, the program throws a RuntimeException with the message "Custom exception: String equals 0".
  • The exception interrupts the normal flow of the program. As a result, the statement System.out.println("This statement won't be executed if an exception is thrown."); is never reached.
  • The output shows the full exception details, including the type of exception (RuntimeException), the custom message (Custom exception: String equals 0), and the location of the error (Main.java:6).

7. Key Differences Between throw and throws

It is essential to differentiate between the throw and throws keywords in Java:

Feature throw throws
Purpose Used to explicitly throw an exception in the code. Declares that a method may throw one or more exceptions.
Location Can be used inside a method body. Used in method signatures.
Checked/Unchecked Used for both checked and unchecked exceptions. Typically used for checked exceptions.
Example throw new RuntimeException("error"); public void myMethod() throws IOException {}

8. Conclusion

The throw keyword in Java is a powerful tool for managing custom exceptions and handling errors in a controlled manner.
By explicitly throwing exceptions when certain conditions are met, you can make your programs more robust and reliable. Always use throw carefully,
as it interrupts the normal flow of the program.

In this article, we explored the basics of the throw keyword, explained how it differs from the throws keyword, and provided example code demonstrating its usage.

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