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.