CHAPTER 22

The Package java.io

Input and output in Java is organized around the concept of streams. A stream is a sequence of items, usually 8-bit bytes, read or written over the course of time.

In the java.io package, all input is done through subclasses of the abstract class InputStream, and all output is done through subclasses of the abstract class OutputStream. The one exception to this rule is the class RandomAccessFile, which handles files that allow random access and perhaps intermixed reading and writing of the file.

For an input stream, the source of data might be a file, a String, an array of bytes, or bytes written to an output stream (typically by another thread). There are also "filter input streams" that take data from another input stream and transform or augment the data before delivering it as input. For example, a LineNumberInputStream passes bytes through verbatim but counts line terminators as they are read.

For an output stream, the sink of data might be a file, an array of bytes, or a buffer to be read as an input stream (typically by another thread). There are also "filter output streams" that transform or augment data before writing it to some other output stream.

An instance of class File represents a path name (a string) that might identify a particular file within a file system. Certain operations on the file system, such as renaming and deleting files, are done by this class rather than through streams.

An instance of class FileDescriptor represents an abstract indication of a particular file within a file system; such file descriptors are created internally by the Java I/O system.

There are two interfaces, DataInput and DataOutput, that support the transfer of data other than bytes or characters, such as long integers, floating-point numbers and strings. The class DataInputStream implements DataInput; the class DataOutputStream implements DataOutput; and RandomAccessFile implements both DataInput and DataOutput.

The class StreamTokenizer provides some simple support for parsing bytes or characters from an input stream into tokens such as identifiers, numbers, and strings, optionally ignoring comments and optionally recognizing or ignoring line terminators.

The hierarchy of classes defined in package java.io is as follows. (Classes whose names are shown here in boldface are in package java.io; the others are in package java.lang and are shown here to clarify subclass relationships.)

Object												§20.1				
	interface DataInput												§22.1
	interface DataOutput												§22.2
	InputStream												§22.3
		FileInputStream												§22.4
		PipedInputStream												§22.5
		ByteArrayInputStream												§22.6
		StringBufferInputStream												§22.7
		SequenceInputStream												§22.8
		FilterInputStream												§22.9
			BufferedInputStream												§22.10
			DataInputStream												§22.11
			LineNumberInputStream												§22.12
			PushBackInputStream												§22.13
	StreamTokenizer												§22.14
	OutputStream												§22.15
		FileOutputStream												§22.16
		PipedOutputStream												§22.17
		ByteArrayOutputStream												§22.18
		FilterOutputStream												§22.19
			BufferedOutputStream												§22.20
			DataOutputStream												§22.21
			PrintStream												§22.22
	RandomAccessFile												§22.23
	File												§22.24
	interface FileNameFilter												§22.25
	FileDescriptor												§22.26
	Throwable												§20.22
		Exception												§20.22
			IOException												§22.27
				EOFException												§22.28
				FileNotFoundException												§22.29
				InterruptedIOException												§22.30
				UTFDataFormatException												§22.31

22.1 The Interface `java.io.DataInput`

The DataInput interface provides for reading bytes from a binary stream and reconstructing from them data in any of the Java primitive types. There is also a facility for reconstructing a String from data in Java modified UTF-8 format.

The DataOutput interface (§22.2) supports the creation of binary output data suitable for reading back in through the DataInput interface.

The DataInput interface is implemented by classes DataInputStream (§22.11) and RandomAccessFile (§22.23).

public interface DataInput {
	public void readFully(byte[] b)
		throws IOException, NullPointerException;
	public void readFully(byte[] b, int off, int len)
		throws IOException, NullPointerException,
			IndexOutOfBoundsException;
	public int skipBytes(int n) throws IOException;
	public boolean readBoolean() throws IOException;
	public byte readByte() throws IOException;
	public int readUnsignedByte() throws IOException;
	public short readShort() throws IOException;
	public int readUnsignedShort() throws IOException;
	public char readChar() throws IOException;
	public int readInt() throws IOException;
	public long readLong() throws IOException;
	public float readFloat() throws IOException;
	public double readDouble() throws IOException;
	public String readLine() throws IOException;
	public String readUTF() throws IOException;
}

It is generally true of all the reading routines in this interface that if end of file is reached before the desired number of bytes has been read, an EOFException (which is a kind of IOException) is thrown. If any byte cannot be read for any reason other than end of file, an IOException other than EOFException is thrown. In particular, an IOException may be thrown if the input stream has been closed (§22.3.6).

