binary formats : Java Glossary

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binary formats

DataOutputStream puts out Java data in big-endian internal binary format. In contrast PrintStream.println and PrintStream.print put out data as human-readable 8-bit ASCII characters.

Java DataOutputStream Binary File Formats

When you create a file via DataOutputStream, what does the binary file look like? It looks like the internal binary RAM format in a big-endian CPU. These are also the internal formats in the Java Virtual Machine.

Everything is stored big endian, MSB (Most Significant Byte) first. (People who cut their teeth on Intel or the MOS 6502 are used to the little endian LSB first format.) Even on Intel hardware Java uses big endian file formats. This permits data interchange with other platforms.

There are no separators between fields. The files are in binary, not readable ASCII.

Method	Type	Size	Description
writeBoolean(boolean v)	boolean	1 byte	8-bit 0x00=false 0x01=true
writeByte(int v)	byte	1 byte	8-bit signed binary integer or 8-bit ASCII char
writeBytes(String s)	bytes	1 byte	8-bit signed binary integers or string of ASCII chars. not null terminated. not in quotes. not counted. not delimited in any way.
writeChar(int v)	char	2 byte	16-bit unsigned binary integer or 16-bit Unicode char.
writeChars(String s)	chars	2 byte	16-bit unsigned binary integers or string of 16-bit Unicode chars. not null terminated. not in quotes. not counted. not delimited in any way.
writeDouble(double v)	double	8 bytes	64-bit IEEE binary 1-bit sign 11-bit base 2 exponent biased+1023 52-bit fraction, lead 1 implied e.g. 3. = 0x4008000000000000 -3. = 0xC008000000000000
writeFloat(float v)	float	4 bytes	32-bit IEEE binary 1-bit sign 8-bit base 2 exponent biased+127 23-bit fraction, lead 1 implied e.g. 3. = 0x404000 -3. = 0xC04000
writeInt(int v)	int	4 bytes	32-bit signed binary e.g. 3 = 0x00 0x00 0x00 0x03 -3 = 0xff 0xff 0xff 0xfd
writeLong(long v)	long	8 bytes	64-bit signed binary
writeShort(int v)	short	2 bytes	16-bit signed binary
writeUTF(String s)	utf	2 bytes	16-bit length count followed by ASCII-7 string. Not null terminated. "ABC" == 0x0003414243 non-ASCII-7 chars use multibyte encodings with first byte having the high bit on.

UTF

UTF is a compact form of Unicode that uses a mixture of 8, 16 and 24-bit codes. Strings are stored as a 16-bit big-endian length count followed by a 7-bit ASCII string. Not null terminated. "ABC" == 0x0003414243. Non-ASCII-7 chars use multibyte encodings with first byte having the high bit on. UTF is an external format. UTF strings are interconverted to ordinary Strings during I/O by readUTF and writeUTF. Unicode-2 supports even 32 bit characters, and UTF has been extended to handle them as well.

Unicode	UTF	bytes required to represent the character
00000000 0xxxxxxx	0xxxxxxx	1
00000yyy yyxxxxxx	110yyyyy 10xxxxxx	2
zzzzyyyy yyxxxxxx	1110zzzz 10yyyyyy 10xxxxxx	3

010 Editor: a binary editor with templates
binary
endian
primitive
println file format
Wotsit

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