throw and throws in Java

These are two related but distinct concepts in Java's exception handling mechanism.

throw keyword

• Used to explicitly throw an exception from a method or block of code
• You can throw either checked or unchecked exceptions
• Syntax: throw new ExceptionType("message");

Example:

public void checkAge(int age) {
    if (age < 18) {
        throw new ArithmeticException("Access denied - You must be at least 18 years old.");
    }
    System.out.println("Access granted");
}

throws keyword

• Used in method signature to declare that a method might throw one or more exceptions
• Primarily used for checked exceptions (though you can use it for unchecked exceptions too)
• Caller of the method must handle or declare these exceptions
• Syntax: returnType methodName() throws Exception1, Exception2 {...}

Example:

public void readFile() throws FileNotFoundException, IOException {
    // code that might throw these exceptions
}

Key Differences

Combined Example

public class Example {
    // Method declares it might throw IOException
    public void processFile(String filePath) throws IOException {
        if (filePath == null) {
            // Explicitly throwing an exception
            throw new IllegalArgumentException("File path cannot be null");
        }

        // Code that might throw IOException
        FileReader file = new FileReader(filePath);
        // ... more code
    }
}

Remember:

• Use throw when you want to generate an exception
• Use throws when your method might propagate an exception that it doesn't handle