22.1.1 public void readFully(byte[] b) throws IOException, NullPointerException;

The general contract of readFully(b) is that it reads some bytes from an input stream and stores them into the buffer array b. The number of bytes read is equal to the length of b.

This method blocks until one of the following conditions occurs:

b.length bytes of input data are available, in which case a normal return is made.
End of file is detected, in which case an EOFException is thrown.
An I/O error occurs, in which case an IOException other than EOFException is thrown.

If b is null, a NullPointerException is thrown.

If b.length is zero, then no bytes are read. Otherwise, the first byte read is stored into element b[0], the next one into b[1], and so on.

If an exception is thrown from this method, then it may be that some but not all bytes of b have been updated with data from the input stream.

22.1.2 public void readFully(byte[] b, int off, int len) throws IOException, NullPointerException, IndexOutOfBoundsException

The general contract of readFully(b, off, len) is that it reads len bytes from an input stream.

This method blocks until one of the following conditions occurs:

len bytes of input data are available, in which case a normal return is made.
End of file is detected, in which case an EOFException is thrown.
An I/O error occurs, in which case an IOException other than EOFException is thrown.

If b is null, a NullPointerException is thrown.

If off is negative, or len is negative, or off+len is greater than the length of the array b, then an IndexOutOfBoundsException is thrown.

If len is zero, then no bytes are read. Otherwise, the first byte read is stored into element b[off], the next one into b[off+1], and so on. The number of bytes read is, at most, equal to len.

If an exception is thrown from this method, then it may be that some but not all bytes of b in positions off through off+len-1 have been updated with data from the input stream.

22.1.3 public int skipBytes(int n) throws IOException

The general contract of skipBytes is that it makes an attempt to skip over n bytes of data from the input stream, discarding the skipped bytes. However, it may skip over some smaller number of bytes, possibly zero. This may result from any of a number of conditions; reaching end of file before n bytes have been skipped is only one possibility. This method never throws an EOFException. The actual number of bytes skipped is returned.

22.1.4 public boolean readBoolean() throws IOException;

The general contract of readBoolean is that it reads one input byte and returns true if that byte is nonzero, false if that byte is zero.

This method is suitable for reading the byte written by the writeBoolean method of interface DataOutput (§22.2.4).

22.1.5 public byte readByte() throws IOException

The general contract of readByte is that it reads and returns one input byte. The byte is treated as a signed value in the range -128 through 127, inclusive.

This method is suitable for reading the byte written by the writeByte method of interface DataOutput (§22.2.5).

22.1.6 public int readUnsignedByte() throws IOException

The general contract of readUnsignedByte is that it reads one input byte, zero- extends it to type int, and returns the result, which is therefore in the range 0 through 255.

This method is suitable for reading the byte written by the writeByte method of interface DataOutput (§22.2.5) if the argument to writeByte was intended to be a value in the range 0 through 255.

22.1.7 public short readShort() throws IOException

The general contract of readShort is that it reads two input bytes and returns a short value. Let a be the first byte read and b be the second byte. The value returned is:

(short)((a << 8) | (b & 0xff))

This method is suitable for reading the bytes written by the writeShort method of interface DataOutput (§22.2.6).

22.1.8 public int readUnsignedShort() throws IOException

The general contract of readUnsignedShort is that it reads two input bytes and returns an int value in the range 0 through 65535. Let a be the first byte read and b be the second byte. The value returned is:

(((a & 0xff) << 8) | (b & 0xff))

This method is suitable for reading the bytes written by the writeShort method of interface DataOutput (§22.2.6) if the argument to writeShort was intended to be a value in the range 0 through 65535.

22.1.9 public char readChar() throws IOException

The general contract of readChar is that it reads two input bytes and returns a char value. Let a be the first byte read and b be the second byte. The value returned is:

(char)((a << 8) | (b & 0xff))

This method is suitable for reading bytes written by the writeChar method of interface DataOutput (§22.2.7).

22.1.10 public int readInt() throws IOException

The general contract of readInt is that it reads four input bytes and returns an int value. Let a be the first byte read, b be the second byte, c be the third byte, and d be the fourth byte. The value returned is:


(((a & 0xff) << 24) | ((b & 0xff) << 16) |
  ((c & 0xff) <<    8) | (d & 0xff))

This method is suitable for reading bytes written by the writeInt method of interface DataOutput (§22.2.8).

