Exceptions: When Things Go Wrong

Checked and Unchecked Exceptions

Java’s exception hierarchy is divided into two types of exceptions. A checked exception is one that can be analyzed by the Java compiler. Checked exceptions are thrown by methods such as the BufferedReader.readLine() method, in which there is a substantial likelihood that something might go wrong. When the compiler encounters one of these method calls, it checks whether the program either handles or declares the exception. Compile-time checking for these exceptions is designed to reduce the number of exceptions that are not properly handled within a program. This improves the security of Java programs.

The throws Clause

The IOException, which we encountered in Chapter 4 , is a checked exception. The Java compiler knows that readLine() is a method that can throw an IOException. A method that contains an expression that might throw a checked exception must either handle the exception or declare it. Otherwise, the compiler would generate a syntax error. The simplest way to avoid such a syntax error is to declare the exception, in our case that means qualifying the method header with the expression throws IOException.

In general, any method that contains an expression that might throw a checked expression must declare the exception. However, because one method can call another method, declaring exceptions can get a little tricky. If a method calls another method that contains an expression that might throw an unchecked exception, then both methods must have a throws clause. For example, consider the following program:

In this case, the doRead() method contains a readLine() expression, which might throw an IOException. Therefore, the doRead() method must declare that it throws IOException. However, because doRead() is called by main(), the main() method must also declare the IOException.

The alternative approach would be to catch the IOException within the body of the method. We will discuss this approach in the next section.

Unchecked Exceptions

An unchecked exception is any exception belonging to a subclass of RuntimeException (Fig. 10.4). Unchecked exceptions are not checked by the compiler. The possibility that some statement or expression will lead to an ArithmeticException or NullPointerException is extremely difficult to detect at compile time. The designers of Java decided that forcing programmers to declare such exceptions would not significantly improve the correctness of Java programs.

Therefore, unchecked exceptions do not have to be handled within a program. And they do not have to be declared in a throws clause. As shown in the chapter’s early divide-by-zero exception example, unchecked exceptions are handled by Java’s default exception handlers, unless your program takes specific steps to handle them directly. In many cases leaving the handling of such exceptions up to Java may be the best course of action, as we will see Section 10.5.