How to Read a .text File in Java

This page discusses the details of reading, writing, creating, and opening files. At that place are a broad array of file I/O methods to choose from. To aid make sense of the API, the following diagram arranges the file I/O methods by complication.

File I/O Methods Arranged from Less Complex to More Complex

On the far left of the diagram are the utility methods readAllBytes, readAllLines, and the write methods, designed for elementary, common cases. To the right of those are the methods used to iterate over a stream or lines of text, such equally newBufferedReader, newBufferedWriter, then newInputStream and newOutputStream. These methods are interoperable with the java.io package. To the right of those are the methods for dealing with ByteChannels, SeekableByteChannels, and ByteBuffers, such equally the newByteChannel method. Finally, on the far right are the methods that apply FileChannel for avant-garde applications needing file locking or memory-mapped I/O.

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Note: The methods for creating a new file enable you lot to specify an optional set up of initial attributes for the file. For example, on a file system that supports the POSIX prepare of standards (such equally UNIX), you can specify a file owner, group owner, or file permissions at the time the file is created. The Managing Metadata page explains file attributes, and how to access and gear up them.

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This page has the following topics:

• The OpenOptions Parameter
• Commonly Used Methods for Small Files
• Buffered I/O Methods for Text Files
• Methods for Unbuffered Streams and Interoperable with coffee.io APIs
• Methods for Channels and ByteBuffers
• Methods for Creating Regular and Temporary Files

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The OpenOptions Parameter

Several of the methods in this section take an optional OpenOptions parameter. This parameter is optional and the API tells you what the default beliefs is for the method when none is specified.

The following StandardOpenOptions enums are supported:

• WRITE – Opens the file for write access.
• Append – Appends the new information to the terminate of the file. This option is used with the WRITE or CREATE options.
• TRUNCATE_EXISTING – Truncates the file to zero bytes. This option is used with the WRITE option.
• CREATE_NEW – Creates a new file and throws an exception if the file already exists.
• CREATE – Opens the file if information technology exists or creates a new file if it does non.
• DELETE_ON_CLOSE – Deletes the file when the stream is airtight. This pick is useful for temporary files.
• SPARSE – Hints that a newly created file will be sparse. This advanced option is honored on some file systems, such as NTFS, where large files with data "gaps" can be stored in a more efficient manner where those empty gaps do not consume disk space.
• SYNC – Keeps the file (both content and metadata) synchronized with the underlying storage device.
• DSYNC – Keeps the file content synchronized with the underlying storage device.

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Commonly Used Methods for Small-scale Files

Reading All Bytes or Lines from a File

If yous have a small-ish file and you would similar to read its entire contents in i laissez passer, you tin apply the readAllBytes(Path) or readAllLines(Path, Charset) method. These methods have care of most of the work for you, such as opening and closing the stream, but are not intended for handling large files. The following code shows how to apply the readAllBytes method:

Path file = ...; byte[] fileArray; fileArray = Files.readAllBytes(file);          

Writing All Bytes or Lines to a File

You can use one of the write methods to write bytes, or lines, to a file.

• write(Path, byte[], OpenOption...)
• write(Path, Iterable< extends CharSequence>, Charset, OpenOption...)

The post-obit lawmaking snippet shows how to use a write method.

Path file = ...; byte[] buf = ...; Files.write(file, buf);          

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Buffered I/O Methods for Text Files

The java.nio.file bundle supports aqueduct I/O, which moves data in buffers, bypassing some of the layers that can bottleneck stream I/O.

Reading a File by Using Buffered Stream I/O

The newBufferedReader(Path, Charset) method opens a file for reading, returning a BufferedReader that tin be used to read text from a file in an efficient way.

The following code snippet shows how to utilise the newBufferedReader method to read from a file. The file is encoded in "United states of america-ASCII."