22.1.11 public long readLong() throws IOException

The general contract of readLong is that it reads eight input bytes and returns a long value. Let a be the first byte read, b be the second byte, c be the third byte, d be the fourth byte, e be the fifth byte, f be the sixth byte, g be the seventh byte, and h be the eighth byte. The value returned is:


(((long)(a & 0xff) << 56) |
  ((long)(b & 0xff) << 48) |
  ((long)(c & 0xff) <<  40) |
  ((long)(d & 0xff) << 32) |
  ((long)(e & 0xff) <<  24) |
  ((long)(f & 0xff) << 16) |
  ((long)(g & 0xff) <<    8) |
  ((long)(h & 0xff)))

This method is suitable for reading bytes written by the writeLong method of interface DataOutput (§22.2.9).

22.1.12 public float readFloat() throws IOException

The general contract of readFloat is that it reads four input bytes and returns a float value. It does this by first constructing an int value in exactly the manner of the readInt method (§22.1.10), then converting this int value to a float in exactly the manner of the method Float.intBitsToFloat (§20.9.23).

This method is suitable for reading bytes written by the writeFloat method of interface DataOutput (§22.2.10).

22.1.13 public double readDouble() throws IOException

The general contract of readDouble is that it reads eight input bytes and returns a double value. It does this by first constructing a long value in exactly the manner of the readlong method (§22.1.11), then converting this long value to a double in exactly the manner of the method Double.longBitsToDouble (§20.10.22).

This method is suitable for reading bytes written by the writeDouble method of interface DataOutput (§22.2.11).

22.1.14 public String readLine() throws IOException

The general contract of readLine is that it reads successive bytes, converting each byte separately into a character, until it encounters a line terminator or end of file; the characters read are then returned as a String. Note that because this method processes bytes, it does not support input of the full Unicode character set.

If end of file is encountered before even one byte can be read, then null is returned. Otherwise, each byte that is read is converted to type char by zero-extension. If the character '\n' is encountered, it is discarded and reading ceases. If the character '\r' is encountered, it is discarded and, if the following byte converts to the character '\n', then that is discarded also; reading then ceases. If end of file is encountered before either of the characters '\n' and '\r' is encountered, reading ceases. Once reading has ceased, a String is returned that contains all the characters read and not discarded, taken in order. Note that every character in this string will have a value less than \u0100, that is, (char)256.

22.1.15 public String readUTF() throws IOException

The general contract of readUTF is that it reads a representation of a Unicode character string encoded in Java modified UTF-8 format; this string of characters is then returned as a String.

First, two bytes are read and used to construct an unsigned 16-bit integer in exactly the manner of the readUnsignedShort method (§22.1.8). This integer value is called the UTF length and specifies the number of additional bytes to be read. These bytes are then converted to characters by considering them in groups. The length of each group is computed from the value of the first byte of the group. The byte following a group, if any, is the first byte of the next group.

If the first byte of a group matches the bit pattern 0xxxxxxx (where x means "may be 0 or 1"), then the group consists of just that byte. The byte is zero-extended to form a character.

If the first byte of a group matches the bit pattern 110xxxxx, then the group consists of that byte a and a second byte b. If there is no byte b (because byte a was the last of the bytes to be read), or if byte b does not match the bit pattern 10xxxxxx, then a UTFDataFormatException is thrown. Otherwise, the group is converted to the character:

(char)(((a & 0x1F) << 6) | (b & 0x3F))

If the first byte of a group matches the bit pattern 1110xxxx, then the group consists of that byte a and two more bytes b and c. If there is no byte c (because byte a was one of the last two of the bytes to be read), or either byte b or byte c does not match the bit pattern 10xxxxxx, then a UTFDataFormatException is thrown. Otherwise, the group is converted to the character:

(char)(((a & 0x0F) << 12) | ((b & 0x3F) << 6) | (c & 0x3F))

If the first byte of a group matches the pattern 1111xxxx or the pattern 10xxxxxx, then a UTFDataFormatException is thrown.

If end of file is encountered at any time during this entire process, then an EOFException is thrown.

After every group has been converted to a character by this process, the characters are gathered, in the same order in which their corresponding groups were read from the input stream, to form a String, which is returned.

