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J2ME Game Development: Issues and Troubleshooting (TM) -Deeper in the Game Logic

by Massimo Perrone (Hanami Solutions)

4.0 Deeper in the game logic

Let's dive further in the game logic.

Inside the game() method, will take place the most disparate tasks, amongst them in the case of our beat-em-up:

public void game()

As you can see everything proceeds whithin the 20 milliseconds in a logical sequence.

First of all we will draw the background and there is no need for explanations about this.

Later on we check if any key has been pressed meanwhile on the device.

This is a delicate operation so i'll briefly sum up the better way to do it.

Canvas class provide us with two extremely useful methods: keyPressed() and keyReleased(). Since the whole game system must be based on a quick feedback from these two methods, best way is to not handle animation process from inside them but solely get input and forward its result to other methods like, (in this case: checkCPU(), updatePlayerStatus() and updateEnemyStatus()) so to make the key press function available again the sooner is possible.

Method keyPressed() shall then contain something like this:

public void keyPressed(int keyCode)
int action = getGameAction(keyCode);

                	if (action==FIRE||keyCode==KEY_NUM5) commandPlayer=BLOCK;
else if (action==LEFT||keyCode==KEY_NUM4) commandPlayer=MOVELEFT;
                	else if (action==RIGHT||keyCode==KEY_NUM6) commandPlayer=MOVERIGHT;
                	else if (action==UP||keyCode==KEY_NUM2) commandPlayer=PUNCH;
                	else if (action==DOWN||keyCode==KEY_NUM8) commandPlayer=KICK;

This way we've quickly received user input and the method has been freed istantly for other key presses. The methods following it will be in charge of handling consequences of user's choice on the game proceedings.

Whithin the code you'll notice variables like "BLOCK", "MOVELEFT", etc.

I've used variables of the byte kind with names matching player status. Of course each variable must have different value, e.g. i've used:


These values will be later be stored, time by time, in "commandPlayer" to know, each moment, what's happening to the player.

In the same way, by mean of method checkCPU() we will have to evaluate according to user's choices, how the artificial intelligence of the current enemy should react and saving in a specific variable ("commandEnemy") what's happening to the current enemy.

By doing so, we will have recorded both the current status of the player and that of his enemy that we will update later with its related methods updatePlayerStatus() and updateCPUStatus().

Inside these two methods, we will take care of, again, by using aforementioned variables, checking if the player and/or his enemy has been hit, if they move rightwards or leftwards, whether they blocked an attack, etc.

At this point, because of what happened up to now, we need to check again if the game must continue, if we reached the end of a round and/or of a match winning it or if it is "game over".

According to the game status that we will handle with appropriate variables, similar to those used for status of player and his enemy, we will know where and how to show the opponents and then finally launch the drawFighters() method to see the result on screen.

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