Read all text from a file
Java 11 added the readString() method to read small files as a String, preserving line terminators:
String content = Files.readString(path, StandardCharsets.US_ASCII);
For versions between Java 7 and 11, here's a compact, robust idiom, wrapped up in a utility method:
static String readFile(String path, Charset encoding)
throws IOException
{
byte[] encoded = Files.readAllBytes(Paths.get(path));
return new String(encoded, encoding);
}
Read lines of text from a file
Java 7 added a convenience method to read a file as lines of text, represented as a List<String>. This approach is "lossy" because the line separators are stripped from the end of each line.
List<String> lines = Files.readAllLines(Paths.get(path), encoding);
Java 8 added the Files.lines() method to produce a Stream<String>. Again, this method is lossy because line separators are stripped. If an IOException is encountered while reading the file, it is wrapped in an UncheckedIOException, since Stream doesn't accept lambdas that throw checked exceptions.
try (Stream<String> lines = Files.lines(path, encoding)) {
lines.forEach(System.out::println);
}
This Stream does need a close() call; this is poorly documented on the API, and I suspect many people don't even notice Stream has a close() method. Be sure to use an ARM-block as shown.
If you are working with a source other than a file, you can use the lines() method in BufferedReader instead.
Memory utilization
The first method, that preserves line breaks, can temporarily require memory several times the size of the file, because for a short time the raw file contents (a byte array), and the decoded characters (each of which is 16 bits even if encoded as 8 bits in the file) reside in memory at once. It is safest to apply to files that you know to be small relative to the available memory.
The second method, reading lines, is usually more memory efficient, because the input byte buffer for decoding doesn't need to contain the entire file. However, it's still not suitable for files that are very large relative to available memory.
For reading large files, you need a different design for your program, one that reads a chunk of text from a stream, processes it, and then moves on to the next, reusing the same fixed-sized memory block. Here, "large" depends on the computer specs. Nowadays, this threshold might be many gigabytes of RAM. The third method, using a Stream<String> is one way to do this, if your input "records" happen to be individual lines. (Using the readLine() method of BufferedReader is the procedural equivalent to this approach.)
Character encoding
One thing that is missing from the sample in the original post is the character encoding. There are some special cases where the platform default is what you want, but they are rare, and you should be able justify your choice.
The StandardCharsets class defines some constants for the encodings required of all Java runtimes:
String content = readFile("test.txt", StandardCharsets.UTF_8);
The platform default is available from the Charset class itself:
String content = readFile("test.txt", Charset.defaultCharset());
Note: This answer largely replaces my Java 6 version. The utility of Java 7 safely simplifies the code, and the old answer, which used a mapped byte buffer, prevented the file that was read from being deleted until the mapped buffer was garbage collected. You can view the old version via the "edited" link on this answer.
Best Answer
I'm not a Java programmer, but I know a fair bit about rendering audio so hopefully the following might be of some help...
Given that you will almost always have a much larger number of samples than available pixels the sensible thing to do would be to draw from a cached reduction or 'summary' of the sample data. This is typically how audio editors (such as Audacity) render audio data. In fact the most common strategy is to compute the number of samples per pixel, then find the maximum and minimum samples for each block of size
SamplesPerPixel, then draw a vertical line between each max-min pair. You might want to cache this reduction, or perhaps a series of such reductions for different zoom levels. Audacity caches to temporary files ('block files') on disk.The above is perhaps something of an oversimplification, however, because in reality you will want to compute the initial max-min pairs from a chunk of fixed size - say 256 samples - rather than from one of size
SamplesPerPixel. Then you can compute further 'on the fly' reductions from that cached reduction. The point is thatSamplesPerPixelwill typically be a dynamic quantity - since the user might resize the canvas at any time (hope that makes sense...).Also remember that when you are drawing to your canvas you will need to scale the sample values by the width and height of the canvas. The best way to do this (in the vertical direction, at least) is to normalize the samples, then multiply by the canvas height. 16-bit audio consists of samples in the range [-32768, 32767], so to normalize just do a floating-point division by 32768. Then reverse the sign (to flip the waveform to the canvas coordinates), add 1 (to compensate for the negative values) and multiply by half the canvas height. That's how I do it, anyway.
This page shows how to build a rudimentary waveform display with Java Swing. I haven't looked at it in detail, but I think it just downsamples the data rather than computing max-min pairs. This will, of course, not provide as accurate a reduction as the max-min method, but it's easier to calculate.
If you want to know how to do things properly you should dig into the Audacity source code (be warned, however - it's fairly gnarly C++). To get a general overview you might look at 'A Fast Data Structure for Disk-Based Audio Editing', by the original author of Audacity, Dominic Mazzoni. You will need to purchase that from CMJ, however.