The writeUTF method of interface DataOutput (§22.2.14) may be used to write data that is suitable for reading by this method.

22.2 The Interface `java.io.DataOutput`

The DataOutput interface provides for converting data from any of the Java primitive types to a series of bytes and writing these bytes to a binary stream. There is also a facility for converting a String into Java modified UTF-8 format and writing the resulting series of bytes.

The DataInput interface (§22.1) can be used to read in and reconstruct Java data from the binary output data produced by the DataOutput interface.

The DataOutput interface is implemented by classes DataOutputStream (§22.21) and RandomAccessFile (§22.23).

public interface DataOutput {
	public void write(int b) throws IOException;
	public void write(byte[] b)
		throws IOException, NullPointerException;
	public void write(byte[] b, int off, int len)
		throws IOException, NullPointerException,
			IndexOutOfBoundsException;
	public void writeBoolean(boolean v) throws IOException;
	public void writeByte(int v) throws IOException;
	public void writeShort(int v) throws IOException;
	public void writeChar(int v) throws IOException;
	public void writeInt(int v) throws IOException;
	public void writeLong(long v) throws IOException;
	public void writeFloat(float v) throws IOException;
	public void writeDouble(double v) throws IOException;
	public void writeBytes(String s)
		throws IOException, NullPointerException;
	public void writeChars(String s)
		throws IOException, NullPointerException;
	public void writeUTF(String s)
		throws IOException, NullPointerException;
}

For all the methods in this interface that write bytes, it is generally true that if a byte cannot be written for any reason, an IOException is thrown.

22.2.1 public void write(int b) throws IOException

The general contract for write is that one byte is written to the output stream. The byte to be written is the eight low-order bits of the argument b. The 24 high-order bits of b are ignored.

22.2.2 public void write(byte[] b) throws IOException, NullPointerException

The general contract for write is that all the bytes in array b are written, in order, to the output stream.

If b is null, a NullPointerException is thrown.

If b.length is zero, then no bytes are written. Otherwise, the byte b[0] is written first, then b[1], and so on; the last byte written is b[b.length-1].

22.2.3 public void write(byte[] b, int off, int len) throws IOException, NullPointerException, IndexOutOfBoundsException

The general contract for write is that len bytes from array b are written, in order, to the output stream.

If b is null, a NullPointerException is thrown.

If off is negative, or len is negative, or off+len is greater than the length of the array b, then an IndexOutOfBoundsException is thrown.

If len is zero, then no bytes are written. Otherwise, the byte b[off] is written first, then b[off+1], and so on; the last byte written is b[off+len-1].

22.2.4 public void writeBoolean(boolean v) throws IOException

The general contract for writeBoolean is that one byte is written to the output stream. If the argument v is true, the value (byte)1 is written; if v is false, the value (byte)0 is written.

The byte written by this method may be read by the readBoolean method of interface DataInput (§22.1.4), which will then return a boolean equal to v.

22.2.5 public void writeByte(int v) throws IOException

The general contract for writeByte is that one byte is written to the output stream to represent the value of the argument. The byte to be written is the eight low- order bits of the argument b. The 24 high-order bits of b are ignored. (This means that writeByte does exactly the same thing as write for an integer argument.)

The byte written by this method may be read by the readByte method of interface DataInput (§22.1.5), which will then return a byte equal to (byte)v.

22.2.6 public void writeShort(int v) throws IOException

The general contract for writeShort is that two bytes are written to the output stream to represent the value of the argument. The byte values to be written, in the order shown, are:


(byte)(0xff & (v >> 8))
(byte)(0xff & v)

The bytes written by this method may be read by the readShort method of interface DataInput (§22.1.7), which will then return a short equal to (short)v.

22.2.7 public void writeChar(int v) throws IOException

The general contract for writeChar is that two bytes are written to the output stream to represent the value of the argument. The byte values to be written, in the order shown, are:


(byte)(0xff & (v >> 8))
(byte)(0xff & v)

The bytes written by this method may be read by the readChar method of interface DataInput (§22.1.9), which will then return a char equal to (char)v.

22.2.8 public void writeInt(int v) throws IOException

The general contract for writeInt is that four bytes are written to the output stream to represent the value of the argument. The byte values to be written, in the order shown, are:


(byte)(0xff & (v >> 24))
(byte)(0xff & (v >> 16))
(byte)(0xff & (v >>    8))
(byte)(0xff & v)

The bytes written by this method may be read by the readInt method of interface DataInput (§22.1.10), which will then return an int equal to v.

