Java Glossary

Last updated 1998 June 16 Roedy Green ©1996-1998 Canadian Mind Products.

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U

UDP
User Datagram Protocol. This protocol is build on top of IP, and allows sending datagrams -- messages of arbitrary length (broken into packets). UDP makes no provision for checking that a message was received or retransmitting it. TCP/IP is built on top of UDP. See IP, TCP/IP.
Unicode
A 16-bit character encoding used in Java. Sometimes called UCS or ISO 10646. Unicode allows Java to handle international characters for most of the world's living languages, including airport symbolic icon, APL, Arabic, Armenian, Bengali, Bopomofo, Chinese (via unified Han), Cyrillic, English, Georgian, Greek, Gujarati, Gurmukhi, Hebrew, Hindi (Devanagari), Japanese (Kanji, Hiragana and Katakana via unified Han), Kannada, Korean (Hangul via unified Han), Lao, mathematics, Maylayalam, Oriya, Tai, Tamil, Telugu and Tibetan. Unicode will make it much easier for non-English speaking programmers to write programs for English speaking users and vice versa. DownloadWord For Windows document giving the full Unicode character set with Java, HTML and Postscript encodings. Nic Fulton has written an applet that can display all 64 thousand Unicode characters including the Chinese/Korean Han. How many of them actually display on your screen depends on the font handling ability of your browser and operating system, and which fonts you have installed. In Java programs, intractable Unicode characters are represented in the form '\uffff', with four hex digits. Ordinary characters like 'A' are actually 16-bit Unicode too. See literal.
unmaintainable code
One of my most popular essays is a tongue in cheek How To Write Unmaintainable Code. Perhaps you would like to submit additions to it.
unsigned
in Java bytes, shorts, ints and longs are all considered signed. The only unsigned type is the 16-bit char. To use the sign bit as an additional data bit you have to promote to the next bigger data type with sign extension then mask off the high order bits. i.e. To get the effect of an 8-bit unsigned:

byte b;
...
int i = b && 0xff;

To get the effect of effect of a 16-bit unsigned use char. To get the effect of a 32-bit unsigned:

int i;
...
long l = i && 0xffffffffL;
update
Clear the area for the component to the background colour by doing a fillrect then paint. update calls paint directly. update routines are often overridden to eliminate the fillrect which will reduce flicker. See paint, repaint.
upload
Send a file. See download. As a memory aid, think of the BBS you are communicating with as being a regal personage perched in a castle high UP on a hill. In the case one person calls another, the caller uploads (sends) and downloads (receives) files.
URL
Uniform Resource Locator, e.g. http://www.hans.org/index.html. It gives the site name and the document name within that site. See URN.
URN
Uniform Resource Number. These will eventually replace URLs. A URN will automatically find the file you want from the closest or least busy server where the file is mirrored/cached. You can think of a URN as analogous to extending the ISBN book cataloging system to cover all the web pages as well. If you think about how paper books are distributed through wholesalers, bookstores and libraries, that will give you a metaphorical framework with which to think about URN caching. It works on much the same economic principles. The main difference is that with a URN, it is very cheap to Xerox a whole "book".
USENET
A method of broadcasting messages to everyone who is interested in a given narrow topic. There are currently about 4000 topics. Only one copy of the message is sent to each computer supporting people interested in that topic. In contrast, a mail list would send individual copies to each user on the list. Usenet topics are for large global, public, communities of interested users. Mail lists are for smaller private groups. Sometimes called "newsgroups".
UTF
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 0xxxxxxx0xxxxxxx1
00000yyy yyxxxxxx110yyyyy 10xxxxxx2
zzzzyyyy yyxxxxxx1110zzzz 10yyyyyy 10xxxxxx3
USENET
A method of broadcasting messages to everyone who is interested in a given narrow topic. There are currently about 4000 topics. Only one copy of the message is sent to each computer supporting people interested in that topic. In contrast, a mail list would send individual copies to each user on the list. Usenet topics are for large global, public, communities of interested users. Mail lists are for smaller private groups. Sometimes called "newsgroups".
UUENCODE
Non-Transparent channels don't simply pass all characters through unmolested. They treat some of the characters as control characters -- commands. In order to send arbitrary text down such channels, you need to convert the special characters into vanilla ones that can get through safely. One technique of converting a file into vanilla before sending is called UUENCODE. see also MIME transparent.
UUCP
Unix To Unix Command Processor/Protocol. This is store and forward protocol for transferring files over Internet. This is how USENET traffic propagates from node to node.



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