Throw vs Throws

The throw and throws keywords are used in Java exception handling, but they serve completely different purposes.

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1. The throw Keyword

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

• It is typically used to throw custom exceptions or explicitly trigger a runtime exception based on certain conditions (like invalid user input).
• You can only throw one exception at a time using throw.

Example

public class LabException5 {

  static void validateAge(int age) {
    if (age < 18) {
      // Explicitly throwing an exception
      throw new ArithmeticException("Not eligible to vote!");
    } else {
      System.out.println("Welcome to the voting system.");
    }
  }

  public static void main(String args[]) {
    validateAge(16); // This will crash the program with the custom message
  }
}

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2. The throws Keyword

The throws keyword is used in a method signature to declare that the method might throw one or more exceptions during its execution.

• It is used primarily for Checked Exceptions to tell the compiler and the caller of the method that they need to handle the exception.
• It does not actually throw an exception; it delegates the responsibility of handling the exception to the caller method.
• You can declare multiple exceptions using a comma-separated list.

Example

import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class LabException6 {

  // Declaring that this method might throw an IOException
  static void readFile() throws IOException {
    File file = new File("nonexistent_file.txt");
    FileReader fr = new FileReader(file); // Might throw FileNotFoundException
  }

  public static void main(String args[]) {
    try {
      // The caller MUST handle the checked exception
      readFile();
    } catch (IOException e) {
      System.out.println("Exception handled: File not found.");
    }
  }
}

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Differences: throw vs throws

Here is a side-by-side comparison of the two keywords:

Feature	throw	throws
Purpose	Used to explicitly throw an exception object.	Used to declare that a method might throw exceptions.
Location	Inside the method body.	In the method signature.
Multiple Exceptions	Can throw only one exception at a time.	Can declare multiple exceptions (comma-separated).
Usage	Followed by an instance (object) of an Exception class (throw new ArithmeticException()).	Followed by the Exception class names (throws IOException, SQLException).
Type of Exception	Mostly used for Custom/Unchecked Exceptions.	Must be used for Checked Exceptions.

[!TIP] Think of throw as the action of causing the problem, and throws as a warning label on the method indicating what problems might occur.