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JVM / Java / Exceptions / Common Runtime Exceptions

1. ArrayIndexOutOfBoundsException

Thrown when accessing an array index that's negative or >= the array's length -- a classic off-by-one bug, especially with loops using <= instead of < against array.length. Unlike a List, an array has no bounds-checked alternative like get(); the index is always used directly.

 
class ArrayIndexOutOfBoundsExceptionExample {
    public static void main(String[] args) {

        int[] numbers = {1, 2, 3};
        // System.out.println(numbers[3]);  // throws: index 3 out of bounds for length 3

        for (int i = 0; i <= numbers.length; i++) { // off-by-one, it should be <
            // System.out.println(numbers[i]);  // fails on the last iteration
        }
    }    
}

2. ClassCastException

Thrown when casting an object to a type it isn't actually an instance of at runtime -- the compiler allows the cast syntactically if there's a valid inheritance relationship, but the actual runtime type is checked when the cast executes. Pattern matching for instanceof (see Versions / Java 17) avoids this by combining the check and cast.

 
class ClassCastExceptionExample {
    public static void main(String[] args) {

        Object obj = "a string";
        // Integer number = (Integer) obj;  // compiles, but throws ClassCastException at runtime

        Object no = 2;
        if (no instanceof Integer) {
            System.out.println(no);
        }
    }
}

3. NumberFormatException

Thrown by Integer.parseInt(), Double.parseDouble(), and similar methods when the input string isn't a valid number -- extra whitespace, letters, or an empty string all trigger it. Since it's a RuntimeException, it's easy to forget to handle when parsing user input or data from an external source.

 
public class NumberFormatExceptionExample {
    public static void main(String[] args) {
        
        // int n = Integer.parseInt("abc");  // throws NumberFormatException
        int n = Integer.parseInt("1");
        System.out.println(n);

        String userInput = "123 ";
        try {
            int safe = Integer.parseInt(userInput.trim());
            System.out.println(safe);
        } catch (NumberFormatException e) {
            System.out.println("Invalid number: " + userInput);
        }
    }
}