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9: Error Handling with Exceptions

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9: Error Handling with Exceptions

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9: Error Handling
with Exceptions

The basic philosophy of Java is that “badly formed code will not be run.”

The ideal time to catch an error is at compile time, before you even try to run the program. However, not all errors can be detected at compile time. The rest of the problems must be handled at run time through some formality that allows the originator of the error to pass appropriate information to a recipient who will know how to handle the difficulty properly. Feedback

C and other earlier languages often had multiple error-handling schemes, and these were generally established by convention and not as part of the programming language. Typically, you returned a special value or set a flag, and the recipient was supposed to look at the value or the flag and determine that something was amiss. However, as the years passed, it was discovered that programmers who use a library tend to think of themselves as invincible—as in, “Yes, errors might happen to others, but not in my code.” So, not too surprisingly, they wouldn’t check for the error conditions (and sometimes the error conditions were too silly to check for[40]). If you were thorough enough to check for an error every time you called a method, your code could turn into an unreadable nightmare. Because programmers could still coax systems out of these languages, they were resistant to admitting the truth: that this approach to handling errors was a major limitation to creating large, robust, maintainable programs. Feedback

The solution is to take the casual nature out of error handling and to enforce formality. This actually has a long history, because implementations of exception handling go back to operating systems in the 1960s, and even to BASIC’s “on error goto.” But C++ exception handling was based on Ada, and Java’s is based primarily on C++ (although it looks more like that in Object Pascal). Feedback

The word “exception” is meant in the sense of “I take exception to that.” At the point where the problem occurs, you might not know what to do with it, but you do know that you can’t just continue on merrily; you must stop, and somebody, somewhere, must figure out what to do. But you don’t have enough information in the current context to fix the problem. So you hand the problem out to a higher context where someone is qualified to make the proper decision (much like a chain of command). Feedback

The other rather significant benefit of exceptions is that they clean up error handling code. Instead of checking for a particular error and dealing with it at multiple places in your program, you no longer need to check at the point of the method call (since the exception will guarantee that someone catches it). And, you need to handle the problem in only one place, the so-called exception handler. This saves you code, and it separates the code that describes what you want to do from the code that is executed when things go awry. In general, reading, writing, and debugging code becomes much clearer with exceptions than when using the old way of error handling. Feedback

Because exception handling is the only official way that Java reports errors, and it is enforced by the Java compiler, there are only so many examples that can be written in this book without learning about exception handling. This chapter introduces you to the code you need to write to properly handle exceptions, and the way you can generate your own exceptions if one of your methods gets into trouble. Feedback

Basic exceptions

An exceptional condition is a problem that prevents the continuation of the method or scope that you’re in. It’s important to distinguish an exceptional condition from a normal problem, in which you have enough information in the current context to somehow cope with the difficulty. With an exceptional condition, you cannot continue processing because you don’t have the information necessary to deal with the problem in the current context. All you can do is jump out of the current context and relegate that problem to a higher context. This is what happens when you throw an exception. Feedback

Division is a simple example. If you’re about to divide by zero, it’s worth checking for that condition. But what does it mean that the denominator is zero? Maybe you know, in the context of the problem you’re trying to solve in that particular method, how to deal with a zero denominator. But if it’s an unexpected value, you can’t deal with it and so must throw an exception rather than continuing along that execution path. Feedback

When you throw an exception, several things happen. First, the exception object is created in the same way that any Java object is created: on the heap, with new. Then the current path of execution (the one you couldn’t continue) is stopped and the reference for the exception object is ejected from the current context. At this point the exception handling mechanism takes over and begins to look for an appropriate place to continue executing the program. This appropriate place is the exception handler, whose job is to recover from the problem so the program can either try another tack or just continue. Feedback

As a simple example of throwing an exception, consider an object reference called t. It’s possible that you might be passed a reference that hasn’t been initialized, so you might want to check before trying to call a method using that object reference. You can send information about the error into a larger context by creating an object representing your information and “throwing” it out of your current context. This is called throwing an exception. Here’s what it looks like:

if(t == null)
  throw new NullPointerException();


This throws the exception, which allows you—in the current context—to abdicate responsibility for thinking about the issue further. It’s just magically handled somewhere else. Precisely where will be shown shortly. Feedback

Exception arguments

Like any object in Java, you always create exceptions on the heap using new, which allocates storage and calls a constructor. There are two constructors in all standard exceptions; The first is the default constructor, and the second takes a string argument so you can place pertinent information in the exception:

  throw new NullPointerException("t = null");


This string can later be extracted using various methods, as you’ll see. Feedback

The keyword throw causes a number of relatively magical things to happen. Typically, you’ll first use new to create an object that represents the error condition. You give the resulting reference to throw. The object is, in effect, “returned” from the method, even though that object type isn’t normally what the method is designed to return. A simplistic way to think about exception handling is as a different kind of return mechanism, although you get into trouble if you take that analogy too far. You can also exit from ordinary scopes by throwing an exception. But a value is returned, and the method or scope exits. Feedback

Any similarity to an ordinary return from a method ends here, because where you return is someplace completely different from where you return for a normal method call. (You end up in an appropriate exception handler that might be far—many levels away on the call stack—from where the exception was thrown.) Feedback

In addition, you can throw any type of Throwable (the exception root class) object that you want. Typically, you’ll throw a different class of exception for each different type of error. The information about the error is represented both inside the exception object and implicitly in the name of the exception class, so someone in the bigger context can figure out what to do with your exception. (Often, the only information is the type of exception, and nothing meaningful is stored within the exception object.) Feedback



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