In this article, we will learn the exception class hierarchy in java.lang package. 

At the top of the exceptions hierarchy is the Throwable class. Every class that can be used for exception handling in Java directly or indirectly inherits from this class. The Throwable class is divided into two major subclasses: 

• Error 
• Exception.

Exceptions Hierarchy in Java 

The figure below illustrates the class hierarchy of the Throwable class and its most significant subclasses.

1. Error Class

The Error class and its subclasses represent abnormal conditions that should generally not be caught in your application. These indicate serious system problems, often JVM related, which an application might not be able to handle. 

Hierarchy of Error Class

The figure below illustrates the class hierarchy of Error Class:

Learn more about below Java built-in errors:

• OutOfMemoryError
• StackOverflowError
• NoClassDefFoundError

2. Exception

Checked Exceptions

These are exceptions that a method can throw but must either catch them or declare them using the throws keyword. They are direct subclasses of the Exception class, except RuntimeException. 

Java built-in checked exceptions:

• FileNotFoundException
• EOFException
• ClassNotFoundException
• SQLException
• NoSuchMethodException
• InterruptedException

The figure below illustrates the class hierarchy of the Exception Class:

Exception Class Example: In this example, FileReader class try to read a file from an invalid location will throw FileNotFoundException exception.

public class FileNotFoundExceptionExample {
    public static void main(String[] args) {
        
        BufferedReader reader = null;
        try {
            reader = new BufferedReader(new FileReader(new File("/invalid/file/location")));
        } catch (FileNotFoundException e) {
            System.err.println("FileNotFoundException caught!");
        }
    }
}

Output:

FileNotFoundException caught!

RuntimeException (Unchecked Exceptions)

These exceptions are not checked at compile-time. They're a direct subclass of the RuntimeException class. 

Java built-in unchecked exceptions:

• NullPointerException
• ArrayIndexOutOfBoundsException
• StringIndexOutOfBoundsException
• ArithmeticException
• IllegalArgumentException
• NumberFormatException
• IllegalStateException
• ClassCastException

The figure below illustrates the class hierarchy of the RuntimeException Class:

RuntimeException Example:  In the below example, the Person class object is not created using the new keyword but it is just declared with a null value. Now we are trying to access the personName field value on the null reference so JVM will throw a NullPointerException exception here.

public class NullPointerExceptionExample {

    public static void main(String[] args) {

        Person personObj = null;

        try {
            String name = personObj.personName; // Accessing the field of a null object
            personObj.personName = "Jon Doe"; // Modifying the field of a null object
        } catch (NullPointerException e) {
            System.err.println("NullPointerException caught!");
        }

    }
}

class Person {

    public String personName;

    public String getPersonName() {
        return personName;
    }

    public void setPersonName(String personName) {
        this.personName = personName;
    }

}

Output:

NullPointerException caught!

Exception Handling Related Posts

• Java Exception Handling Guide
• How the Exception Handling Works in Java
• Three Types of Exceptions in Java
• Exceptions Hierarchy in Java
• Java Chained Exceptions
• java.lang.Throwable Class in Java
• Exception Handling Keywords in Java
• Java try/catch Block
• Java throw Keyword
• Java throws Keyword
• Java finally Block
• The try-with-resources Statement
• Java Exception Handling Best Practices