Wednesday, August 31, 2016

JAVA 10 - Arrays

Way back in JAVA 4 we looked at declaring and using variables - and we've been using them ever since.

Sometimes though we need a multiple grouping of an item.  For instance, name - you have your own name obviously.  But remember back in school and your teacher called a register for class to see who was present?

For something like that you need a grouping of names ... which we call an array.

Here are a couple of examples,

String[] rollcall = { "Pugh", "Pugh", "Barney McGrew", "Cuthbert", "Dibble", "Grubb" };


  • the [] square brackets declare we're calling an array of Strings.  
  • rollcall is the name of the array
  • the {} brackets hold our list of items
  • "Pugh" - each item in quotes is an item in our array - we have 6 in all

Elsewhere, we can extract an item from our array by going ...


  • name - is the name of the array, obviously
  • index - is the nth number in the array.  Now an important thing to know about it index is it starts from 0.


  • rollcall[0] returns our first item "Pugh"
  • rollcall[5] returns our last item "Grubb"
  • rollcall[6] produces an error

You could also declare arrays for double or ints,

int[] eyesFingerKneesToes = { 2, 10, 2, 11 };

double[] percentComplete = { 22.3, 30.2, 82,1 };

You can also declare an array as empty, and then add values to it later ...

eyesFingerKneesToes = new int[4];
eyesFingerKneesToes[0] = 2;
eyesFingerKneesToes[1] = 10;
eyesFingerKneesToes[2] = 2;
eyesFingerKneesToes[3] = 11;

What's important to know though - once you've declared your array, you can't make it bigger.  It's a bit of a limitation of arrays, but next time we'll talk about ways around it.

Competition Draw

Here's an example I know you've been dying to see - yes, I'm using software for my competition draw!

Building on some unit testing we did a while ago, I've a class (shhh ... spoilers) which allows me to choose a random number.

Here's the JUnit test though I use this to do my draw ...

Here I declare an array of entrants who replied to my blog.

I can access methods and data within the entrants array, so when I choose entrants.length, it tells me the number of entrants in that array (I know I could count, but this is more fun).

I use that to define the maximum number I want, and get a random number winnerNum back.

I then reference the Twitter handle of the winner by using entrants[winnerNum].

From this we clearly see ...

Well done Alaine Miller!

Was it a fair draw?

I actually had another check inside my JUnit tests - which you might remember from looking at unit tests here.

This function is a much more complex in it's use of loops and arrays ...

This runs 800,000 draws for 8 entrance, making sure they're all within 1% of each other.  It does this by creating countArray - where it counts the number of 0, 1 ... 8 results.  So countArray[4] tells how many times the number 4 has come up etc.

I loop through 800,000 rolls, then loop through the values 0 ... 8 to make sure each is about 1/8 of the number of selections.

That's as fair as I can get it!

You can find the code for this here.

Extension material

Read more about arrays here.

If experimenting around with this yourself, you will need to import java.util.Arrays.


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