Recognizing Exception Classes

Recognizing Exception Classes

The OCP exam expects you to recognize common exception class names, know whether each is checked or unchecked, and understand the conditions that cause them.

Common Checked Exceptions

Checked exceptions extend Exception (but not RuntimeException). The compiler requires every calling method to either catch them or declare them with throws.

Class Package Typical Cause
IOException java.io General I/O failure (reading/writing streams, files)
FileNotFoundException java.io Attempt to open a file that does not exist
SQLException java.sql Database access error or invalid SQL
ParseException java.text Failure to parse a string into a date/number
CloneNotSupportedException java.lang clone() called on object that does not implement Cloneable 
java
import java.io.*;

public void loadConfig(String path) throws IOException {
    // FileNotFoundException is a subclass of IOException
    FileReader reader = new FileReader(path);   // throws FileNotFoundException
    int ch;
    while ((ch = reader.read()) != -1) {        // throws IOException
        System.out.print((char) ch);
    }
    reader.close();
}

Common Unchecked Exceptions (RuntimeException subclasses)

No throws declaration is required, but these will crash a program if not handled.

Class Typical Cause
NullPointerException Calling a method or accessing a field on a null reference
ArrayIndexOutOfBoundsException Accessing an array element at an index < 0 or >= length
StringIndexOutOfBoundsException charAt() or substring() called with an invalid index
ClassCastException Casting an object to an incompatible type
ArithmeticException Illegal arithmetic operation, most commonly integer division by zero
NumberFormatException Parsing a string that does not represent a valid number
IllegalArgumentException Method receives an argument that violates its contract
IllegalStateException Method called at an inappropriate time (object in wrong state) 
java
// NullPointerException
String s = null;
s.length();   // throws NullPointerException

// ArrayIndexOutOfBoundsException
int[] arr = new int[3];
arr[5] = 10;  // throws ArrayIndexOutOfBoundsException

// ClassCastException
Object obj = "hello";
Integer i = (Integer) obj;   // throws ClassCastException

// ArithmeticException
int result = 10 / 0;   // throws ArithmeticException: / by zero

// NumberFormatException — subclass of IllegalArgumentException
int n = Integer.parseInt("abc");   // throws NumberFormatException

// IllegalArgumentException
public void setAge(int age) {
    if (age < 0) throw new IllegalArgumentException("Age cannot be negative");
}

// IllegalStateException
Iterator<String> it = list.iterator();
it.remove();   // throws IllegalStateException if next() not called first

Error Classes

Error subclasses represent severe JVM-level problems. Application code should not attempt to catch or recover from them.

Class Typical Cause
OutOfMemoryError JVM cannot allocate memory for a new object
StackOverflowError Infinite or excessively deep recursion exhausts the call stack
VirtualMachineError General JVM internal error (parent of the above two) 
java
// StackOverflowError — infinite recursion
public void recurse() {
    recurse();   // eventually throws StackOverflowError
}

// OutOfMemoryError — allocating more memory than the heap allows
int[] huge = new int[Integer.MAX_VALUE];   // throws OutOfMemoryError

Exam tip: StackOverflowError and OutOfMemoryError are Error subclasses, not Exception subclasses. A catch (Exception e) block will not catch them.

Exception Hierarchy Quick Reference

plaintext
Throwable
├── Error (unchecked, do not catch)
│   ├── OutOfMemoryError
│   └── StackOverflowError
└── Exception
    ├── IOException  (checked)
    │   └── FileNotFoundException  (checked)
    ├── SQLException  (checked)
    ├── ParseException  (checked)
    └── RuntimeException  (unchecked)
        ├── NullPointerException
        ├── ArrayIndexOutOfBoundsException
        ├── ClassCastException
        ├── ArithmeticException
        ├── NumberFormatException
        │     (extends IllegalArgumentException)
        ├── IllegalArgumentException
        └── IllegalStateException

Key Rules Summary

  • Checked exceptions (IOException, SQLException, etc.) require throws declarations or catch blocks.
  • NullPointerException, ClassCastException, and ArrayIndexOutOfBoundsException are the most frequently seen unchecked exceptions on the exam.
  • NumberFormatException is a subclass of IllegalArgumentException.
  • FileNotFoundException is a subclass of IOException — catching IOException also catches FileNotFoundException.
  • Error subclasses (OutOfMemoryError, StackOverflowError) are neither checked exceptions nor RuntimeExceptions — they sit in a completely separate branch.

References