OpenJDK 27 · java.io.Reader · APIdia, the JavaDoc alternative

Modifier and Type	Field and Description
protected Object	lock The object used to synchronize operations on this stream.
private static final int	maxSkipBufferSize Maximum skip-buffer size
private char[]	skipBuffer Skip buffer, null until allocated
private static final int	TRANSFER_BUFFER_SIZE

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

Method Summary

Modifier and Type	Method and Description
public abstract void	close() Redeclares java.io.Closeable.close. Closes the stream and releases any system resources associated with it.
public void	mark(int 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. readAheadLimit) Marks the present position in the stream.
public boolean	Returns: true if and only if this stream supports the mark operation. markSupported() Tells whether this stream supports the mark() operation.
public static Reader	Returns: a `Reader` which reads no characters nullReader() Returns a new `Reader` that reads no characters.
public static Reader	Returns: a `Reader` which reads characters from `cs` of(final CharSequence `CharSequence` providing the character stream. cs) Returns a `Reader` that reads characters from a `CharSequence`.
public int	Returns: The number of characters added to the buffer, possibly zero, or -1 if this source of characters is at its end read(CharBuffer the buffer to read characters into target) Implements java.lang.Readable.read. Attempts to read characters into the specified character buffer.
public int	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 read() Reads a single character.
public int	Returns: The number of characters read, or -1 if the end of the stream has been reached read(char[] Destination buffer cbuf) Reads characters into an array.
public abstract int	Returns: The number of characters read, or -1 if the end of the stream has been reached read(char[] Destination buffer cbuf, int Offset at which to start storing characters off, int Maximum number of characters to read len) Reads characters into a portion of an array.
public String	Returns: a `String` containing all remaining characters readAllAsString() Reads all remaining characters into a string.
public List<String>	Returns: the remaining characters as lines of text stored in an unmodifiable `List` of `String`s in the order they are read readAllLines() Reads all remaining characters as lines of text.
public boolean	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. ready() Tells whether this stream is ready to be read.
public void	reset() Resets the stream.
public long	Returns: The number of characters actually skipped skip(long The number of characters to skip n) Skips characters.
public long	Returns: the number of characters transferred transferTo(Writer the writer, non-null out) Reads all characters from this reader and writes the characters to the given writer in the order that they are read.

Inherited from java.lang.Object:: clone equals finalize getClass hashCode notify notifyAll toString wait wait wait

lock	back to summary
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.

maxSkipBufferSize	back to summary
private static final int maxSkipBufferSize Maximum skip-buffer size

skipBuffer	back to summary
private char[] skipBuffer Skip buffer, null until allocated

TRANSFER_BUFFER_SIZE	back to summary
private static final int TRANSFER_BUFFER_SIZE

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

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

Method Detail

close	back to summary
public abstract void close() throws IOException Redeclares java.io.Closeable.close. 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. Exceptions IOException: If an I/O error occurs

back to summary

public abstract void close() throws IOException

Redeclares java.io.Closeable.close.

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.

Exceptions

IOException:: If an I/O error occurs

mark	back to summary
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:int 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. Exceptions IOException: If the stream does not support mark(), or if some other I/O error occurs

mark

back to summary

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:int: 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.

Exceptions

IOException:: If the stream does not support mark(), or if some other I/O error occurs

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

markSupported

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public boolean markSupported()

Tells whether this stream supports the mark() operation. The default implementation always returns false. Subclasses should override this method.

Returns:boolean: true if and only if this stream supports the mark operation.

nullReader back to summary

nullReader	back to summary
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:Reader a `Reader` which reads no characters Since 11

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:Reader: a Reader which reads no characters
Since: 11

of back to summary

of	back to summary
public static Reader of(final 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 `CharSequence` providing the character stream. Returns:Reader a `Reader` which reads characters from `cs` Exceptions NullPointerException: if `cs` is `null` Since 24

public static Reader of(final 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: CharSequence providing the character stream.

