When to catch the Exception vs When to throw the Exceptions?

Question

I have been coding in Java for a while now. But sometimes, I don't understand when I should throw the exception and when should I catch the exception. I am working on a project in which there are lot of methods. The hierarchy is something like this-

Method A will call Method B and Method B will call some Method C and Method C will call Method D and Method E.

So currently what I am doing is- I am throwing exceptions in all the methods and catching it in Method A and then logging as an error.

But I am not sure whether this will be the right way to do it? Or should I start catching exceptions in all the Methods. So that is why this confusion started in my- When should I catch the Exception vs When should I throw the exceptions. I know it's a silly question but somehow I am struggling to understand this major concept.

Can someone give me a detailed example of When to catch the Exception vs When to throw the Exceptions so that my concepts gets cleared on this? And in my case, should I keep on throwing the exception and then catch it in the main calling Method A?

Answer 1

You should catch the exception when you are in the method that knows what to do.

For example, forget about how it actually works for the moment, let's say you are writing a library for opening and reading files.

So you have a class, say:

public class FileInputStream extends InputStream {
    public FileInputStream(String filename) { }
}

Now, lets say the file doesn't exist. What should you do? If you're struggling to think of the answer, that's because there isn't one... the FileInputStream doesn't know what to do about that problem. So it throws it up the chain, i.e.:

public class FileInputStream extends InputStream {
    public FileInputStream(String filename) throws FileNotFoundException { }
}

Now, lets say someone's using your library. They might have code that looks like this:

public class Main {
    public static void main(String... args) {
        String filename = "foo.txt";
        try {
            FileInputStream fs = new FileInputStream(filename);

            // The rest of the code
        } catch (FileNotFoundException e) {
            System.err.println("Unable to find input file: " + filename);
            System.err.println("Terminating...");
            System.exit(3);
        }
    }
}

Here, the programmer knows what to do, so they catch the exception and handle it.