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The JavaReader class, java.io.Reader
, is the base class for all Reader
subclasses in the Java IO API. A Java Reader
is like a Java InputStream except that it is character based rather than byte based. In other words, a Java Reader
is intended for reading text (characters), whereas an InputStream
is intended for reading raw bytes.
Readers and Sources
The read method of Reader Class in Java is used to read a single character from the stream. This method blocks the stream till: It has taken some input from the stream. Some IOException has occurred; It has reached the end of the stream while reading. Java 7 and later versions are not supported by these older versions of Mac OS X. If you have problems with Java 6, contact Apple Technical Support. Java.com does not provide a download for these systems. Mac OS X Version 10.7.3 (Lion) or newer. Java 7 and later versions can run on your Mac. Oracle Java (Version 7 and later versions) requires an.
A Reader
is typically connected to some source of data like a file, char array, network socket etc. This is also explained in more detail in the Java IO Overview text.
Characters in Unicode
Today, many applications use Unicode (UTF-8 or UTF-16) to store text data. It may take one or more bytes to represent a single character in UTF-8. In UTF-16 each character takes 2 bytes to represent. Therefore, when reading text data, a single byte in the data may not correspond to one character in UTF. If you just read one byte at a time of UTF-8 data via an InputStream
and try to convert each byte into a char
, you may not end up with the text you expected.
To solve this problem we have the Reader
class. The Reader
class is capable of decoding bytes into characters. You need to tell the Reader
what character set to decode. This is done when you instantiate the Reader
(actually, when you instantiate one of its subclasses).
Java Reader Subclasses
You will normally use a Reader
subclass rather than a Reader
directly. Java IO contains a lot of Reader
subclasses. Here is a list of the Java Reader
subclasses:
Here is an example of creating a Java FileReader
which is a subclass of Java Reader
:
Read Characters From a Reader
The read()
method of a Java Reader
returns an int which contains the char
value of the next character read. If the read()
method returns -1, there is no more data to read in the Reader
, and it can be closed. That is, -1 as int value, not -1 as byte or char value. There is a difference here!
Here is an example of reading all characters from a Java Reader
:
Countdown to 2021. Notice how the code example first reads a single character from the Java Reader
and checks if the char numerical value is equal to -1. If not, it processes that char
and continues reading until -1 is returned from the Reader
read()
method.
Read Array of Characters From Reader
The Java Reader
class also has a read()
method that takes a char
array as parameter, as well as a start offset and length. The char
array is where the read()
method will read the characters into. The offset parameter is where in the char
array the read()
method should start reading into. The length parameter is how many characters the read()
method should read into the char
array from the offset and forward. Here is an example of reading an array of characters into a char
array with a Java Reader
:
The read(char[], offset, length)
method returns the number of characters read into the char
array, or -1 if there are no more characters to read in the Reader
, for instance if the end of the file the Reader
is connected to has been reached.
Read Performance
Reading an array of characters at a time is faster than reading a single character at a time from a Java Reader
. The difference can easily be a factor 10 or more in performance increase, by reading an array of characters rather than reading a single character at a time.
The exact speedup gained depends on the size of the char
array you read, and the OS, hardware etc. of the computer you are running the code on. You should study the hard disk buffer sizes etc. of the target system before deciding. However buffer sizes of 8KB and up will give a good speedup. However, once your char
array exceeds the capacity of the underlying OS and hardware, you won't get a bigger speedup from a bigger char
array.
You will probably have to experiment with different byte array size and measure read performance, to find the optimal char
array size.
Transparent Buffering via BufferedReader
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You can add transparent, automatic reading and buffering of an array of bytes from a Reader
using a Java BufferedReader . The BufferedReader
reads a chunk of chars
into a char
array from the underlying Reader
. You can then read the bytes one by one from the BufferedReader
and still get a lot of the speedup that comes from reading an array of chars
rather than one character at a time. Here is an example of wrapping a Java Reader
in a BufferedReader
:
Notice, that a BufferedReader
is a Reader
subclass and can be used in any place where an Reader
can be used.
