Read and Write Text File in One Java File

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 assistance make sense of the API, the following diagram arranges the file I/O methods by complexity.

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

On the far left of the diagram are the utility methods readAllBytes, readAllLines, and the write methods, designed for simple, common cases. To the right of those are the methods used to iterate over a stream or lines of text, such as 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 as the newByteChannel method. Finally, on the far right are the methods that use FileChannel for avant-garde applications needing file locking or retentiveness-mapped I/O.

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

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This page has the following topics:

• The OpenOptions Parameter
• Ordinarily 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 department take an optional OpenOptions parameter. This parameter is optional and the API tells you what the default behavior is for the method when none is specified.

The following StandardOpenOptions enums are supported:

• WRITE – Opens the file for write admission.
• Append – Appends the new data to the end of the file. This option is used with the WRITE or CREATE options.
• TRUNCATE_EXISTING – Truncates the file to nada bytes. This option is used with the WRITE pick.
• CREATE_NEW – Creates a new file and throws an exception if the file already exists.
• CREATE – Opens the file if it exists or creates a new file if it does not.
• DELETE_ON_CLOSE – Deletes the file when the stream is airtight. This option is useful for temporary files.
• SPARSE – Hints that a newly created file volition exist thin. This avant-garde option is honored on some file systems, such equally NTFS, where large files with data "gaps" can exist 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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Normally Used Methods for Pocket-sized Files

Reading All Bytes or Lines from a File

If you take a small-ish file and you would like to read its unabridged contents in one pass, you tin use the readAllBytes(Path) or readAllLines(Path, Charset) method. These methods have intendance of most of the work for y'all, such every bit opening and closing the stream, but are not intended for handling large files. The following code shows how to use the readAllBytes method:

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

Writing All Bytes or Lines to a File

Yous can use 1 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 packet supports channel 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 can exist used to read text from a file in an efficient manner.

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

Charset charset = Charset.forName("U.s.-ASCII"); endeavor (BufferedReader reader = Files.newBufferedReader(file, charset)) {     String line = null;     while ((line = reader.readLine()) != nil) {         System.out.println(line);     } } take hold of (IOException 10) {     System.err.format("IOException: %southward%north", x); }          

Writing a File by Using Buffered Stream I/O

You tin can employ 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 "U.s.a.-ASCII" using this method:

Charset charset = Charset.forName("U.s.a.-ASCII"); String s = ...; attempt (BufferedWriter writer = Files.newBufferedWriter(file, charset)) {     writer.write(southward, 0, s.length()); } catch (IOException ten) {     System.err.format("IOException: %s%due north", ten); }          

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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 lot can use the newInputStream(Path, OpenOption...) method. This method returns an unbuffered input stream for reading bytes from the file.

Path file = ...; try (InputStream in = Files.newInputStream(file);     BufferedReader reader =       new BufferedReader(new InputStreamReader(in))) {     String line = null;     while ((line = reader.readLine()) != aught) {         System.out.println(line);     } } catch (IOException x) {     Organisation.err.println(x); }          

Creating and Writing a File by Using Stream I/O

You tin can create a file, suspend 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 up options are specified, and the file does not 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 following instance opens a log file. If the file does not be, it is created. If the file exists, information technology is opened for appending.

import static coffee.nio.file.StandardOpenOption.*; import java.nio.file.*; import java.io.*;  public form LogFileTest {    public static void main(String[] args) {      // Convert the string to a     // byte array.     String s = "Hello 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, information.length);     } catch (IOException 10) {       System.err.println(x);     }   } }          

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Methods for Channels and ByteBuffers

Reading and Writing Files past Using Aqueduct I/O

While stream I/O reads a character at a fourth dimension, aqueduct I/O reads a buffer at a time. The ByteChannel interface provides bones read and write functionality. A SeekableByteChannel is a ByteChannel that has the capability to maintain a position in the channel and to alter that position. A SeekableByteChannel likewise supports truncating the file associated with the channel and querying the file for its size.

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

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

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

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Note: The newByteChannel methods render an instance of a SeekableByteChannel. With a default file organization, you lot tin can bandage this seekable byte aqueduct to a FileChannel providing admission to more advanced features such mapping a region of the file directly into retentiveness for faster access, locking a region of the file so other processes cannot admission information technology, or reading and writing bytes from an absolute position without affecting the channel'south current position.

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Both newByteChannel methods enable y'all to specify a list of OpenOption options. The same open options used past the newOutputStream methods are supported, in addition to 1 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 aqueduct for writing. If none of these options are specified, then the channel 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     try (SeekableByteChannel sbc = Files.newByteChannel(path)) {         terminal int BUFFER_CAPACITY = ten;         ByteBuffer buf = ByteBuffer.allocate(BUFFER_CAPACITY);          // Read the bytes with the proper encoding for this platform. If         // you skip this step, you lot might see foreign or illegible         // characters.         String encoding = Arrangement.getProperty("file.encoding");         while (sbc.read(buf) > 0) {             buf.flip();             Arrangement.out.print(Charset.forName(encoding).decode(buf));             buf.articulate();         }     }     }          

The following case, 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 it already exists. The log file is created with read/write permissions for owner and read only permissions for group.

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

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Methods for Creating Regular and Temporary Files

Creating Files

You can create an empty file with an initial set of attributes by using the createFile(Path, FileAttribute<?>) method. For instance, if, at the time of creation, you desire a file to have a detail set of file permissions, use the createFile method to do then. If you exercise not 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 procedure more than secure against malicious code.

The following code snippet creates a file with default attributes:

Path file = ...; effort {     // Create the empty file with default permissions, etc.     Files.createFile(file); } catch (FileAlreadyExistsException x) {     System.err.format("file named %s" +         " already exists%n", file); } grab (IOException 10) {     // Some other sort of failure, such as permissions.     Organization.err.format("createFile fault: %southward%due north", x); }          

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

Y'all can also create a new file by using the newOutputStream methods, as described in Creating and Writing a File using Stream I/O. If you lot open a new output stream and close it immediately, an empty file is created.

Creating Temporary Files

You can create a temporary file using one of the post-obit createTempFile methods:

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

The first method allows the code to specify a directory for the temporary file and the 2nd method creates a new file in the default temporary-file directory. Both methods allow you to specify a suffix for the filename and the first method allows yous to too specify a prefix. The post-obit code snippet gives an example of the second method:

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

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

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

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

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

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