Two Types of Exceptions

There are two kinds of exceptions in Java: checked exceptions and unchecked exceptions. In this lesson, I'll provide some examples to understand the distinction.

Checked Exceptions

Consider this code:

public void doSomething() {
	doThisFirst();
	// more code here!
}

If doThisFirst method throws a checked exception, the compiler will not let you simply call it! You must handle the exception by either of these techniques:

1. Using try/catch block

public void doSomething()  {
  try { 
    doThisFirst();
  } catch (SomeCheckedException e) {
    // Deal with the exception.
  }
  // more code here!
}

2. Allow the doSomething method to throw the exception raised by doThisFirst

public void doSomething() throws SomeCheckedException {
  doThisFirst();
  // more code here!
}

Java verifies checked exceptions at compile-time and forces you to provide a handling mechanism for it.

Unchecked Exceptions

All Java exceptions are checked unless they subtype the Error class or the RuntimeException class.

The compiler does not force you to handle unchecked exceptions. For instance, the compiler will let you simply call the doThisFirst() if it (may potentially) throws an unchecked exception:

public void doSomething() {
	doThisFirst();
	// more code here!
}

Why two types of Exceptions?

The distinction between checked v.s. unchecked exception is specific to Java. In C++, for instance, all exceptions are unchecked, so it is up to the programmer to handle them or not.

Read Unchecked Exceptions — The Controversy on Oracle's website for more information. In a nutshell, Java believes it must force programmers to handle exceptions. But, on the other hand, it makes allowance to declare errors that could happen, but rarely do, as unchecked exceptions. So, the programmer is not frustrated to, e.g., have to write try/catch every time they use division in a program even though division by zero causes ArithmeticException.