Skip Characters
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The Java Reader
class has a method named skip()
which can be used to skip over a number of characters in the input that you do not want to read. You pass the number of characters to skip as parameter to the skip()
method. Here is an example of skipping characters from a Java Reader
:
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This example tells the Java Reader
to skip over the next 24 characters in the Reader
. The skip()
method returns the actual number of characters skipped. In most cases that will be the same number as you requested skipped, but in case there are less characters left in the Reader
than the number you request skipped, the returned number of skipped characters can be less than the number of characters you requested skipped.
Closing a Reader
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When you are finished reading characters from a Reader
you should remember to close it. Closing an Reader
is done by calling its close()
method. Here is how closing an Reader
looks:
You can also use the Java try with resources construct introduced in Java 7. Here is how to use and close a InputStreamReader
looks with the try-with-resources construct:
Notice how there is no longer any explicit close()
method call. The try-with-resources construct takes care of that.
- All Implemented Interfaces:
Closeable
,AutoCloseable
,Readable
- Direct Known Subclasses:
BufferedReader
,CharArrayReader
,FilterReader
,InputStreamReader
,PipedReader
,StringReader
,URLReader
- Since:
- 1.1
- See Also:
BufferedReader
,LineNumberReader
,CharArrayReader
,InputStreamReader
,FileReader
,FilterReader
,PushbackReader
,PipedReader
,StringReader
,Writer
Field Summary
Fields Modifier and Type Field Description protected Object
lock
The object used to synchronize operations on this stream.Constructor Summary
Constructors Modifier Constructor Description protected
Reader()
Creates a new character-stream reader whose critical sections will synchronize on the reader itself.protected
Reader(Object lock)
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)
boolean
markSupported()
Tells whether this stream supports the mark() operation.static Reader
nullReader()
int
read()
Reads a single character.int
read(char[] cbuf)
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.boolean
ready()
void
reset()
Resets the stream.long
skip(long n)
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 java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Field Details
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 thanthis
or a synchronized method.
Constructor Details
Reader
Creates a new character-stream reader whose critical sections will synchronize on the reader itself.Reader
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
Returns a newReader
that reads no characters. The returned stream is initially open. The stream is closed by calling theclose()
method. Subsequent calls toclose()
have no effect.While the stream is open, the
read()
,read(char[])
,read(char[], int, int)
,read(Charbuffer)
,ready()
,skip(long)
, andtransferTo()
methods all behave as if end of stream has been reached. After the stream has been closed, these methods all throwIOException
.The
markSupported()
method returnsfalse
. Themark()
andreset()
methods throw anIOException
.The
object
used to synchronize operations on the returnedReader
is not specified.- Returns:
- a
Reader
which reads no characters - Since:
- 11
read
publicintread(CharBuffer target) throws IOExceptionAttempts 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.- Specified by:
read
in interfaceReadable
- Parameters:
target
- the buffer to read characters into- Returns:
- The number of characters added to the buffer, or -1 if this source of characters is at its end
- Throws:
IOException
- if an I/O error occursNullPointerException
- if target is nullReadOnlyBufferException
- if target is a read only buffer- Since:
- 1.5
read
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
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.- 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 abstractintread(char[] cbuf,int off,int len) throws IOExceptionReads 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.- Parameters:
cbuf
- Destination bufferoff
- Offset at which to start storing characterslen
- Maximum number of characters to read- Returns:
- The number of characters read, or -1 if the end of the stream has been reached
- Throws:
IOException
- If an I/O error occursIndexOutOfBoundsException
- Ifoff
is negative, orlen
is negative, orlen
is greater thancbuf.length - off
skip
Skips characters. This method will block until some characters are available, an I/O error occurs, or the end of the stream is reached.- Parameters:
n
- The number of characters to skip- Returns:
- The number of characters actually skipped
- Throws:
IllegalArgumentException
- Ifn
is negative.IOException
- If an I/O error occurs
ready
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
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
publicvoidmark(int readAheadLimit) throws IOExceptionMarks 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
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
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 interfaceAutoCloseable
- Specified by:
close
in interfaceCloseable
- Throws:
IOException
- If an I/O error occurs
transferTo
publiclongtransferTo(Writer out) throws IOExceptionReads 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 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 writingNullPointerException
- ifout
isnull
- Since:
- 10