Difference between Checked and Unchecked exceptions in Java

Java distinguishes between checked exceptions and unchecked exceptions. This distinction is important, because the Java compiler enforces a catch-or-declare requirement for checked exceptions. An exception’s type determines whether it’s checked or unchecked. All exception types that are direct or indirect subclasses of class RuntimeException (package java.lang) are unchecked exceptions. These are typically caused by defects in your program’s code. Examples of unchecked exceptions include ArrayIndexOutOfBoundsExceptions (discussed in Chapter 7) and ArithmeticExceptions (shown in Fig. 11.3). All classes that inherit from class Exception but not class RuntimeException are considered to be checked exceptions. Such exceptions are typically caused by conditions that are not under the control of the program — for example, in file processing, the program can’t open a file because the file does not exist. Classes that inherit from class Error are considered to be unchecked.

The compiler checks each method call and method declaration to determine whether

the method throws checked exceptions. If so, the compiler verifies that the checked exception is caught or is declared in a throws clause. We show how to catch and declare checked exceptions in the next several examples. Recall from Section 11.3 that the throws clause specifies the exceptions a method throws. Such exceptions are not caught in the method’s body. To satisfy the catch part of the catch-or-declare requirement, the code that generates the exception must be wrapped in a try block and must provide a catch handler for the checked-exception type (or one of its superclass types). To satisfy the declare part of the catch-or-declare requirement, the method containing the code that generates the exception must provide a throws clause containing the checked-exception type after its parameter list and before its method body. If the catch-or-declare requirement is not satisfied, the compiler will issue an error message indicating that the exception must be caught or declared. This forces you to think about the problems that may occur when a method that throws checked exceptions is called.

Unlike checked exceptions, the Java compiler does not check the code to determine

whether an unchecked exception is caught or declared. Unchecked exceptions typically can be prevented by proper coding. For example, the unchecked ArithmeticException exception can be avoided if the method ensures that the denominator is not zero before attempting to perform the division. Unchecked exceptions are not required to be listed in a method’s throws clause—even if they are, it’s not required that such exceptions be caught by an application.