Java Throws Keyword

A low-level method that reads a file often can't sensibly handle a missing file: it doesn't know whether to ask the user, use a default, or give up. Its caller knows. The throws clause in a method's header says, "this method may throw these checked exceptions; whoever calls me must deal with them." It's a promise written into the method's signature, and the compiler holds every caller to it.

Passing Responsibility Up

import java.io.FileNotFoundException;
import java.io.FileReader;
 
public class Report {
    static FileReader openMarks(String file) throws FileNotFoundException {
        System.out.println("opening " + file);
        return new FileReader(file);
    }
 
    static void printReport(String file) throws FileNotFoundException {
        openMarks(file);
        System.out.println("report printed");
    }
 
    public static void main(String[] args) {
        try {
            printReport("advitha-marks.txt");
        } catch (FileNotFoundException e) {
            System.out.println("No marks file yet, showing an empty report");
        }
    }
}

opening advitha-marks.txt
No marks file yet, showing an empty report

How it works

1. openMarks declares throws FileNotFoundException: it doesn't catch the problem, it passes it on.
2. printReport calls openMarks, so it must also catch or declare. It declares too, passing the problem further up.
3. main is the place that knows what to do (show an empty report), so it catches it.

Catch or Declare?

Rules of throws

Common Mistakes

throws Exception on every method. It hides real information and forces callers to catch everything.

Declaring and never handling anywhere. Somebody, eventually main, must decide what to do.

Thinking throws throws something. It only declares; throw actually throws.

Interview Corner

Practice

1. Easy: Write static void pause(long ms) throws InterruptedException that calls Thread.sleep(ms), and call it from main with a try-catch.
2. Harder: Add a third level to Report.java between printReport and main. Which methods need throws, and which one catches?

Key Takeaways