File I/O - Absolute Java - page 619

from each other.) The situation is the same for all the other write methods that
write primitive types to binary files. However, the method writeUTF uses differing
numbers of bytes to store different strings in a file. Longer strings require more
bytes than shorter strings. This can present a problem to Java, because there are no
separators between data items in a binary file. The way that Java manages to make
this work is by writing some extra information at the start of each string. This extra
information tells how many bytes are used to write the string, so readUTF knows how
many bytes to read and convert. (The method readUTF will be discussed a little later
in this chapter, but, as you may have already guessed, it reads a String value that was
written using the UTF coding method.)
The situation with writeUTF is even a little more complicated than what we
discussed in the previous paragraph. Notice that we said that the information at the
start of the string code in the file tells how many bytes to read, not how many characters
are in the string . These two figures are not the same. With the UTF way of encoding,
different characters are encoded in different numbers of bytes. However, all the ASCII
characters are stored in just one byte, and you are undoubtedly using only ASCII
characters, so this difference is more theoretical than real to you now.
Reading Simple Data from a Binary File
The stream class ObjectInputStream is used to read from a file that has been
written to using ObjectOutputStream . Display 10.15 gives some of the most
commonly used methods for this class. If you compare that table with the methods for
ObjectOutputStream given in Display 10.14, you will see that each output method
in ObjectOutputStream has a corresponding input method in ObjectInputStream .
For example, if you write an integer to a file using the method writeInt of
ObjectOutputStream , then you can read that integer back with the method readInt
of ObjectInputStream . If you write a number to a file using the method writeDouble
of ObjectOutputStream , then you can read that number back with the method
readDouble of ObjectInputStream , and so forth. Display 10.16 gives an example of
using readInt in this way.
Self-Test Exercises
28. How do you open the binary fi le bindata.dat so that it is connected to an
output stream of type ObjectOutputStream that is named outputThisWay ?
29. Give two statements that will write the values of the two double variables
v1 and v2 to the fi le bindata.dat . Use the stream outputThisWay that you
created as the answer to Self-Test Exercise 28.
30. Give a statement that will write the string value "Hello" to the fi le
bindata.dat . Use the stream outputThisWay that you created as the answer
to Self-Test Exercise 28.
31. Give a statement that will close the stream outputThisWay created as the
answer to Self-Test Exercise 28.