Charset charset = Charset.forName("United states-ASCII"); try (BufferedReader reader = Files.newBufferedReader(file, charset)) {     String line = nil;     while ((line = reader.readLine()) != zilch) {         Organization.out.println(line);     } } catch (IOException x) {     System.err.format("IOException: %s%n", ten); }          

Writing a File past Using Buffered Stream I/O

Yous can utilize the newBufferedWriter(Path, Charset, OpenOption...) method to write to a file using a BufferedWriter.

The following lawmaking snippet shows how to create a file encoded in "Usa-ASCII" using this method:

Charset charset = Charset.forName("U.s.-ASCII"); String due south = ...; try (BufferedWriter writer = Files.newBufferedWriter(file, charset)) {     writer.write(s, 0, s.length()); } catch (IOException x) {     System.err.format("IOException: %south%n", x); }          

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Methods for Unbuffered Streams and Interoperable with java.io APIs

Reading a File by Using Stream I/O

To open a file for reading, you tin use the newInputStream(Path, OpenOption...) method. This method returns an unbuffered input stream for reading bytes from the file.

Path file = ...; effort (InputStream in = Files.newInputStream(file);     BufferedReader reader =       new BufferedReader(new InputStreamReader(in))) {     Cord line = naught;     while ((line = reader.readLine()) != null) {         System.out.println(line);     } } catch (IOException x) {     Arrangement.err.println(ten); }          

Creating and Writing a File by Using Stream I/O

You tin create a file, append to a file, or write to a file by using the newOutputStream(Path, OpenOption...) method. This method opens or creates a file for writing bytes and returns an unbuffered output stream.

The method takes an optional OpenOption parameter. If no open options are specified, and the file does non exist, a new file is created. If the file exists, information technology is truncated. This option is equivalent to invoking the method with the CREATE and TRUNCATE_EXISTING options.

The post-obit example opens a log file. If the file does not be, information technology is created. If the file exists, it is opened for appending.

import static java.nio.file.StandardOpenOption.*; import java.nio.file.*; import java.io.*;  public class LogFileTest {    public static void main(String[] args) {      // Convert the string to a     // byte array.     Cord s = "Howdy World! ";     byte data[] = s.getBytes();     Path p = Paths.get("./logfile.txt");      try (OutputStream out = new BufferedOutputStream(       Files.newOutputStream(p, CREATE, APPEND))) {       out.write(data, 0, data.length);     } catch (IOException 10) {       Arrangement.err.println(ten);     }   } }          

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Methods for Channels and ByteBuffers

Reading and Writing Files by Using Channel I/O

While stream I/O reads a grapheme at a time, channel I/O reads a buffer at a time. The ByteChannel interface provides basic read and write functionality. A SeekableByteChannel is a ByteChannel that has the capability to maintain a position in the channel and to change that position. A SeekableByteChannel also supports truncating the file associated with the channel and querying the file for its size.

The capability to movement to different points in the file and then read from or write to that location makes random access of a file possible. See Random Access Files for more information.

There are 2 methods for reading and writing channel I/O.

• newByteChannel(Path, OpenOption...)
• newByteChannel(Path, Set<? extends OpenOption>, FileAttribute<?>...)

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Annotation: The newByteChannel methods return an instance of a SeekableByteChannel. With a default file system, y'all can bandage this seekable byte aqueduct to a FileChannel providing access to more advanced features such mapping a region of the file directly into retentiveness for faster access, locking a region of the file and so other processes cannot access information technology, or reading and writing bytes from an absolute position without affecting the aqueduct's current position.

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Both newByteChannel methods enable you to specify a listing of OpenOption options. The aforementioned open options used by the newOutputStream methods are supported, in addition to one more option: READ is required because the SeekableByteChannel supports both reading and writing.

Specifying READ opens the channel for reading. Specifying WRITE or Suspend opens the channel for writing. If none of these options are specified, then the aqueduct is opened for reading.

