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/OpenJDK 25

Class Reader

java.lang.Object
java.io.Reader
All Implemented Interfaces:
Closeable, AutoCloseable, Readable
Direct Known Subclasses:
BufferedReader, CharArrayReader, FilterReader, InputStreamReader, PipedReader, StringReader
public abstract class Reader extends Object implements Readable, Closeable
Abstract class for reading character streams. The only methods that a subclass must implement are read(char[], int, int) and close(). Most subclasses, however, will override some of the methods defined here in order to provide higher efficiency, additional functionality, or both.
Since:
1.1
See Also:

Field Summary

Modifier and Type Field Description
protected Object lock
The object used to synchronize operations on this stream.

Constructor Summary

Reader()
Reader(Object lock)
Modifier Constructor Description
protected
Creates a new character-stream reader whose critical sections will synchronize on the reader itself.
protected
Creates a new character-stream reader whose critical sections will synchronize on the given object.

Method Summary

Modifier and Type Method Description
abstract void close()
Closes the stream and releases any system resources associated with it.
void mark(int readAheadLimit)
Marks the present position in the stream.
boolean markSupported()
Tells whether this stream supports the mark() operation.
static Reader nullReader()
Returns a new Reader that reads no characters.
static Reader of(CharSequence cs)
Returns a Reader that reads characters from a CharSequence.
int read()
Reads a single character.
int read(char[] cbuf)
Reads characters into an array.
abstract int read(char[] cbuf, int off, int len)
Reads characters into a portion of an array.
int read(CharBuffer target)
Attempts to read characters into the specified character buffer.
String readAllAsString()
Reads all remaining characters into a string.
List<String> readAllLines()
Reads all remaining characters as lines of text.
boolean ready()
Tells whether this stream is ready to be read.
void reset()
Resets the stream.
long skip(long n)
Skips characters.
long transferTo(Writer out)
Reads all characters from this reader and writes the characters to the given writer in the order that they are read.

Methods declared in class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Details

lock

protected Object lock
The object used to synchronize operations on this stream. For efficiency, a character-stream object may use an object other than itself to protect critical sections. A subclass should therefore use the object in this field rather than this or a synchronized method.

Constructor Details

Reader

protected Reader()
Creates a new character-stream reader whose critical sections will synchronize on the reader itself.

Reader

protected Reader(Object lock)
Creates a new character-stream reader whose critical sections will synchronize on the given object.
Parameters:
lock - The Object to synchronize on.

Method Details

nullReader

public static Reader nullReader()
Returns a new Reader that reads no characters. The returned stream is initially open. The stream is closed by calling the close() method. Subsequent calls to close() have no effect.

While the stream is open, the read(), read(char[]), read(char[], int, int), read(CharBuffer), ready(), skip(long), and transferTo() methods all behave as if end of stream has been reached. After the stream has been closed, these methods all throw IOException.

The markSupported() method returns false. The mark() and reset() methods throw an IOException.

The object used to synchronize operations on the returned Reader is not specified.

Returns:
a Reader which reads no characters
Since:
11

of

public static Reader of(CharSequence cs)
Returns a Reader that reads characters from a CharSequence. The reader is initially open and reading starts at the first character in the sequence.

The returned reader supports the mark() and reset() operations.

The resulting reader is not safe for use by multiple concurrent threads. If the reader is to be used by more than one thread it should be controlled by appropriate synchronization.

If the sequence changes while the reader is open, e.g. the length changes, the behavior is undefined.

Parameters:
cs - CharSequence providing the character stream.
Returns:
a Reader which reads characters from cs
Throws:
NullPointerException - if cs is null
Since:
24

read

public int read(CharBuffer target) throws IOException
Attempts to read characters into the specified character buffer. The buffer is used as a repository of characters as-is: the only changes made are the results of a put operation. No flipping or rewinding of the buffer is performed. If the length of the specified character buffer is zero, then no characters will be read and zero will be returned.
Specified by:
read in interface Readable
Parameters:
target - the buffer to read characters into
Returns:
The number of characters added to the buffer, possibly zero, or -1 if this source of characters is at its end
Throws:
IOException - if an I/O error occurs
NullPointerException - if target is null
ReadOnlyBufferException - if target is a read only buffer, even if its length is zero
Since:
1.5

read

public int read() throws IOException
Reads a single character. This method will block until a character is available, an I/O error occurs, or the end of the stream is reached.

Subclasses that intend to support efficient single-character input should override this method.

Returns:
The character read, as an integer in the range 0 to 65535 (0x00-0xffff), or -1 if the end of the stream has been reached
Throws:
IOException - If an I/O error occurs

read

public int read(char[] cbuf) throws IOException
Reads characters into an array. This method will block until some input is available, an I/O error occurs, or the end of the stream is reached.

If the length of cbuf is zero, then no characters are read and 0 is returned; otherwise, there is an attempt to read at least one character. If no character is available because the stream is at its end, the value -1 is returned; otherwise, at least one character is read and stored into cbuf.

Parameters:
cbuf - Destination buffer
Returns:
The number of characters read, or -1 if the end of the stream has been reached
Throws:
IOException - If an I/O error occurs

read

public abstract int read(char[] cbuf, int off, int len) throws IOException
Reads characters into a portion of an array. This method will block until some input is available, an I/O error occurs, or the end of the stream is reached.

If len is zero, then no characters are read and 0 is returned; otherwise, there is an attempt to read at least one character. If no character is available because the stream is at its end, the value -1 is returned; otherwise, at least one character is read and stored into cbuf.