22.2.9 public void writeLong(long v) throws IOException

The general contract for writeLong is that four bytes are written to the output stream to represent the value of the argument. The byte values to be written, in the order shown, are:


(byte)(0xff & (v >> 56))
(byte)(0xff & (v >> 48))
(byte)(0xff & (v >> 40))
(byte)(0xff & (v >> 32))
(byte)(0xff & (v >> 24))
(byte)(0xff & (v >> 16))
(byte)(0xff & (v >>    8))
(byte)(0xff & v)

The bytes written by this method may be read by the readLong method of interface DataInput (§22.1.11), which will then return a long equal to v.

22.2.10 public void writeFloat(float v) throws IOException

The general contract for writeFloat is that four bytes are written to the output stream to represent the value of the argument. It does this as if it first converts this float value to an int in exactly the manner of the Float.floatToIntBits method (§20.9.22) and then writes the int value in exactly the manner of the writeInt method (§22.2.8).

The bytes written by this method may be read by the readFloat method of interface DataInput (§22.1.12), which will then return a float equal to v.

22.2.11 public void writeDouble(double v) throws IOException

The general contract for writeDouble is that eight bytes are written to the output stream to represent the value of the argument. It does this as if it first converts this double value to a long in exactly the manner of the Double.doubleToLongBits method (§20.10.21) and then writes the long value in exactly the manner of the writeLong method (§22.2.9).

The bytes written by this method may be read by the readDouble method of interface DataInput (§22.1.13), which will then return a double equal to v.

22.2.12 public void writeBytes(String s) throws IOException, NullPointerException

The general contract for writeBytes is that for every character in the string s, taken in order, one byte is written to the output stream.

If s is null, a NullPointerException is thrown.

If s.length is zero, then no bytes are written. Otherwise, the character s[0] is written first, then s[1], and so on; the last character written is s[s.length-1]. For each character, one byte is written, the low-order byte, in exactly the manner of the writeByte method (§22.2.5). The high-order eight bits of each character in the string are ignored.

22.2.13 public void writeChars(String s) throws IOException, NullPointerException

The general contract for writeChars is that every character in the string s is written, in order, to the output stream, two bytes per character.

If s is null, a NullPointerException is thrown.

If s.length is zero, then no characters are written. Otherwise, the character s[0] is written first, then s[1], and so on; the last character written is s[s.length-1]. For each character, two bytes are actually written, high-order byte first, in exactly the manner of the writeChar method (§22.2.7).

22.2.14 public void writeUTF(String s) throws IOException, NullPointerException

The general contract for writeUTF is that two bytes of length information are written to the output stream, followed by the Java modified UTF representation of every character in the string s.

If s is null, a NullPointerException is thrown.

Each character in the string s is converted to a group of one, two, or three bytes, depending on the value of the character.

If a character c is in the range '\u0001' through '\u007f', it is represented by one byte:

(byte)c

If a character c is '\u0000' or is in the range '\u0080' through '\u07ff', then it is represented by two bytes, to be written in the order shown:


(byte)(0xc0 | (0x1f & (c >> 6)))
(byte)(0x80 | (0x3f & c))

If a character c is in the range '\u0800' through '\uffff', then it is represented by three bytes, to be written in the order shown:


(byte)(0xc0 | (0x0f & (c >> 12)))
(byte)(0x80 | (0x3f & (c >>    6)))
(byte)(0x80 | (0x3f & c))

First, the total number of bytes needed to represent all the characters of s is calculated. If this number is larger than 65535, then a UTFDataFormatError is thrown. Otherwise, this length is written to the output stream in exactly the manner of the writeShort method (§22.2.6); after this, the one-, two-, or three-byte representation of each character in the string s is written.

The bytes written by this method may be read by the readUTF method of interface DataInput (§22.1.15), which will then return a String equal to s.

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Java Language Specification (HTML generated by Suzette Pelouch on February 24, 1998)
Copyright © 1996 Sun Microsystems, Inc. All rights reserved
Please send any comments or corrections to doug.kramer@sun.com

The Package java.io

22.1 The Interface java.io.DataInput

22.2 The Interface java.io.DataOutput

22.1 The Interface `java.io.DataInput`

22.2 The Interface `java.io.DataOutput`