How to Read a File in Java Using a Mac

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

File I/O Methods Bundled from Less Complex to More than Complex

On the far left of the diagram are the utility methods readAllBytes, readAllLines, and the write methods, designed for unproblematic, 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 coffee.io packet. To the correct 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 retentivity-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 example, on a file system that supports the POSIX ready of standards (such as UNIX), you tin 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 set 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 java.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 yous what the default behavior 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 data to the terminate 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 option.
• 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 non.
• DELETE_ON_CLOSE – Deletes the file when the stream is closed. This option is useful for temporary files.
• Sparse – Hints that a newly created file will exist sparse. This advanced option is honored on some file systems, such as NTFS, where large files with information "gaps" can be stored in a more than efficient fashion where those empty gaps do not eat 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 Files

Reading All Bytes or Lines from a File

If you take a small-ish file and y'all would like to read its entire contents in one pass, you can use the readAllBytes(Path) or readAllLines(Path, Charset) method. These methods take intendance of most of the work for y'all, such every bit opening and closing the stream, just 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 tin employ 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 following code snippet shows how to utilise a write method.

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

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Buffered I/O Methods for Text Files

The coffee.nio.file package 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 can exist used to read text from a file in an efficient manner.

The following lawmaking snippet shows how to utilise the newBufferedReader method to read from a file. The file is encoded in "US-ASCII."

Charset charset = Charset.forName("US-ASCII"); attempt (BufferedReader reader = Files.newBufferedReader(file, charset)) {     Cord line = null;     while ((line = reader.readLine()) != null) {         System.out.println(line);     } } catch (IOException x) {     System.err.format("IOException: %s%n", x); }          

Writing a File by Using Buffered Stream I/O

You lot tin can use the newBufferedWriter(Path, Charset, OpenOption...) method to write to a file using a BufferedWriter.

The post-obit code snippet shows how to create a file encoded in "US-ASCII" using this method:

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

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Methods for Unbuffered Streams and Interoperable with coffee.io APIs

Reading a File by Using Stream I/O

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

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

Creating and Writing a File past 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 up 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 following example opens a log file. If the file does non exist, it is created. If the file exists, information technology is opened for appending.

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

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Methods for Channels and ByteBuffers

Reading and Writing Files past Using Channel I/O

While stream I/O reads a character at a time, channel I/O reads a buffer at a fourth dimension. The ByteChannel interface provides basic read and write functionality. A SeekableByteChannel is a ByteChannel that has the adequacy 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 motion to unlike points in the file and then read from or write to that location makes random access of a file possible. See Random Admission Files for more information.

There are ii methods for reading and writing aqueduct I/O.

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

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Note: The newByteChannel methods return an case of a SeekableByteChannel. With a default file organisation, you can cast this seekable byte channel to a FileChannel providing access to more than advanced features such mapping a region of the file directly into memory for faster access, locking a region of the file and then other processes cannot access information technology, or reading and writing bytes from an absolute position without affecting the channel's current position.

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Both newByteChannel methods enable you 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 Append opens the channel for writing. If none of these options are specified, and so 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     effort (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 step, you might run across foreign or illegible         // characters.         String encoding = System.getProperty("file.encoding");         while (sbc.read(buf) > 0) {             buf.flip();             System.out.print(Charset.forName(encoding).decode(buf));             buf.clear();         }     }     }          

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 merely permissions for grouping.

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

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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 cosmos, y'all want a file to have a detail set of file permissions, use the createFile method to do so. If yous do not specify whatever attributes, the file is created with default attributes. If the file already exists, createFile throws an exception.

In a single atomic performance, 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 = ...; try {     // Create the empty file with default permissions, etc.     Files.createFile(file); } catch (FileAlreadyExistsException x) {     System.err.format("file named %south" +         " already exists%northward", file); } take hold of (IOException x) {     // Some other sort of failure, such as permissions.     System.err.format("createFile error: %south%n", x); }          

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

Y'all can likewise 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 up a new output stream and shut it immediately, an empty file is created.

Creating Temporary Files

You can create a temporary file using one of the following createTempFile methods:

• createTempFile(Path, String, String, FileAttribute<?>)
• createTempFile(Cord, 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 first method allows you to also specify a prefix. The following code 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 x) {     Organisation.err.format("IOException: %s%n", x); }          

The result of running this file would be something like the post-obit:

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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