What Is Simplest Way to Read a File into String

What is simplest way to read a file into String?

Yes, you can do this in one line (though for robust IOException handling you wouldn't want to).

String content = new Scanner(new File("filename")).useDelimiter("\\Z").next();
System.out.println(content);

This uses a java.util.Scanner, telling it to delimit the input with \Z, which is the end of the string anchor. This ultimately makes the input have one actual token, which is the entire file, so it can be read with one call to next().

There is a constructor that takes a File and a String charSetName (among many other overloads). These two constructor may throw FileNotFoundException, but like all Scanner methods, no IOException can be thrown beyond these constructors.

You can query the Scanner itself through the ioException() method if an IOException occurred or not. You may also want to explicitly close() the Scanner after you read the content, so perhaps storing the Scanner reference in a local variable is best.

See also

• Java Tutorials - I/O Essentials - Scanning and formatting

Related questions

• Validating input using java.util.Scanner - has many examples of more typical usage

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Third-party library options

For completeness, these are some really good options if you have these very reputable and highly useful third party libraries:

Guava

com.google.common.io.Files contains many useful methods. The pertinent ones here are:

• String toString(File, Charset)
  
  • Using the given character set, reads all characters from a file into a String
• List<String> readLines(File, Charset)
  
  • ... reads all of the lines from a file into a List<String>, one entry per line

Apache Commons/IO

org.apache.commons.io.IOUtils also offer similar functionality:

• String toString(InputStream, String encoding)
  
  • Using the specified character encoding, gets the contents of an InputStream as a String
• List readLines(InputStream, String encoding)
  
  • ... as a (raw) List of String, one entry per line

Related questions

• Most useful free third party Java libraries (deleted)?

How do I create a Java string from the contents of a file?

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, encoding);

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

If your file is small enough relative to your available memory, reading the entire file at once might work fine. However, if your file is too large, reading one line at a time, processing it, and then discarding it before moving on to the next could be a better approach. Stream processing in this way can eliminate the total file size as a factor in your memory requirement.

Character encoding

One thing that is missing from the sample in the original post is the character encoding. This encoding generally can't be determined from the file itself, and requires meta-data such as an HTTP header to convey this important information.

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());

There are some special cases where the platform default is what you want, but they are rare. You should be able justify your choice, because the platform default is not portable. One example where it might be correct is when reading standard input or writing standard output.

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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.

How to read the content of a file to a string in C?

I tend to just load the entire buffer as a raw memory chunk into memory and do the parsing on my own. That way I have best control over what the standard lib does on multiple platforms.

This is a stub I use for this. you may also want to check the error-codes for fseek, ftell and fread. (omitted for clarity).

char * buffer = 0;
long length;
FILE * f = fopen (filename, "rb");

if (f)
{
  fseek (f, 0, SEEK_END);
  length = ftell (f);
  fseek (f, 0, SEEK_SET);
  buffer = malloc (length);
  if (buffer)
  {
    fread (buffer, 1, length, f);
  }
  fclose (f);
}

if (buffer)
{
  // start to process your data / extract strings here...
}

How can I read a whole file into a string variable

Use ioutil.ReadFile:

func ReadFile(filename string) ([]byte, error)

ReadFile reads the file named by filename and returns the contents. A successful call
returns err == nil, not err == EOF. Because ReadFile reads the whole file, it does not treat
an EOF from Read as an error to be reported.

You will get a []byte instead of a string. It can be converted if really necessary:

s := string(buf)

What's a reasonable way to read an entire text file as a single string?

What about IO.read()?

Edit: IO.read(), as an added bonus, closes the file for you.

Reading a plain text file in Java

ASCII is a TEXT file so you would use Readers for reading. Java also supports reading from a binary file using InputStreams. If the files being read are huge then you would want to use a BufferedReader on top of a FileReader to improve read performance.

Go through this article on how to use a Reader

I'd also recommend you download and read this wonderful (yet free) book called Thinking In Java

In Java 7:

new String(Files.readAllBytes(...))

(docs)
or

Files.readAllLines(...)

(docs)

In Java 8:

Files.lines(..).forEach(...)

(docs)

Most basic way to read file contents into a string in javascript

This is a simple approach, place the script tag at the end of the body. This will ensure the page is fully loaded before it executes.

<script>
function done()
{
    var fileContents = this.response;
}

var xmlhttp;
xmlhttp=new XMLHttpRequest();
xmlhttp.addEventListener("load", done, false);
xmlhttp.open("GET","/contents.txt",true);
xmlhttp.send();
</script>
</body>

How to read a text file into a string variable and strip newlines?

You could use:

with open('data.txt', 'r') as file:
    data = file.read().replace('\n', '')

Or if the file content is guaranteed to be one-line

with open('data.txt', 'r') as file:
    data = file.read().rstrip()

How to read an entire file to a string using C#?

How about File.ReadAllText:

string contents = File.ReadAllText(@"C:\temp\test.txt");

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