330 Java Tips.pdf - FTP Server

String, text, numbers, I/O II part
inherit from Object (or anything else, for that matter). To put primitives (byte, short,
int, long, float, double, boolean, char) into something that requires an Object, use
Java's wrapper classes.
The wrapper classes are Double, Integer, Long, Boolean, etc., and are basically an
object "wrapped" around a primitive type. You make a Double object by:
Double d = new Double (myDouble);
and to get the actual value back,
double z = d.doubleValue();
It works the same way for all the rest of the primitive/wrapper pairs.
by Trevor Hill
Q: is there a mod (x, y) function that returns the remainder when x is divided by
y? Something equivalent to fmod(x,y) in C?
Answer: a = x%y;
I'm having trouble figuring out how to convert characters to their ASCII value in
java
Q: I'm having trouble figuring out how to convert characters to their ASCII
value in java. Is there a class like NumberFormat that will do it?
Answer: I can't see any problem here:
char ch = 'A'; // character 'A'
int i = (int)ch; // ASCII value for 'A' (=>65)
Yes. And just be aware that ASCII only runs from 0 through 127. Anything
higher needs to be addressed differently, since Java is using Unicode values.
Q: I'm writing a program that makes I/O on a (huge) file.
Due to its size, I have to directly jump to a given line number so that I can make a
readLine(). Is there a fast way to do so without traversing all the previous lines?
Note that I've tried the LineNumberReader class but it only keeps track of line
numbers, and does not allow me to go to a specific position in the stream.
Answer, Part 1:
No, there is no easy way because the exact beginning of a line isn't stored
anywhere. It's determined by the content of the file, and nobody is keeping track of
the line feeds, carriage returns etc.
If the file doesn't change, you could find out the offset of each line relative to the
beginning of the file once in a separate run over the original file and store these
offset values in a binary file that has one long value for each line.
You can then load these long values into an array and access it whenever you need
a line. You then seek to that position and read the line directly. If there are too many
long offsets to be kept in memory, you still have a speed advantage; if you want the
offset of line i (i zero-based, so the first line is 0, the second 1 and so on), you seek
to i * 8 (that's the size of a long) with a RandomAccessFile, load the offset value, go
that position in the data file and load the line.
With intelligent caching, you could avoid quite some disk access.
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