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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Java arrays - are they true arrays or not?

Java's arrays are "true" arrays, in that they occupy sequential memory locations in which they are looked up by position number, so that access can be very fast. But that does mean that once an array has been defined to be a certain size, it cannot be resized in situ.

Fortunately, Java is a dynamically running language, with variables being created, used, released and garbage collected (so that memory is available for reuse within the same process) as the program runs - so you don't have to decide how big each of your arrays needs to be at the time you compile your program.

On our Java Bootcamp training course, I set an exercise that uses this reuse capability in a practical way - in Pascal's triangle, each row can be derived from the previous row and each row contains one more element than the previous row. So, we start off initialising previous and to nothing and declaring current:
  int [] current;
  int [] previous = {};

then, within a loop we define current as being one longer than previous:
  current = new int[previous.length+1];
we then do the calculations to work out the new values, and at the end of the loop we take the new array (current) and also apply the previous name to it - in the process, releasing the previous previous for garbage collection:
  previous = current;

The full source code of this example is [here].

Java does not (strictly) support two dimensional arrays - where each of the one dimensional arrays is arranged sequentially in memory directly after the previous one. Rather, it supports arrays of arrays. This might sound like a semantic difference, but it's of great use. For a tiny reduction in efficiency in accessing rectangular blocks of data, the programmer gains the ability to do everything he could using true 2 dimensional arrays within the language, AND the ability to create arrays which are differently shaped - as (for example) in another implementation of Pascal's Triangle.

The enclosing array is set up as follows:
  int [][] tri = new int[20][];
and then each row is set up to a differing length in a way that is not possible with true 2 dimensional arrays:
  for (int r=0; r<tri.length; r++) {
  tri[r] = new int[r+1];

There's a full source code example here [here].

(written 2010-02-23, updated 2010-03-03)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
J705 - Java - Arrays
  [1497] Training Season Starts again! - (2008-01-07)
  [1498] Java is a dynamic language .... (and comparison) - (2008-01-08)
  [1614] When an array is not an array - (2008-04-17)
  [3038] Setting up individual variables, and arrays, in Java - some commented examples - (2010-11-09)
  [3039] Fresh Paint - Java Arrays - (2010-11-09)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [4347] Arrays in Java - an introduction for newcomers - (2014-12-10)
  [4413] Binomial Coefficient (Pascal Triangle) objects in Java - (2015-02-03)
  [4428] Using the lead - passing arrays and other collections in Java - (2015-02-16)


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Reading and writing cookies in Java Servlets and JSPs
Calculation within objects - early, last minute, or cached?
Getting a phone line changed ...
Length, size or capacity in Java?
Java arrays - are they true arrays or not?
Removing duplicates from a MySQL table
Compile but do not run PHP - syntax check only
Optimising and caching your MySQL enquiries
Counting rows in joined MySQL tables
Relating tables with joins in MySQL
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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