Merge contents of file of files starting with a prefix

Question

How to merge files starting with a pattern from a folder in Java.

I have files as below:

UW110_file_1.dat

ROW1   DATA1  SOMESTUFF1
ROW2   DATA2  SOMESTUFF2

UW110_file_2.dat

ROW3   DATA3  SOMESTUFF3
ROW4   DATA4  SOMESTUFF4

UW110_file_2.hdr

ROW3   DATA3  SOMESTUFF3
ROW4   DATA4  SOMESTUFF4

I need to check all files starting with UW110 that has an extension of .dat and merge them.

In this case i'll have to create a file as below

UW110_file

ROW1   DATA1  SOMESTUFF1
ROW2   DATA2  SOMESTUFF2
ROW3   DATA3  SOMESTUFF3
ROW4   DATA4  SOMESTUFF4

Answer 1

Create a File object with the path to where the files are located, let's call it datFolder

public String getMergedFiles(File datFolder) {
    ArrayList<File> files = new ArrayList<>();
    files.addAll(Arrays.asList(datFolder.listFiles((f) -> f.getName().endsWith(".dat") && f.getName().startsWith("UW110"))));

    StringBuilder sb = new StringBuilder();

    for (File datFile : files) {
        InputStream is = new FileInputStream(datFile);
        BufferedReader buf = new BufferedReader(new InputStreamReader(is));

        String line = buf.readLine();
        while(line != null) {
            sb.append(line).append("\n");
            line = buf.readLine();
        }

        buf.close();
        is.close();
    }

    return sb.toString();
}

public void writeToFile(String input, File file) {
    PrintWriter out = new PrintWriter(file);
    try{
        out.println(input);
    }catch(Exception e){
        e.printStackTrace();
    }
    out.close();
}

// Somewhere in your code
File folder = new File("myFolder");
writeToFile(getMergedFiles(folder), new File("outputFile.dat"));

This makes use of a lambda though, look up how they work if you're not using java 8, should be easy turning it into a loop.

Didn't test this code but i believe it should work. If not, probably a quick fix should do it.

You can simply write the string this method returns to a file and you should have your final file!

Enjoy :)