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Exception Handling in java (throw & throws)

#java #backenddevelopment #programming #beginners

title

throw

  • The throw keyword is used to explicitly throw an exception. By default, all predefined exceptions in Java are created and thrown implicitly. If we want to throw an exception manually, we use the *throw * keyword.

  • We can throw either checked or unchecked exceptions using throw.

  • When *throw * is executed, the method terminates immediately, and further code is not executed.

  • We must create a new exception object using throw new ExceptionType(...).

  • Only objects of classes that extend Throwable (or its subclasses) can be thrown.

throw error Syntax 

throw new ExceptionType("Error message");
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Example 

public class Mastering_backend { public static void main(String[] args) { int age = 15; if (age < 18) { throw new IllegalArgumentException("Age must be 18 or above"); } System.out.println("You are eligible to vote."); } }
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In this example, we explicitly throw an IllegalArgumentException if the age is less than 18.

Output 

Exception in thread "main" java.lang.IllegalArgumentException: Age must be 18 or above at Main.main(Main.java:5)
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throws

  • The throws keyword in Java is used to declare exceptions that a method might throw.

  • It specifies a list of exceptions that can be thrown during the execution of the method.

pictorial exampleSyntax 

returnType methodName(parameters) throws ExceptionType1, ExceptionType2 { // method body }
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public void readFile() throws IOException { // code that might throw IOException }
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public void processFile() throws IOException, ParseException { // code that might throw either IOException or ParseException }
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Example 

import java.io.IOException; public class MasteringBackend { // Method that declares it might throw an IOException public static void readFile(String fileName) throws IOException { if (fileName == null || fileName.isEmpty()) { // Throwing a checked exception throw new IOException("File name cannot be empty"); } System.out.println("Reading file: " + fileName); } // Method that throws an unchecked exception public static void validateAge(int age) { if (age < 18) { // Throwing an unchecked exception (no need to declare with throws) throw new IllegalArgumentException("Age must be 18 or above"); } System.out.println("Age is valid for access."); } public static void main(String[] args) { System.out.println("== Mastering Backend =="); // Example 1: Handling checked exception using try-catch try { readFile(""); // Will throw IOException } catch (IOException e) { System.out.println("Caught IOException: " + e.getMessage()); } // Example 2: Unchecked exception (optional to handle, but can crash if not caught) try { validateAge(15); // Will throw IllegalArgumentException } catch (IllegalArgumentException e) { System.out.println("Caught IllegalArgumentException: " + e.getMessage()); } System.out.println("Program ended successfully."); } }
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In this example, the throws keyword is used in the method signature to declare checked exceptions like IOException, while unchecked exceptions like IllegalArgumentException do not need to be declared with throws.

Output 

== Mastering Backend == Caught IOException: File name cannot be empty Caught IllegalArgumentException: Age must be 18 or above Program ended successfully.
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throw vs throws

  • throw – The throw keyword is used to actually throw an exception.

  • throws – The throws keyword is used to declare a list of exceptions that a method might throw.

differences between throw and throws

Example: 

// Using throws public void readFile() throws IOException, SQLException { // Inside this method, you can use throw throw new IOException("File not found"); }
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In this example, ** throw ** is used to throw an exception (new IOException("File not found")), and throws is used to declare that the method may throw that exception.

User Defined exception

A user-defined exception is a custom exception created by the programmer by extending the Exception class (for checked exceptions) or *RuntimeException * class (for unchecked exceptions).

Syntax 

// For a checked exception class MyException extends Exception { public MyException(String message) { super(message); } } // For an unchecked exception class MyRuntimeException extends RuntimeException { public MyRuntimeException(String message) { super(message); } }
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Example 

// Custom exception class class InvalidBookingException extends Exception { public InvalidBookingException(String message) { super(message); } } public class MasteringBackend { public static void bookSeat(int seatNumber) throws InvalidBookingException { if (seatNumber <= 0 || seatNumber > 100) { throw new InvalidBookingException("Seat number " + seatNumber + " is invalid!"); } System.out.println("Seat " + seatNumber + " booked successfully."); } public static void main(String[] args) { try { bookSeat(105); } catch (InvalidBookingException e) { System.out.println("Booking failed: " + e.getMessage()); } } }
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In this example, we created a custom exception by extending the Exception class, named *InvalidBookingException.
*

Output 

Booking failed: Seat number 105 is invalid!
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Have a great one!!!

Author: Solomon Eseme

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