Reference types - Data Structures and Problem Solving Using Java - page 59

whether there are no exceptions, handled exceptions, or unhandled excep-
tions. The code to handle the close is complex because

1. fileIn must be declared outside of the try block in order to be
visible in the finally block.
2. fileIn must be initialized to null to avoid compiler complaints
about a possible uninitialized variable.

3. Prior to calling close , we must check that fileIn is not null to

avoid generating a NullPointerException ( fileIn would be null
if the file was not found, resulting in an IOException prior to its
assignment).

4. In some instances, but not in our example, close might itself

throw a checked exception, and would then require an additional
try/catch block.

Formatted file output is similar to file input. FileWriter , PrintWriter , and
println replace FileReader , Scanner , and nextLine , respectively. Figure 2.20
illustrates a program that double-spaces files that are specified on the com-
mand line (the resulting files are placed in a file with a .ds extension).
This description of Java I/O, while enough to do basic formatted I/O,
hides an interesting object-oriented design that is discussed in more detail in
Section 4.5.3.

FileWriter is used
for file output.

summary

This chapter examined reference types. A reference is a variable that stores
either the memory address where an object resides or the special reference
null . Only objects may be referenced; any object can be referenced by several
reference variables. When two references are compared via == , the result is
true if both references refer to the same object. Similarly, = makes a reference
variable reference another object. Only a few other operations are available.
The most significant is the dot operator, which allows the selection of an
object's method or access of its internal data.
Because there are only eight primitive types, virtually everything of con-
sequence in Java is an object and is accessed by a reference. This includes
String s, arrays, exception objects, data and file streams, and a string tokenizer.
The String is a special reference type because + and += can be used for con-
catenation. Otherwise, a String is like any other reference; equals is required to
test if the contents of two String s are identical. An array is a collection of iden-
tically typed values. The array is indexed starting at 0, and index range checking
is guaranteed to be performed. Arrays can be expanded dynamically by using