The static modifier is used for identifiers that apply to a class as a whole, rather than
to a particular instance of a class. Constants should be declared to be static, because
then there will be only one copy of the constant, instead of one copy for each instance
of the class. To access a static identifier outside of its class, the class identifier—
rather than an object—is the qualifier.
An exception is an object that signals a special situation, usually that an error
has occurred. An exception can be handled with try/catch blocks. The sequence of
statements in the try block is executed. If, during execution, an exception is thrown
(indicating that an error has occurred), the appropriate catch block is executed to
specify what, if anything, is to be done.
File output is similar to console-oriented output, except that a PrintWriter object
is explicitly created to write to the specified output file. The output is not immediately
sent to the output file, but rather to a buffer. At the conclusion of file processing, the
buffer is flushed to the output file by a call to the close method.
To increase a programmer’s confidence that a method is correct—that is, satisfies
the method’s specifications—the method can be tested in a program that repeatedly
invokes the method with a variety of argument values. For testing all of a
class’s methods in concert, a program called a driver is created. Testing is bottomup:
low-level classes are tested first; ultimately, the entire project is run against system
tests.
The Java run-time, also known as the Java Virtual Machine, is a program that
interprets and executes the bytecode output from a Java compiler. Among other tasks,
the Java Virtual Machine is responsible for preinitialization of fields and deallocation
of inaccessible objects.
A package is a collection of related classes. An identifier with no visibility modifier
is said to have default visibility. Java is “package friendly.” That is, an identifier
with default visibility can be accessed by any object (or class, in the case of a
static member) in the same package as the class in which the identifier is declared.
If a given class’s identifier has protected visibility, that identifier can be accessed
in any subclass of the given class, even in a different package. Unfortunately, that
identifier may also be accessed in any class—even if not a subclass—within the
given package’s class.
The equals method in the Object class should be overridden for any class C
whose instances might become elements of a collection. The overriding method
invokes the instanceof method to return false for any argument object that is not an
instance of class C, and then casts the Object class to class C in order to make the
appropriate comparisons.
