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Tips for Using Exceptions

There is a certain amount of controversy about the proper use of exceptions. Some programmers believe that all checked exceptions are a nuisance, others can't seem to throw enough of them. We think that exceptions (even checked exceptions) have their place, and offer you these tips for their proper use.

  1. Exception handling is not supposed to replace a simple test.

    As an example of this, we wrote some code that uses the built-in Stack class. The code in Example 11-3 tries 1,000,000 times to pop an empty stack. It first does this by finding out whether the stack is empty.

    if (!s.empty()) s.pop();

    Next, we tell it to pop the stack no matter what. Then, we catch the EmptyStackException that tells us that we should not have done that.

    catch (EmptyStackException e)

    On our test machine, we got the timing data in Table 11-1.

    Table 11-1. Timing Data



    154 milliseconds

    10739 milliseconds

    As you can see, it took far longer to catch an exception than it did to perform a simple test. The moral is: Use exceptions for exceptional circumstances only.

  2. Do not micromanage exceptions.

    Many programmers wrap every statement in a separate try block.

    OutputStream out;
    Stack s;
    for (i = 0; i < 100; i++)
          n = s.pop();
       catch (EmptyStackException s)
          // stack was empty
       catch (IOException e)
          // problem writing to file

    This approach blows up your code dramatically. Think about the task that you want the code to accomplish. Here we want to pop 100 numbers off a stack and save them to a file. (Never mind why—it is just a toy example.) There is nothing we can do if a problem rears its ugly head. If the stack is empty, it will not become occupied. If the file contains an error, the error will not magically go away. It therefore makes sense to wrap the entire task in a try block. If any one operation fails, you can then abandon the task.

       for (i = 0; i < 100; i++)
          n = s.pop();
    catch (IOException e)
       // problem writing to file
    catch (EmptyStackException s)
       // stack was empty

    This code looks much cleaner. It fulfills one of the promises of exception handling, to separate normal processing from error handling.

  3. Make good use of the exception hierarchy.

    Don't just throw a RuntimeException. Find an appropriate subclass or create your own.

    Don't just catch Throwable. It makes your code hard to read and maintain.

    Respect the difference between checked and unchecked exceptions. Checked exceptions are inherently burdensome—don't throw them for logic errors. (For example, the reflection library gets this wrong. Callers often need to catch exceptions that they know can never happen.)

    Do not hesitate to turn an exception into another exception that is more appropriate. For example, when you parse an integer in a file, catch the NumberFormatException and turn it into a subclass of IOException or MySubsystemException.

  4. Do not squelch exceptions.

    In Java, there is the tremendous temptation to shut up exceptions. You write a method that calls a method that might throw an exception once a century. The compiler whines because you have not declared the exception in the throws list of your method. You do not want to put it in the throws list because then the compiler will whine about all the methods that call your method. So you just shut it up:

    public Image loadImage(String s)
          code that threatens to throw checked exceptions
       catch (Exception e)
       {} // so there

    Now your code will compile without a hitch. It will run fine, except when an exception occurs. Then, the exception will be silently ignored. If you believe that exceptions are at all important, you should make some effort to handle them right.

  5. When you detect an error, "tough love" works better than indulgence.

    Some programmers worry about throwing exceptions when they detect errors. Maybe it would be better to return a dummy value rather than throw an exception when a method is called with invalid parameters. For example, should Stack.pop return null rather than throw an exception when a stack is empty? We think it is better to throw a EmptyStackException at the point of failure than to have a NullPointerException occur at later time.

  6. Propagating exceptions is not a sign of shame.

    Many programmers feel compelled to catch all exceptions that are thrown. If they call a method that throws an exception, such as the FileInputStream constructor or the readLine method, they instinctively catch the exception that may be generated. Often, it is actually better to propagate the exception instead of catching it:

    public void readStuff(String filename) throws IOException // not a sign of shame!
       InputStream in = new FileInputStream(filename);
       . . .

    Higher-level methods are often better equipped to inform the user of errors or to abandon unsuccessful commands.

Rules 5 and 6 can be summarized as "throw early, catch late."

Example 11-3. ExceptionalTest.java

 1. import java.util.*;
 3. public class ExceptionalTest
 4. {
 5.    public static void main(String[] args)
 6.    {
 7.       int i = 0;
 8.       int ntry = 1000000;
 9.       Stack s = new Stack();
10.       long s1;
11.       long s2;
13.       // test a stack for emptiness ntry times
14.       System.out.println("Testing for empty stack");
15.       s1 = new Date().getTime();
16.       for (i = 0; i <= ntry; i++)
17.          if (!s.empty()) s.pop();
18.       s2 = new Date().getTime();
19.       System.out.println((s2 - s1) + " milliseconds");
21.       // pop an empty stack ntry times and catch the
22.       // resulting exception
23.       System.out.println("Catching EmptyStackException");
24.       s1 = new Date().getTime();
25.       for (i = 0; i <= ntry; i++)
26.       {
27.          try
28.          {
29.             s.pop();
30.          }
31.          catch(EmptyStackException e)
32.          {
33.          }
34.       }
35.       s2 = new Date().getTime();
36.       System.out.println((s2 - s1) + " milliseconds");
37.    }
38. }
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