What are exceptions in Java? Explain the difference between checked and unchecked exceptions and when to use each category.

Answer

Exceptions are a mechanism for error handling in Java. Every thrown object is a subclass of Throwable. There are two main categories:

• Checked exceptions (subclasses of Exception, excluding RuntimeException): must be explicitly handled or declared in the method signature via throws.
• Unchecked exceptions (subclasses of RuntimeException): explicit handling is not required and can occur anywhere (e.g., NullPointerException).

When to use?

• Checked — for expected errors that can be handled (e.g., file operations, network errors).
• Unchecked — for errors that indicate programming defects (e.g., incorrect array index).

Code Example

public void readFile(String path) throws IOException {
    // Checked exception — requires handling
    Files.readAllLines(Paths.get(path));
}

public void divide(int a, int b) {
    int c = a / b; // Unchecked — ArithmeticException
}

Tricky Question

Can a method that does not declare throws throw a checked exception?

Answer: No, the compiler will not allow throwing a checked exception without declaring it in the method signature (throws). However, an unchecked exception can be thrown without any warnings.

void foo() {
    throw new IOException(); // Compilation error
}

Examples of real errors due to misunderstanding of the topic

---

Story

In an internet bank, the customer decided to ignore the handling of SQLExceptions using unchecked wrappers. This led to invisible errors during driver updates, resulting in data desynchronization.

---

Story

In a logging application, the log file remained locked due to an unhandled checked exception (FileNotFoundException), which was "lost" in the call chain; the error was not handled in time, and the application could no longer write to logs.

---

Story

In a trading platform, the team implemented a global catch(Throwable) to "never crash." As a result, critical unchecked exceptions were masked — the application continued to run in an inconsistent state, leading to complex bugs and data loss.