Returns:Reader

a Reader which reads characters from cs

Exceptions

NullPointerException:: if cs is null

Since

read	back to summary
public int read(CharBuffer target) throws IOException Implements java.lang.Readable.read. 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. Parameters target:CharBuffer the buffer to read characters into Returns:int The number of characters added to the buffer, possibly zero, or -1 if this source of characters is at its end Exceptions 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

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public int read(CharBuffer target) throws IOException

Implements java.lang.Readable.read.

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.

Parameters

target:CharBuffer: the buffer to read characters into

Returns:int

The number of characters added to the buffer, possibly zero, or -1 if this source of characters is at its end

Exceptions

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 back to summary

read	back to summary
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:int 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 Exceptions IOException: If an I/O error occurs

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:int

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

Exceptions

IOException:: If an I/O error occurs

read back to summary

read	back to summary
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:char[] Destination buffer Returns:int The number of characters read, or -1 if the end of the stream has been reached Exceptions IOException: If an I/O error occurs

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:char[]: Destination buffer

Returns:int

The number of characters read, or -1 if the end of the stream has been reached

Exceptions

IOException:: If an I/O error occurs

read back to summary

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

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:char[]: Destination buffer
off:int: Offset at which to start storing characters
len:int: Maximum number of characters to read

Returns:int

The number of characters read, or -1 if the end of the stream has been reached

Exceptions

IOException:: If an I/O error occurs
IndexOutOfBoundsException:: If off is negative, or len is negative, or len is greater than cbuf.length - off

readAllAsString back to summary

readAllAsString	back to summary
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:String a `String` containing all remaining characters Exceptions IOException: If an I/O error occurs OutOfMemoryError: If the number of remaining characters exceeds the implementation limit for `String`. Since 25 See Also `readAllLines`, `java.nio.file.Files#readString`

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:String

a String containing all remaining characters

Exceptions

IOException:: If an I/O error occurs
OutOfMemoryError:: If the number of remaining characters exceeds the implementation limit for String.

Since

readAllLines back to summary

readAllLines	back to summary
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:List<String> the remaining characters as lines of text stored in an unmodifiable `List` of `String`s in the order they are read Exceptions IOException: If an I/O error occurs OutOfMemoryError: If the number of remaining characters exceeds the implementation limit for `String`. Since 25 See Also `String#lines`, `readAllAsString`, `java.nio.file.Files#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.

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:List<String>

the remaining characters as lines of text stored in an unmodifiable List of Strings in the order they are read

Exceptions

IOException:: If an I/O error occurs
OutOfMemoryError:: If the number of remaining characters exceeds the implementation limit for String.

Since

ready	back to summary
public boolean ready() throws IOException Tells whether this stream is ready to be read. Returns:boolean 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. Exceptions IOException: If an I/O error occurs

ready

back to summary

public boolean ready() throws IOException

Tells whether this stream is ready to be read.

Returns:boolean

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.

Exceptions

IOException:: If an I/O error occurs

reset	back to summary
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(). Exceptions 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

reset

back to summary

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().

Exceptions

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

skip back to summary

skip	back to summary
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:long The number of characters to skip Returns:long The number of characters actually skipped Exceptions IOException: If an I/O error occurs IllegalArgumentException: If `n` is negative.

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:long: The number of characters to skip

Returns:long

The number of characters actually skipped

Exceptions

IOException:: If an I/O error occurs
IllegalArgumentException:: If n is negative.

transferTo back to summary

transferTo	back to summary
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:Writer the writer, non-null Returns:long the number of characters transferred Exceptions IOException: if an I/O error occurs when reading or writing NullPointerException: if `out` is `null` Since 10

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:Writer: the writer, non-null

Returns:long

the number of characters transferred

Exceptions

IOException:: if an I/O error occurs when reading or writing
NullPointerException:: if out is null

Since

public abstract Class Reader

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