The following code snippet reads a file and prints it to standard output:

public static void readFile(Path path) throws IOException {      // Files.newByteChannel() defaults to StandardOpenOption.READ     endeavor (SeekableByteChannel sbc = Files.newByteChannel(path)) {         final int BUFFER_CAPACITY = 10;         ByteBuffer buf = ByteBuffer.allocate(BUFFER_CAPACITY);          // Read the bytes with the proper encoding for this platform. If         // you skip this pace, you might run into foreign or illegible         // characters.         Cord encoding = System.getProperty("file.encoding");         while (sbc.read(buf) > 0) {             buf.flip();             System.out.print(Charset.forName(encoding).decode(buf));             buf.articulate();         }     }     }          

The post-obit example, written for UNIX and other POSIX file systems, creates a log file with a specific set of file permissions. This code creates a log file or appends to the log file if information technology already exists. The log file is created with read/write permissions for owner and read merely permissions for group.

import static java.nio.file.StandardOpenOption.*; import java.nio.*; import java.nio.channels.*; import java.nio.file.*; import coffee.nio.file.aspect.*; import java.io.*; import java.util.*;  public course LogFilePermissionsTest {    public static void main(String[] args) {        // Create the set of options for appending to the file.     Set up<OpenOption> options = new HashSet<OpenOption>();     options.add together(APPEND);     options.add(CREATE);      // Create the custom permissions attribute.     Set<PosixFilePermission> perms =       PosixFilePermissions.fromString("rw-r-----");     FileAttribute<Set<PosixFilePermission>> attr =       PosixFilePermissions.asFileAttribute(perms);      // Convert the string to a ByteBuffer.     String due south = "Hullo World! ";     byte data[] = s.getBytes();     ByteBuffer bb = ByteBuffer.wrap(data);          Path file = Paths.get("./permissions.log");      attempt (SeekableByteChannel sbc =       Files.newByteChannel(file, options, attr)) {       sbc.write(bb);     } catch (IOException 10) {       System.out.println("Exception thrown: " + 10);     }   } }          

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Methods for Creating Regular and Temporary Files

Creating Files

You lot can create an empty file with an initial ready of attributes past using the createFile(Path, FileAttribute<?>) method. For instance, if, at the fourth dimension of creation, y'all want a file to accept a item set of file permissions, use the createFile method to practise so. If you exercise non specify any attributes, the file is created with default attributes. If the file already exists, createFile throws an exception.

In a single atomic operation, the createFile method checks for the existence of the file and creates that file with the specified attributes, which makes the process more secure against malicious lawmaking.

The post-obit lawmaking snippet creates a file with default attributes:

Path file = ...; try {     // Create the empty file with default permissions, etc.     Files.createFile(file); } catch (FileAlreadyExistsException x) {     System.err.format("file named %southward" +         " already exists%northward", file); } catch (IOException x) {     // Some other sort of failure, such equally permissions.     System.err.format("createFile error: %s%n", ten); }          

POSIX File Permissions has an case that uses createFile(Path, FileAttribute<?>) to create a file with pre-set permissions.

You can besides create a new file past using the newOutputStream methods, as described in Creating and Writing a File using Stream I/O. If you open a new output stream and close it immediately, an empty file is created.

Creating Temporary Files

Yous tin create a temporary file using one of the following createTempFile methods:

• createTempFile(Path, Cord, Cord, FileAttribute<?>)
• createTempFile(String, String, FileAttribute<?>)

The first method allows the lawmaking to specify a directory for the temporary file and the second method creates a new file in the default temporary-file directory. Both methods allow you to specify a suffix for the filename and the start method allows you lot to also specify a prefix. The post-obit lawmaking snippet gives an example of the second method:

try {     Path tempFile = Files.createTempFile(null, ".myapp");     System.out.format("The temporary file" +         " has been created: %s%n", tempFile) ; } catch (IOException ten) {     System.err.format("IOException: %s%n", 10); }          

The result of running this file would be something like the following:

The temporary file has been created: /tmp/509668702974537184.myapp          

The specific format of the temporary file proper noun is platform specific.

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Source: https://docs.oracle.com/javase/tutorial/essential/io/file.html

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