What is the difference between a `checked` and an `unchecked` exception in Java?

Q: What is the difference between a `checked` and an `unchecked` exception in Java?

Learn the answer to "What is the difference between a `checked` and an `unchecked` exception in Java?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a backend engineer at a fintech company. You are writing a new service that needs to interact with a third-party API. The API can throw a variety of different exceptions, and you need to decide how to handle them.

The Challenge

Explain the difference between a checked and an unchecked exception in Java. What are the pros and cons of each approach, and how would you decide which one to use for a given exception?

Wrong Approach

A junior engineer might not be aware of the difference between a `checked` and an `unchecked` exception. They might just catch all exceptions and log them, which would not be a very robust solution.

Addresses symptoms, not root cause

Right Approach

A senior engineer would be able to provide a detailed explanation of the differences between a `checked` and an `unchecked` exception. They would also be able to explain the trade-offs between each approach and would have a clear recommendation for which one to use in a given situation.

Step 1: Understand the Key Differences

Feature	`checked` Exception	`unchecked` Exception
Compiler	The compiler checks that the exception is handled.	The compiler does not check that the exception is handled.
`throws`	A method that can throw a `checked` exception must declare it in its `throws` clause.	A method that can throw an `unchecked` exception does not need to declare it in its `throws` clause.
Subclasses	Subclasses of `Exception` (but not `RuntimeException`).	Subclasses of `RuntimeException`.
Use Cases	For recoverable errors that the caller should be able to handle.	For programming errors that should not be caught.

Step 2: Choose the Right Tool for the Job

For our use case, we should use a checked exception for any exceptions that can be thrown by the third-party API. This will force the caller to handle the exception, which will make our code more robust.

We should use an unchecked exception for any programming errors in our own code, such as a NullPointerException.

Step 3: Code Examples

Here are some code examples that show the difference between the two approaches:

checked Exception:

public void myMethod() throws MyCheckedException {
    // ...
    throw new MyCheckedException("Something went wrong.");
}

unchecked Exception:

public void myMethod() {
    // ...
    throw new MyUncheckedException("Something went wrong.");
}

Systematic, production-ready debugging

Practice Question

You are writing a library and you want to make sure that the caller handles a particular error. Which of the following would you use?

Questions