Exceptions are objects, and objects are defined using classes. The root class for exceptions is java.lang.Throwable. The preceding section used the classes ArithmeticException and InputMismatchException. Are there any other types of exceptions you can use? Can you define your own exception classes? Yes. There are many predefined exception classes in the Java API. Figure below shows some of them.

The class names Error, Exception, and RuntimeException are somewhat confusing. All three of these classes are exceptions, and all of the errors occur at runtime.

The Throwable class is the root of exception classes. All Java exception classes inherit directly or indirectly from Throwable. You can create your own exception classes by extending Exception or a subclass of Exception.

The exception classes can be classified into three major types: system errors, exceptions, and runtime exceptions.

• System errors are thrown by the JVM and are represented in the Error class. The Error class describes internal system errors, though such errors rarely occur. If one does, there is little you can do beyond notifying the user and trying to terminate the program gracefully. Examples of subclasses of Error are listed in Table below.

• Exceptions are represented in the Exception class, which describes errors caused by your program and by external circumstances. These errors can be caught and handled by your program. Examples of subclasses of Exception are listed in Table below.

• Runtime exceptions are represented in the RuntimeException class, which describes programming errors, such as bad casting, accessing an out-of-bounds array, and numeric errors. Runtime exceptions are generally thrown by the JVM. Examples of subclasses are listed in Table below.

RuntimeException, Error, and their subclasses are known as unchecked exceptions. All other exceptions are known as checked exceptions, meaning that the compiler forces the programmer to check and deal with them in a try-catch block or declare it in the method header.

In most cases, unchecked exceptions reflect programming logic errors that are unrecoverable. For example, a NullPointerException is thrown if you access an object through a reference variable before an object is assigned to it; an IndexOutOfBoundsException is thrown if you access an element in an array outside the bounds of the array. These are logic errors that should be corrected in the program. Unchecked exceptions can occur anywhere in a program. To avoid cumbersome overuse of try-catch blocks, Java does not mandate that you write code to catch or declare unchecked exceptions.