Concatenating a number of .txt files in Java

Question

I have a number of .txt files. I would like to concatenate those and generate a text file.
How would I do it in Java?

Following is the case

file1.txt file2.txt

Concatenation results into

file3.txt

Such that the contents of file1.txt is followed by file2.txt.

yes i used to do it with cat in unix command. but i would like to do it in my program. — thetna, May 20 '12 at 17:14
@casperOne This is a very real question, several have understood it, and provided useful information, like the pointer to the FileUtils class. — Eric Wilson, May 02 '13 at 18:26
Yeah, definitely a real question. Asked exactly what I wanted to know, and the answers below were helpful. — csjacobs24, Sep 15 '15 at 15:29
+1. It is quite legitimate question, given that the shortest pure-Java answer is 11 lines of code. Compare with #cat file1 file2 > file3 in UNIX shell. — badbishop, Oct 07 '17 at 06:36

score 39 · Answer 1 · edited Jun 20 '20 at 09:12

Using Apache Commons IO

You could use the Apache Commons IO library. This has the FileUtils class.

// Files to read
File file1 = new File("file1.txt");
File file2 = new File("file2.txt");

// File to write
File file3 = new File("file3.txt");

// Read the file as string
String file1Str = FileUtils.readFileToString(file1);
String file2Str = FileUtils.readFileToString(file2);

// Write the file
FileUtils.write(file3, file1Str);
FileUtils.write(file3, file2Str, true); // true for append

There are also other methods in this class that could help accomplish the task in a more optimal way (eg using streams or lists).

Using Java 7+

If you are using Java 7+

public static void main(String[] args) throws Exception {
    // Input files
    List<Path> inputs = Arrays.asList(
            Paths.get("file1.txt"),
            Paths.get("file2.txt")
    );

    // Output file
    Path output = Paths.get("file3.txt");

    // Charset for read and write
    Charset charset = StandardCharsets.UTF_8;

    // Join files (lines)
    for (Path path : inputs) {
        List<String> lines = Files.readAllLines(path, charset);
        Files.write(output, lines, charset, StandardOpenOption.CREATE,
                StandardOpenOption.APPEND);
    }
}

[This question](http://stackoverflow.com/questions/1605332/java-nio-filechannel-versus-fileoutputstream-performance-usefulness) describes significantly better performance using Channels instead of reading everything into memory. You can also use Guava's ByteSource.concat to get an InputStream of everything concatenated, then pass it into FileChannel.transferTo — Meredith, Feb 04 '16 at 00:42

score 17 · Accepted Answer · edited Apr 09 '19 at 00:32

17

Read file-by-file and write them to target file. Something like the following:

    OutputStream out = new FileOutputStream(outFile);
    byte[] buf = new byte[n];
    for (String file : files) {
        InputStream in = new FileInputStream(file);
        int b = 0;
        while ( (b = in.read(buf)) >= 0)
            out.write(buf, 0, b);
        in.close();
    }
    out.close();

edited Apr 09 '19 at 00:32

CryptoFool

21,719
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answered May 20 '12 at 17:19

AlexR

114,158
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I like this one more as it extensible for more than two files and doesn't have to hold as much in memory. – Brian Risk Mar 24 '17 at 17:59
@alexr can you extend it a little bit more and explain it step by step? – JoelBonetR Mar 09 '18 at 09:44
The outer loop iterates over files. Inner loop reads the file's content using n-bytes long buffer. Just take a look on any tutorial on java streams for details. – AlexR Mar 10 '18 at 11:34

Samer Makary · Answer 3 · 2018-08-29T07:06:15.943

4

this works fine for me.

// open file input stream to the first file file2.txt
InputStream in = new FileInputStream("file1.txt");
byte[] buffer = new byte[1 << 20];  // loads 1 MB of the file
// open file output stream to which files will be concatenated. 
OutputStream os = new FileOutputStream(new File("file3.txt"), true);
int count;
// read entire file1.txt and write it to file3.txt
while ((count = in.read(buffer)) != -1) {
    os.write(buffer, 0, count);
    os.flush();
}
in.close();
// open file input stream to the second file, file2.txt
in = new FileInputStream("file2.txt");
// read entire file2.txt and write it to file3.txt
while ((count = in.read(buffer)) != -1) {
    os.write(buffer, 0, count);
    os.flush();
}
in.close();
os.close();

edited Aug 29 '18 at 07:06

answered May 20 '12 at 17:29

Samer Makary

1,815
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Can you explain your code step by step please? – JoelBonetR Mar 09 '18 at 10:05
@JoelBonetR I added some comments explaining this old code. However, I think some answers above are better. – Samer Makary Aug 29 '18 at 07:07

score 1 · Answer 4 · answered May 20 '12 at 17:16

You mean you need one file with the content of other text files? Then, read each and every file(you can do it in a loop), save their content in a StringBuffer/ArrayList, and generate the final .txt file by flushing those saved texts in StringBuffer/ArrayList to the final .txt file.

Don't worry, this is an easy task. Just get used to the given system, then you are OK :)

score 0 · Answer 5 · answered May 20 '12 at 17:16

Sounds like homework...

Open File 1
Open File 2
Create/Open File 3
Read from File 1 and Write into File 3
Close File 1
Read From File 2 and Write into File 3
Close File 2
Close File 3

If you need to know how to create/open/read/write/close files in Java, search for documentation. That information should be widely available.

Concatenating a number of .txt files in Java

Following is the case

5 Answers5

Using Apache Commons IO

Using Java 7+

Linked