Errors and Exceptions

Data Types and Exceptions in Java

Jim White

Java Developer

Exceptions vs Errors

Error is a serious issue or problem
Applications cannot handle and recover from errors
- Errors cause the application to "crash" or stop
- Example: an out of memory problem is an error in Java
Applications can handle and recover from exceptions
- Try to access elements outside the array size is an exception.

Example Error

import java.util.*;

public class CauseOutOfMemory {
    public static void main(String[] args) {
        List<Long> numbers = new ArrayList<Long>();
        long counter = 0;
        while (true) {
            numbers.add(counter);
            counter++;
        }
    }
}

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.base/java.lang.Long.valueOf(Long.java:1204)
    at foo/exceptions.CauseOutOfMemory.main(CauseOutOfMemory.java:12)

Two types of exceptions

	Checked Exception	Runtime ("unchecked") Exception
Require handling	Yes	No
General cause	Things outside our control	Our programming mistakes
Recovery	Can usually recover when anticipated	Usually unrecoverable
Recovery mechanism	try/catch or throws	Best to code carefully to avoid them
Example exception	File not found	Array index out of bounds

Checked and Runtime exceptions

Exception subclasses not also subclasses of RuntimeException are checked exceptions
- Must be dealt with (i.e. handled)
- Ex: file not found (FileNotFoundException)
RuntimeException superclass for all unchecked exceptions
- Do not require handling
- Ex: accessing an array index that does not exist (IndexOutOfBoundsException)
- Ex: trying to divide by zero (ArithmeticException)

RuntimeException

RuntimeException Subclass	Description of when thrown
ArithmeticException	Bad arithmetic calculations like dividing by 0
IndexOutOfBoundsException	Index used on an array, String, etc. is out of range
NegativeArraySizeException	Trying to create an array with a negative size

int y = 5/0; // ArithmeticException causing code
int[] list = new int[-1];  //NegativeArraySizeException causing code

Exception in thread "main" java.lang.ArithmeticException: / by zero
Exception in thread "main" java.lang.NegativeArraySizeException: -1

¹ See https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/RuntimeException.html

"Checked" Exception

public class LoadClass {
    public static void main(String[] args) {
        Class myClass = Class.forName("com.mysql.Driver");
    }
}

LoadClass.java:3: error: unreported exception ClassNotFoundException; 
must be caught or declared to be thrown
                Class myClass = Class.forName("com.mysql.Driver");
                                             ^
1 error

¹ See https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Exception.html

Let's practice!

Data Types and Exceptions in Java