Difference between throws and throw in Java

1. Introduction

In Java, exception handling is a powerful mechanism that handles runtime errors to maintain normal application flow. Two keywords used in exception handling are throw and throws. 

The throw keyword is used to explicitly throw an exception from a method or any block of code. 

On the other hand, throws is used in a method's signature to declare that the method may throw exceptions that must be caught or declared by the calling method.

2. Key Points

1. throw is used within a method to throw an exception.

2. throws is used in a method declaration to indicate that it can throw specified exceptions.

3. throw initializes a new exception object and then throws it.

4. throws works with multiple exceptions, allowing a method to declare that it may throw more than one type of exception.

3. Differences

throws	throw
Used in the method signature to declare that the method may throw one or more exceptions.	Used within the method body to actually throw an exception.
Indicates to the caller of the method that they need to handle or declare the exception.	Instantiates an exception object and terminates the method unless caught with a try-catch block.
Does not throw an exception by itself; it simply warns the caller of the potential for the method to throw an exception.	Physically throws an exception, changing the flow of control through the program.
Can declare multiple exceptions separated by commas.	Can only throw one exception at a time.
Used for checked exceptions that are checked at compile-time (except for RuntimeException and its subclasses).	Can be used to throw both checked and unchecked exceptions.
Allows the method to pass the responsibility of handling the exception to the method that called it.	Requires the method to create and throw an exception object explicitly.

4. Example

public class ExceptionHandling {

    // Method that uses throw
    public static void simulateError() {
        // Step 1: Use throw to simulate an error by throwing an ArithmeticException
        throw new ArithmeticException("Demo exception - this is thrown using throw");
    }

    // Method that declares exceptions using throws
    public static void riskyMethod() throws ArithmeticException, NullPointerException {
        // Step 2: The method does not throw any exception itself, but it could do, hence the throws declaration
    }

    public static void main(String[] args) {
        try {
            simulateError();
        } catch (ArithmeticException e) {
            System.out.println(e.getMessage());
        }

        try {
            riskyMethod();
        } catch (Exception e) {
            // Step 3: Catch any exceptions that might be thrown by riskyMethod
            System.out.println(e.getMessage());
        }
    }
}

Output:

Demo exception - this is thrown using throw

Explanation:

1. simulateError method uses the throw keyword to explicitly throw an ArithmeticException.

2. riskyMethod method uses the throws keyword to declare that it might throw either an ArithmeticException or a NullPointerException.

3. The main method catches the ArithmeticException thrown by simulateError and would also catch any exceptions thrown by riskyMethod.

5. When to use?

Use throw when you want to create an exception and pass it to the runtime to be handled.

Use throws when you want to declare that your method could throw exceptions but do not want to handle them within the same method.