Re: Create grid

Eric Sosman <esosman@ieee-dot-org.invalid>
Mon, 18 Jan 2010 14:48:42 -0500
On 1/18/2010 11:42 AM, Fencer wrote:

Hello, I'm writing a simulator that involves the concept of a grid of
tiles. There are a number of different tile types and each tile must be
of one of those types.

I want a method that can create a new grid from a text file. Say the
file has the following content:
the method should create a 3x3 grid and the type of any given tile
is determined by its corresponding character in the input file.

I wanted this method to work both on real text files but also on
file-like objects (inspired by python's StringIO), so I came up with this:

     The code looks reasonable (as far as it goes: You've still
not dealt with actually creating the tiles). I've interlarded
a few fairly minor comments:

package controller;


public class Controller {
public void init(final Reader reader) throws IOException {
LineNumberReader rdr = new LineNumberReader(reader);

String line = "";

     Why bother to initialize `line'? Since the very first
thing you'll do with it is assign it a different value, the
as-initialized value is never used at all. Initialization
can even make things worse: For example, suppose you had
another String variable `lion' lying around, and you wrote
`lion = rdr.readLine()' by accident. If `line' were not
initialized, javac would complain if you then tried to use
the value -- but with initialization, javac won't complain
and it may take you longer to discover the error.

     Initialize when the initial value might actually be
used for something; otherwise, don't.

while ((line = rdr.readLine()) != null) {

     The principle of "restrict each variable to its narrowest
useful scope" says that `line' really shouldn't exist after
the end of the `while' loop. One way to do this is to create
a brand-new scope just for `line':

        String line;
        while ((line = ...) != null) {

Or, there's a slicker way:

    for (String line; (line = ...) != null; ) {

for (int column = 0; column < line.length(); ++column) {

     Some kind of sanity-checking of the length and content of
the input line would be a good idea. It is almost never a good
idea to swallow external inputs without examination. (See also
"SQL injection attack.")

int lineNum = rdr.getLineNumber() - 1;

     Since `lineNum' will have the same value for every character
in the current line, it would make more sense to compute it once
before the inner loop starts than to re-compute the exact same
thing over and over again. For a three-by-three grid this won't
make any noticeable difference, but keep the "compute once, save
for later" idea in mind.

     Also, dragging out all the machinery of LineNumberReader
just to get a count (a count you need to adjust, no less) seems
like overkill. Why not just use your own `int' variable, start
it at zero, and increment for each line?

char c = line.charAt(column);
createTile(lineNum, column, c);
[... test harness snipped ...]

     Summary: Looks reasonable to me; room for small improvements.

Eric Sosman

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