Exception hierarchy
- Java course
- Beginner
- Lesson 109/116
- 5 min
Learn RuntimeException, IOException and others.
The exception hierarchy
All exceptions descend from Throwable. Error represents unrecoverable system failures; Exception can be handled by the programmer. The RuntimeException subclass is considered unchecked.
Throwable
├─ Error
└─ Exception
├─ RuntimeException (unchecked)
└─ IOException (checked)Checked exceptions must be caught or declared with throws at compile time. IOException is an example. Unchecked ones are under RuntimeException; catching them is optional.
Common exception types
Some common exceptions: NullPointerException (using a null object), ArithmeticException (division by zero), ArrayIndexOutOfBoundsException (out of bounds index) and NumberFormatException (invalid number string).
Integer.parseInt("abc");
// NumberFormatException
int[] arr = new int[3];
int x = arr[5];
// ArrayIndexOutOfBoundsExceptionThese all fall under RuntimeException; they are unchecked. Most can be prevented by careful coding; null checks, boundary checks and input validation are examples.
Practice Catch an invalid number
Define String s = "abc";. Inside try call Integer.parseInt(s). Catch NumberFormatException and print Sayı değil.
Starter code
public class Main {
public static void main(String[] args) {
String s = "abc";
// attempt parseInt, catch NumberFormatException, print a message
}
}Sample solution
try {
int n = Integer.parseInt(s);
System.out.println(n);
} catch (NumberFormatException e) {
System.out.println("Sayı değil");
}Expected output
Sayı değil
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which of the following is an unchecked exception?
- IOException
- Error
- RuntimeException Correct answer
- Throwable
Explanation: RuntimeException and its subclasses are unchecked; catching them is optional.
2 Match each exception to its category
| Term | Meaning |
|---|---|
IOException |
Checked |
RuntimeException |
Unchecked |
Error |
System error |
NullPointerException |
null object error |
3 Catch NumberFormatException
try {
Integer.parseInt(s);
} catch (____ e) {
System.out.println("Sayı değil");
}try {
Integer.parseInt(s);
} catch (NumberFormatException e) {
System.out.println("Sayı değil");
}Explanation: NumberFormatException represents an invalid number string.