Read Data From Text File Into Dictionary in Java

This page discusses the details of reading, writing, creating, and opening files. In that location are a broad array of file I/O methods to cull from. To aid make sense of the API, the post-obit diagram arranges the file I/O methods by complexity.

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 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 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 advanced applications needing file locking or memory-mapped I/O.

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Annotation: The methods for creating a new file enable you to specify an optional gear up of initial attributes for the file. For case, on a file system that supports the POSIX set of standards (such as UNIX), yous 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 gear up them.

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This folio has the following topics:

• The OpenOptions Parameter
• Unremarkably Used Methods for Pocket-size 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 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 post-obit 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 zero bytes. This selection 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 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 be sparse. This advanced pick 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 exercise 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 Modest Files

Reading All Bytes or Lines from a File

If you accept a small-ish file and you would like to read its entire contents in 1 pass, yous can utilize the readAllBytes(Path) or readAllLines(Path, Charset) method. These methods take care of near of the work for yous, such as opening and closing the stream, but are not intended for handling big files. The following code shows how to utilise the readAllBytes method:

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

Writing All Bytes or Lines to a File

You tin 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 code snippet shows how to utilize 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 parcel 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 mode.

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

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

Writing a File by Using Buffered Stream I/O

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

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

Charset charset = Charset.forName("The states-ASCII"); String s = ...; try (BufferedWriter writer = Files.newBufferedWriter(file, charset)) {     author.write(s, 0, s.length()); } catch (IOException x) {     System.err.format("IOException: %s%n", 10); }          

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Methods for Unbuffered Streams and Interoperable with java.io APIs

Reading a File past Using Stream I/O

To open a file for reading, y'all tin utilise 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))) {     String line = null;     while ((line = reader.readLine()) != nada) {         Organisation.out.println(line);     } } catch (IOException x) {     Arrangement.err.println(10); }          

Creating and Writing a File by Using Stream I/O

You can create a file, suspend to a file, or write to a file past 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, it 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, it is opened for appending.

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

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

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

• newByteChannel(Path, OpenOption...)
• newByteChannel(Path, Fix<? 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 advanced features such mapping a region of the file direct into retentiveness for faster access, locking a region of the file so other processes cannot access it, or reading and writing bytes from an absolute position without affecting the channel's electric 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 add-on to i more than option: READ is required considering the SeekableByteChannel supports both reading and writing.

Specifying READ opens the channel for reading. Specifying WRITE or APPEND 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 information technology 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 see 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 example, written for UNIX and other POSIX file systems, creates a log file with a specific gear up of file permissions. This lawmaking 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 grouping.

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

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Methods for Creating Regular and Temporary Files

Creating Files

You tin can create an empty file with an initial set of attributes by using the createFile(Path, FileAttribute<?>) method. For example, if, at the time of creation, you want a file to take a particular set of file permissions, use the createFile method to do so. If you exercise not specify whatever attributes, the file is created with default attributes. If the file already exists, createFile throws an exception.

In a single atomic functioning, the createFile method checks for the existence of the file and creates that file with the specified attributes, which makes the procedure more 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 10) {     System.err.format("file named %s" +         " already exists%due north", file); } catch (IOException 10) {     // Another sort of failure, such as permissions.     System.err.format("createFile fault: %s%n", x); }          

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

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

Creating Temporary Files

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

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

The first method allows the code 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 as well 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) {     Arrangement.err.format("IOException: %south%north", 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 name is platform specific.

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

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