Time is an illusion. Lunchtime doubly so.
— Douglas Adams
We are starting a new, probably more or less regular, thing today: answering viewer questions.
In the first of the Q&A episodes, I am talking about timers: How to arrange something to happen after a specific time delay. This principle is generally useful for many games and simulations. We also get bit more practice with int variables.
Concepts discussed: time delay, variables, counting
If life gives you lemons – keep them. Because, hey. Free lemons.
— Anonymous
In this episode, we’re continuing from the last episode’s theme and investigate and compare an alternative structure for making our Ball and Counter objects cooperate. We also have a first brief look at casting.
Recognising alternatives is a good skill, and it slowly eases us into discussions about code quality, which we will get to in more detail later.
The download today (below) is the scenario as it is at the end of this episode. However, try to add a counter into one of your own scenarios to make sure you are fully taking in what’s going on here.
Concepts discussed: object interaction, casting, score counting
Download: breakout-v3.zip (the scenario at the end of this episode)
Daring ideas are like chessmen moved forward; they may be beaten,
but they may start a winning game.
— Goethe
In this and the next episode, we’re looking a bit more into how to set up object structures, and how to get them to interact. This is one of the most fundamental concepts of object-oriented programming, so you need to get your head around this.
We discuss this here with the example of integrating our counter object into a scenario.
Concepts discussed: object interaction, object references, score counting, constructors, fields
Download: breakout-v2.zip (the scenario at the end of this episode)
Teamwork is essential; it allows you to blame someone else.
— Finagles’ 8th Rule
One of the frequently asked questions is how to display text in Greenfoot scenarios. Today I’ll answer that question.
I discuss two things: How to display some static text (text that does not change, such as instructions), and how to display text that should change during the program run. We’ll do that by making our own score counter.
Concepts discussed: Dynamic images, text display, GreenfootImage, score counting
We learn wisdom from failure much more than from success. We often discover
what will do, by finding out what will not do; and probably he who never made a
mistake never made a discovery.
— Samuel Smiles
We’re talking about errors today. I’m sure you all have – like me – got your fair share of errors when you were programming. I’ll talk a bit about how to go about investigating what went wrong when you get an error.
In doing this, we’re discussing the important distinction of synax errors and semantic errors, and we’re learning to deal with both. This includes reading stack traces.
Concepts discussed: syntax errors, runtime errors, stack trace, exception
The ideals which have lighted me on my way and time after time given me new
courage to face life cheerfully, have been Truth, Goodness, and Beauty. . . . The
ordinary objects of human endeavour — property, outward success, luxury — have
always seemed to me contemptible.
— Albert Einstein
One more quick one about image effects: a smoke effect to be precise. I’ll show you how to create this really cool effect just with what you know already. Hopefully, this will give you some ideas of more cool effects that you can do on your own.
Concepts discussed: image manipulation, visual effects, animation
Download: The project can be downloaded from the Greenfoot book website.
A positive attitude may not solve all your problems,
but it will annoy enough people to make it worth the effort.
— attributed to Herman Oliver Albright
A last little bit before we leave our coloured circles behind: an animation. In this case, it’s a very simple one: fading out the circles. But it shows an important technique: The appearance of an actor can change all the time, if we want to. This is the basis for many visual effects.
And, we finally see an if-statement with an else clause! I can’t believe that it took this long for this to pop up.
Concepts discussed: transparency, dynamically changing images, if-statement
Mathematicians are like Frenchmen: whatever you say to them they translate
into their own language and forthwith it is something entirely different.
— Goethe, Maximen und Reflexionen
Tracing control flow (either through control structures, such as if-statements or loops) or through method calls, is hard for beginners.
This episode shows you how to use the debugger to make this clearer.
Concepts discussed: control flow, tracing, breakpoints, debugger
We are accustomed to see men deride what they do not understand, and snarl
at the good and beautiful because it lies beyond their sympathies.
— Goethe
Okay, time to get a little more professional again: Let’s clean up our code a little.
Writing in good style is really important, so today I’ll show you a bit about refactoring. Especially, we want to create short, clear methods. (The technical term for it is cohesion – we want highly cohesive methods.)
Concepts discussed: methods, refactoring, style, cohesion
For every action, there is an equal and opposite criticism.
— Steven Wright
We’re getting a bit further into image manipulation. I’ll show you how an actor can change appearance by dynamically changing its image while the program is running.
Concepts discussed: image manipulation, random numbers