Parameters:
cbuf - Destination buffer
off - Offset at which to start storing characters
len - Maximum number of characters to read
Returns:
The number of characters read, or -1 if the end of the stream has been reached
Throws:
IndexOutOfBoundsException - If off is negative, or len is negative, or len is greater than cbuf.length - off
IOException - If an I/O error occurs

readAllLines

public List<String> readAllLines() throws IOException
Reads all remaining characters as lines of text. This method blocks until all remaining characters have been read and end of stream is detected, or an exception is thrown. This method does not close the reader.

When this reader reaches the end of the stream, further invocations of this method will return an empty list.

A line is either a sequence of zero or more characters followed by a line terminator, or it is a sequence of one or more characters followed by the end of the stream. A line does not include the line terminator.

A line terminator is one of the following: a line feed character "\n" (U+000A), a carriage return character "\r" (U+000D), or a carriage return followed immediately by a line feed "\r\n" (U+000D U+000A).

The behavior for the case where the reader is asynchronously closed, or the thread interrupted during the read, is highly reader specific, and therefore not specified.

If an I/O error occurs reading from the stream then it may do so after some, but not all, characters have been read. Consequently the stream may not be at end of stream and may be in an inconsistent state. It is strongly recommended that the reader be promptly closed if an I/O error occurs.

API Note:
This method is intended for simple cases where it is appropriate and convenient to read the entire input into a list of lines. It is not suitable for reading input from an unknown origin, as this may result in the allocation of an arbitrary amount of memory.
Returns:
the remaining characters as lines of text stored in an unmodifiable List of Strings in the order they are read
Throws:
IOException - If an I/O error occurs
OutOfMemoryError - If the number of remaining characters exceeds the implementation limit for String.
Since:
25
See Also:

readAllAsString

public String readAllAsString() throws IOException
Reads all remaining characters into a string. This method blocks until all remaining characters including all line separators have been read and end of stream is detected, or an exception is thrown. The resulting string will contain line separators as they appear in the stream. This method does not close the reader.

When this reader reaches the end of the stream, further invocations of this method will return an empty string.

The behavior for the case where the reader is asynchronously closed, or the thread interrupted during the read, is highly reader specific, and therefore not specified.

If an I/O error occurs reading from the stream then it may do so after some, but not all, characters have been read. Consequently the stream may not be at end of stream and may be in an inconsistent state. It is strongly recommended that the reader be promptly closed if an I/O error occurs.

API Note:
This method is intended for simple cases where it is appropriate and convenient to read the entire input into a String. It is not suitable for reading input from an unknown origin, as this may result in the allocation of an arbitrary amount of memory.
Returns:
a String containing all remaining characters
Throws:
IOException - If an I/O error occurs
OutOfMemoryError - If the number of remaining characters exceeds the implementation limit for String.
Since:
25
See Also:

skip

public long skip(long n) throws IOException
Skips characters. This method will block until some characters are available, an I/O error occurs, or the end of the stream is reached. If the stream is already at its end before this method is invoked, then no characters are skipped and zero is returned.
Parameters:
n - The number of characters to skip
Returns:
The number of characters actually skipped
Throws:
IllegalArgumentException - If n is negative.
IOException - If an I/O error occurs

ready

public boolean ready() throws IOException
Tells whether this stream is ready to be read.
Returns:
True if the next read() is guaranteed not to block for input, false otherwise. Note that returning false does not guarantee that the next read will block.
Throws:
IOException - If an I/O error occurs

markSupported

public boolean markSupported()
Tells whether this stream supports the mark() operation. The default implementation always returns false. Subclasses should override this method.
Returns:
true if and only if this stream supports the mark operation.

mark

public void mark(int readAheadLimit) throws IOException
Marks the present position in the stream. Subsequent calls to reset() will attempt to reposition the stream to this point. Not all character-input streams support the mark() operation.
Parameters:
readAheadLimit - Limit on the number of characters that may be read while still preserving the mark. After reading this many characters, attempting to reset the stream may fail.
Throws:
IOException - If the stream does not support mark(), or if some other I/O error occurs

reset

public void reset() throws IOException
Resets the stream. If the stream has been marked, then attempt to reposition it at the mark. If the stream has not been marked, then attempt to reset it in some way appropriate to the particular stream, for example by repositioning it to its starting point. Not all character-input streams support the reset() operation, and some support reset() without supporting mark().
Throws:
IOException - If the stream has not been marked, or if the mark has been invalidated, or if the stream does not support reset(), or if some other I/O error occurs

close

public abstract void close() throws IOException
Closes the stream and releases any system resources associated with it. Once the stream has been closed, further read(), ready(), mark(), reset(), or skip() invocations will throw an IOException. Closing a previously closed stream has no effect.
Specified by:
close in interface AutoCloseable
Specified by:
close in interface Closeable
Throws:
IOException - If an I/O error occurs

transferTo

public long transferTo(Writer out) throws IOException
Reads all characters from this reader and writes the characters to the given writer in the order that they are read. On return, this reader will be at end of the stream. This method does not close either reader or writer.

This method may block indefinitely reading from the reader, or writing to the writer. The behavior for the case where the reader and/or writer is asynchronously closed, or the thread interrupted during the transfer, is highly reader and writer specific, and therefore not specified.

If the total number of characters transferred is greater than Long.MAX_VALUE, then Long.MAX_VALUE will be returned.

If an I/O error occurs reading from the reader or writing to the writer, then it may do so after some characters have been read or written. Consequently the reader may not be at end of the stream and one, or both, streams may be in an inconsistent state. It is strongly recommended that both streams be promptly closed if an I/O error occurs.

Parameters:
out - the writer, non-null
Returns:
the number of characters transferred
Throws:
IOException - if an I/O error occurs when reading or writing
NullPointerException - if out is null
Since:
10

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/io/Reader.html