Files

Classpath

To load a file from the classpath: Resource in JAR.

Collections

Note: A File object does not compare as expected when used as a key in a map. Better to convert the file into a normalised string format before using as a key. For sample code:

../../misc/howto/java/FolderCompare.java.txt

For the same issue in C# see Folders.

Concatenate Folder and File

Make sure you include a slash at the start of the file (/src/site/apt/):

File projectRoot = new File("/home/patrick/documents/my-memory");
File aptFolder = new File(projectRoot + "/src/site/apt/");

Copy File

(Also see Read File below)…

Is much easier to use the copyFile method from the Commons IO, Apache, Jakarta Project: Commons IO.

If you don’t want to use Commons IO, (we think) the file has to be copied one byte at a time:

import java.io.*;

public class Copy {
    public static void main(String[] args) throws IOException {
        File inputFile = new File("farrago.txt");
        File outputFile = new File("outagain.txt");

        FileReader in = new FileReader(inputFile);
        FileWriter out = new FileWriter(outputFile);
        int c;

        while ((c = in.read()) != -1)
           out.write(c);

        in.close();
        out.close();
    }
}

Invalid Characters

Code Snippets, Filename - Replace Invalid Characters: File.

Iterate through files in a folder…

Alternatively use Apache Commons Directory Walker: Commons IO.

File [] files  = new File(SITES_PATH).listFiles();

for (int i = 0; i < files.length; i++) {
    File f = files[i];
    if (f.isDirectory()) {
        logger.info("addSites() " + f.toString());
    }
}

If you want to filter the list of files:

private void listFiles(final File folder) {
    FilenameFilter filter = new FilenameFilter() {
        public boolean accept(final File dir, final String name) {
            return name.toLowerCase().endsWith(".jar");
        }
    };
    String [] fileList = folder.list(filter);
    for (int i = 0; i < fileList.length; i++) {
        logger.info("Found [" + fileList[i] + "]");
        ...
        }

Note:

list returns a list of strings with the file name.
listFiles returns a list of files.

The accept method appears to have two signatures:

public boolean accept(final File file) {

Move File

Note: Also see Commons IO, FileUtils, Move File: Commons IO.

http://javaalmanac.com/egs/java.io/MoveFile.html

File toFile = new File(...
boolean success = fromFile.renameTo(toFile);
if (!success) {
    // File was not successfully moved
}

Read File

Note: See Commons IO, FileUtils, Read a File: Commons IO.

http://www.devdaily.com/java/edu/pj/pj010017/index.shtml

File inputFile = new File("in.txt");
FileReader fr = new FileReader(inputFile);
BufferedReader br = new BufferedReader(fr);
String line;
while ((line = br.readLine()) != null) {
    System.out.println(line);
}

http://java.sun.com/j2se/1.4.2/docs/api/java/io/BufferedReader.html

It is advisable to wrap a BufferedReader around any Reader whose read() operations may be costly, such as FileReaders and InputStreamReaders.

Temporary File

See JavaDocs for createTempFile in java.io.File

Example from The Java Developers Almanac 1.4 - Creating a Temporary File

try {
    // Create temp file.
    File temp = File.createTempFile("prefix", ".suffix");
    // or...
    // File temp = File.createTempFile("prefix", ".suffix", "folder");

    // Delete temp file when program exits (use with care)!
    // temp.deleteOnExit();

    // Write to temp file
} catch (IOException e) {
}

Write File

Example from The Java Developers Almanac 1.4 - Writing to a File

BufferedWriter out = null;
try {
    out = new BufferedWriter(new FileWriter("outfilename"));
    out.write("aString");
} catch (IOException e) {
} finally {
    try {
        if (out != null) {
            out.close();
        }
    } catch (IOException e) {
    }
}