Checked vs Unchecked Exceptions

Some problems are nobody's fault and can happen to perfectly correct code: the file you need was deleted, the database is down, the network is slow. Others mean the code itself is wrong: calling a method on null, dividing by zero, reading past the end of an array. Java treats these two groups differently. For the first group, checked exceptions, the compiler makes sure you've planned for them. For the second, unchecked exceptions, it trusts you to fix the bug instead.

The Compiler Checks One Kind

import java.io.FileReader;
 
public class Checked {
    public static void main(String[] args) {
        FileReader reader = new FileReader("marks.txt");
    }
}

Checked.java:5: error: unreported exception FileNotFoundException; must be caught or declared to be thrown
        FileReader reader = new FileReader("marks.txt");
                            ^
1 error

FileReader's constructor declares that it may throw FileNotFoundException, a checked exception. The program won't even compile until you do one of two things:

import java.io.FileNotFoundException;
import java.io.FileReader;
 
public class CatchOrDeclare {
    static void option1() {
        try {
            new FileReader("marks.txt");
        } catch (FileNotFoundException e) {
            System.out.println("Option 1, caught: " + e.getMessage());
        }
    }
 
    static void option2() throws FileNotFoundException {
        new FileReader("marks.txt");
    }
 
    public static void main(String[] args) {
        option1();
        try {
            option2();
        } catch (FileNotFoundException e) {
            System.out.println("Option 2, caller caught: " + e.getMessage());
        }
        int n = Integer.parseInt("42");
        System.out.println("Unchecked code compiles with no try: " + n);
    }
}

Option 1, caught: marks.txt (No such file or directory)
Option 2, caller caught: marks.txt (No such file or directory)
Unchecked code compiles with no try: 42

How it works

1. Catch it: option1 handles the problem itself.
2. Declare it: option2 adds throws FileNotFoundException, passing the responsibility to its caller, main, which then catches it.
3. Integer.parseInt can throw NumberFormatException, but that's unchecked, so no try is required.

Which Is Which

Why Java Has Both

The idea is simple: if a problem can happen even when your code is correct, the compiler reminds you to plan for it. If a problem only happens because of a bug, forcing a try around every division or every method call would bury the code in noise, so Java leaves those unchecked. You prevent them by writing correct code: checking for null, validating arguments, testing indexes.

Common Mistakes

Adding throws Exception to every method to silence the compiler. It hides which problems can really happen.

Catching an unchecked exception instead of fixing the bug. catch (NullPointerException e) usually means a missing null check.

Thinking "unchecked" means "can't be caught". Any exception can be caught; unchecked only means the compiler doesn't insist.

Interview Corner

Practice

1. Easy: Classify each as checked or unchecked: FileNotFoundException, ClassCastException, SQLException, IllegalStateException, InterruptedException, StackOverflowError.
2. Harder: Write a method that calls Thread.sleep(500) (which throws the checked InterruptedException). Make it compile once by catching and once by declaring.

Key Takeaways