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Screenshot Core Java 2: Volume I - Fundamentals

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 11.  Exceptions and Debugging


Assertions are a commonly used idiom for defensive programming. Suppose you are convinced that a particular property is fulfilled, and you rely on that property in your code. For example, you may be computing

double y = Math.sqrt(x);

You are certain that x is not negative. Perhaps it is the result of another computation that can't have a negative result, or it is a parameter of a method that requires its callers to supply only positive inputs. Still, you want to double-check rather than having confusing "not a number" floating-point values creep into your computation. You could, of course, throw an exception:

if (x < 0) throw new IllegalArgumentException("x < 0");

But this code stays in the program, even after testing is complete. If you have lots of checks of this kind, the program runs quite a bit slower than it should. The assertion mechanism allows you to put in checks during testing, and to have them automatically removed in the production code. As of SDK 1.4, the Java language has a new keyword assert. There are two forms:

assert condition;


assert condition : expression;

Both statements evaluate the condition and throw an AssertionError if it is false. In the second statement, the expression is passed to the constructor of the AssertionError object and turned into a message string.

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The sole purpose of the expression part is to produce a message string. The AssertionError object does not store the actual expression value, so you can't query it later. As the SDK documentation states with paternalistic charm, doing so "would encourage programmers to attempt to recover from assertion failure, which defeats the purpose of the facility."

To assert that x is nonnegative, you can simply use the statement
assert x >= 0;

Or you can pass the actual value of x into the AssertionError object, so that it gets displayed later.

assert x >= 0 : x;

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The assert macro of the C language turns the assertion condition into a string that is printed if the assertion fails. For example, if assert(x >= 0) fails, it prints that "x >= 0" is the failing condition. In Java, the condition is not automatically part of the error report. If you want to see it, you have to pass it as a string into the AssertionError object: assert x >= 0 : "x >= 0".

Since assert is a new keyword, you have to tell the compiler that you are going to take advantage of it. Use the -source 1.4 option, like this:
javac -source 1.4 MyClass.class

Of course, in some future version of the SDK, support for assertions will become the default.

Enabling and Disabling Assertions

By default, assertions are disabled. You enable them by running the program with the -enableassertions or -ea option:

java -enableassertions MyApp

Note that you do not have to recompile your program to enable or disable assertions. Enabling or disabling assertions is a function of the class loader. When assertions are disabled, the class loader strips out the assertion code so that it won't slow down execution. You can even turn on assertions in specific classes, or in entire packages. For example,

java -ea:MyClass -ea:com.mycompany.mylib... MyApp

This command turns on assertions for the class MyClass and all classes in the com.mycompany.mylib package and its subpackages. The option -ea... turns on assertions in all classes of the default package. You can also disable assertions in certain classes and packages with the -disableassertions or -da option:

java -ea:... -da:MyClass MyApp

Some classes are not loaded by a class loader but directly by the virtual machine. You can use these switches to selectively enable or disable assertions in those classes. However, the -ea and -da switches that enable or disable all assertions do not apply to the "system classes" without class loaders. Use the -enablesystemassertions/-esa switch to enable assertions in system classes. It is also possible to control the assertion status of class loaders programmatically. See the API notes at the end of this section.

Usage Hints for Assertions

The Java language gives you three mechanisms to deal with system failures:

  • Throwing an exception;
  • Logging;
  • Assertions.

When should you choose assertions? Keep these points in mind:

  • Assertion failures are intended to be fatal, unrecoverable errors.
  • Assertion checks are turned on only during development and testing. (This is sometimes jokingly described as "wearing a life jacket when you are close to shore, and throwing it overboard once you are in the middle of the ocean.")

Therefore, you would not use assertions for signaling recoverable conditions to another part of the program, or for communicating problems to the program user. Assertions should only be used to locate internal program errors during testing. Let's look at a common scenario—the checking of method parameters. Should you use assertions to check for illegal index values or null references? To answer that question, you have to look at the documentation of the method. For example, consider the Arrays.sort method from the standard library.

 Sorts the specified range of the specified array into
 ascending numerical order. The range to be sorted extends
 from fromIndex, inclusive, to toIndex, exclusive.
 @param a the array to be sorted.
 @param fromIndex the index of the first element (inclusive) to
 be sorted.
 @param toIndex the index of the last element (exclusive) to be
 @throws IllegalArgumentException if fromIndex > toIndex
 @throws ArrayIndexOutOfBoundsException if fromIndex < 0 or
 toIndex > a.length
static void sort(int[] a, int fromIndex, int toIndex)

The documentation states that the method throws an exception if the index values are incorrect. That behavior is part of the contract that the method makes with its callers. If you implement the method, you have to respect that contract and throw the indicated exceptions. It would not be appropriate to use assertions instead. Should you assert that a is not null? That is not appropriate either. The method documentation is silent on the behavior of the method when a is null. The callers have the right to assume that the method will return successfully in that case and not throw an assertion error. However, suppose the method contract had been slightly different:

@param a the array to be sorted. (Must not be null)

Now the callers of the method have been put on notice that it is illegal to call the method with a null array. Then the method may start with the assertion

assert(a != null);

Computer scientists call this kind of contract a precondition. The original method had no preconditions on its parameters—it promised a well-defined behavior in all cases. The revised method has a single precondition, that a is not null. If the caller fails to fulfill the precondition, then all bets are off and the method can do anything it wants. In fact, with the assertion in place, the method has a rather unpredictable behavior when it is called illegally. It sometimes throws an assertion error, and sometimes a null pointer exception, depending on how its class loader is configured.

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In JDK 1.4, the Arrays.sort method throws a NullPointerException if you call it with a null array. That is a bug, either in the specification or the implementation.

Many programmers use comments to document their underlying assumptions. The SDK documentation contains a good example:
if (i % 3 == 0)
 . . .
else if (i % 3 == 1)
 . . .
else // (i % 3 == 2)
 . . .

In this case, it makes a lot of sense to use an assertion instead.

if (i % 3 == 0)
 . . .
else if (i % 3 == 1)
 . . .
 assert(i % 3 == 2);
 . . .

Of course, it would make even more sense to think through the issue a bit more thoroughly. What are the possible values of i % 3? If i is positive, the remainders must be 0, 1, or 2. If i is negative, then the remainders can be -1 or -2. Thus, the real assumption is that i is not negative. A better assertion would be

assert(i >= 0);

before the if statement. At any rate, this example shows a good use of assertions as a self-check for the programmer. As you can see, assertions are a tactical tool for testing and debugging, whereas logging is a strategic tool for the entire life cycle of a program

java.lang.ClassLoader 1.0

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  • void setDefaultAssertionStatus(boolean b) 1.4

    enables or disables assertions for all classes loaded by this class loader that don't have an explicit class or package assertion status.

  • void setClassAssertionStatus(String className, boolean b) 1.4

    enables or disables assertions for the given class and its inner classes.

  • void setPackageAssertionStatus(String packageName, boolean b) 1.4

    enables or disables assertions for all classes in the given package and its subpackages.

  • void clearAssertionStatus() 1.4

    removes all explicit class and package assertion status settings, and disables assertions for all classes loaded by this class loader.

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