Introduction

• The throws keyword is used to specify the types of exceptions that a method might throw during execution.
• It tells the caller of the method that this method may result in an exception and the caller should handle it.
• Use :
  • It is used to declare checked exceptions (like IOException, SQLException etc).
  • It is used to inform the caller method about possible exceptions.
  • It is used to avoid handling exceptions inside the method itself (i.e., pass the responsibility to the caller).
• Syntax :

  • return_type methodName(parameters) throws ExceptionType1, ExceptionType2, ...
    {
        // method body
    }

  • Here, ExceptionType1, ExceptionType2, ... are the types of exceptions that the method declares it might throw.
• Example :

  • import java.io.FileInputStream;
    import java.io.IOException;
    
    public class ThrowsDemo
    {
        // Method declares that it may throw IOException
        void readFile() throws IOException
        {
            // Using try-with-resources ensures FileInputStream is closed automatically
            try (FileInputStream fis = new FileInputStream("test.txt"))
            {
                // Read first byte from the file
                int data = fis.read();
                System.out.println("First byte of the file: " + data);
            }
        }
    
        public static void main(String[] args)
        {
            try
            {
                ThrowsDemo obj = new ThrowsDemo();
    
                // Caller method handles the IOException thrown by readFile()
                obj.readFile();
            }
            catch (IOException e)
            {
                // Exception is caught and handled here
                System.out.println("Exception handled: " + e);
            }
        }
    }

    Output:

    Exception handled: java.io.FileNotFoundException: test.txt (The system cannot find the file specified)

  • Explanation:
    • readFile() method declares throws IOException.
      • It means this method might throw an IOException while reading the file.
    • In main() method, we call readFile() inside a try-catch block.
      • If the file test.txt is not found, an exception will be caught.
    • If the file exists, it will print the first line.

Points to remember for "throws" Keyword:

1. The throws keyword is used in a method declaration to declare the exceptions that the method might throw.
2. It can declare one or multiple exceptions, separated by commas.
   • void myMethod() throws IOException, SQLException { }
3. It is mainly used for checked exceptions (exceptions checked at compile-time), but can also declare unchecked exceptions (though unnecessary).
4. Using throws does not throw an exception by itself; it only informs the caller about possible exceptions.
5. If a method calls another method that declares checked exceptions, it must either handle the exception using try-catch or declare it using throws.
6. It cannot be used inside a block of code; it is only valid in method or constructor declarations.