The Error Class

When the JVM detects a serious abnormal condition in the program, it raises an exception of type Error. When you get an exception of Error or its subtypes, the exception is not meant for you to handle. The best course of action is to let the program crash! Why? Let’s discuss a trivial example to understand this.

Assume that you try to run a program that does not exist! For example, consider the UnCheckedExceptionExample3 class that you saw in Listing 11-16; if you make a mistake in the capitalization of the class name and try to invoke it, you’ll get NoClassDefFoundError.

D:\ > java UncheckedExceptionExample3

Exception in thread "main" java.lang.NoClassDefFoundError: UncheckedExceptionExample3 (wrong name: UnCheckedExceptionExample3)

at java.lang.ClassLoader.defineClass1(Native Method)

at java.lang.ClassLoader.defineClass(ClassLoader.java:791)

at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)

at java.net.URLClassLoader.access$100(URLClassLoader.java:71) at java.net.URLClassLoader$1.run(URLClassLoader.java:361)

at java.net.URLClassLoader$1.run(URLClassLoader.java:355)

at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:354) at java.lang.ClassLoader.loadClass(ClassLoader.java:423)

at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308) at java.lang.ClassLoader.loadClass(ClassLoader.java:356)

at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:480)

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Chapter 11 ■ Exceptions and Assertions

As the helpful part of the stack trace indicates (“wrong name: UnCheckedExceptionExample3”), you have given a class to load that does not exist by the given name (note that names in Java are case-sensitive). So, the JVM responded with a NoClassDefFoundError.

Let’s consider a programming example to understand how an error could occur. Assume that you’re writing a recursive method to calculate the factorial of a number and forget to put in the right termination condition (see Listing 11-18).

Listing 11-18.  NonTerminatingRecursion.java

class NonTerminatingRecursion {

        // factorial is a recursive call

        static int factorial(int n) {

                int result = 0;

                // Assume that the following termination condition statement is missing ...

                // if(n == 0) return 1;

                result = factorial(n - 1) * n;

                return result;

        }

        public static void main(String ... args) {

                System.out.println("factorial of 4 is: " + factorial(4));

        }

}

When run, this program crashes after throwing this exception:

Exception in thread "main" java.lang.StackOverflowError

        at NonTerminatingRecursion.factorial(NonTerminatingResursion.java:7)

        at NonTerminatingRecursion.factorial(NonTerminatingResursion.java:7)

         [... this at "NonTerminatingRecursion.factorial(NonTerminatingResursion.java:7)" is repeated a large number of times...]

For each method call, the JVM creates a runtime structure called a stack frame in its stack area. Since the recursive call, the JVM keeps creating such stack frames, and after some time, it exhausts the stack area. At this point, the JVM cannot continue its execution, so it throws the StackOverflowError. When you get a StackOverflowError, you can almost be sure that it is a programming error that caused this exception. You need to fix the program to avoid raising this exception.

Exceptions of type Error indicate an abnormal condition in the program. There is no point in catching this exception and trying to continue execution and pretending nothing has happened. It is a really bad practice to do so!

Table 11-3 provides a list of important subclasses of the Error class.

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