How to create a Java String from the contents of a file?

Does anyone have a better/different way to read a file into a string in Java?

private String readFile( String file ) throws IOException {
    BufferedReader reader = new BufferedReader( new FileReader (file));
    String         line = null;
    StringBuilder  stringBuilder = new StringBuilder();
    String         ls = System.getProperty("line.separator");

    while( ( line = reader.readLine() ) != null ) {
        stringBuilder.append( line );
        stringBuilder.append( ls );

    return stringBuilder.toString();


asked Jun 18, 2015 in Core java by rajesh
0 votes

1 Answer

0 votes

Read all text from a file

Here's a compact, robust idiom for Java 7, 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. 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);

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.

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 define 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());
answered Jun 18, 2015 by rajesh