The Movement Routine in Java

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The Movement Routine
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Movement occurs by creating a special game thread This thread will continue running as long as the game is in progress Any game actions can be handled in this loop We can create a game thread in Listing 173 Listing 173 Creating a Game Thread
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public class GameThread extends Thread { public void run() { while (running) { long time = SystemcurrentTimeMillis(); moveSprites(); // Check collisions repaint(); serviceRepaints(); time = SystemcurrentTimeMillis() - time; try { if (time < DELAY) Threadsleep(DELAY - (int)time); } catch (Exception ex) { } } } }
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The moveSprite() routine, which actually handles the movement of each sprite, is detailed in Listing 174 Listing 174 Moving the Sprites
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private void moveSprites() { switch (key) { case CanvasLEFT: if (FloatgetInteger(playergetX()) > (Cachewidth - ROAD_WIDTH) / 2) playersetX(Floatsub(playergetX(), FloatcreateFloat(2))); break; case CanvasRIGHT: if (FloatgetInteger(playergetX()) + playergetWidth() < (Cachewidth + ROAD_WIDTH) / 2) playersetX(Floatadd(playergetX(), FloatcreateFloat(2))); break; } for (int i = 0; i < enemyListsize(); i++)
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{ Sprite sprite = enemyListgetSprite(i); spritesetY(Floatadd(spritegetY(), spritegetVy())); } }
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Finally, we make sure to paint all our sprites in the paint() method of GameCanvas This can be found in Listing 175 Listing 175 Painting Sprites
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public void paint(Graphics gr) { "" // All the other paint functions happen here // // Then we paint the enemy list enemyListpaint(g); // And then the player playerpaint(g); // And then drop it all onto the real screen grdrawImage(scene, 0, 0, GraphicsTOP | GraphicsLEFT); }
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The movement magic happens in the moveSprites() method This method does the following:
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Move the hero sprite The race car moves either to the left if the player is holding down the left arrow key, or to the right if the player is holding down the right arrow key Movement occurs by adding or subtracting 2 pixels (as a floating-point number) from the vehicle's current x position If the race car hits either side of the track, movement stops Move opponent sprites Opponent movement, for now, is pretty simple Each enemy car is moved a set number of pixels down the screen The enemy's current y position is increased by the enemy's current y velocity An enemy can be slowed down or sped up by changing the y velocity value
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Piecing It All Together
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We now have all sorts of sprites moving around the game screen All that remains now is to handle sprites when they interact, and to determine how the game ends
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Handling Collision Detection
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Whenever your car touches an enemy car, it loses some energy That means collision detection is essential Because our Sprite class already has collision detection routines, it's just a matter of detecting and handling collisions A method could be created as follows:
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private void checkCollision() { if (enemyListcollide(player)) playersetEnergy(playergetEnergy() COLLIDE_ENERGY); if (playergetEnergy() <= 0) running = false;
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This method checks for a collision If one occurs, the COLLIDE_ENERGY value is deducted from the player's energy If the energy reaches zero, then we stop running the game loop It's important for players to know where they stand in a game at any given time As such, the energy value is drawn at the top of the screen at all times We'll also let the player know how much time has passed:
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public void paint(Graphics gr) { gdrawString("E:" + playergetEnergy(), (getWidth() ROAD_WIDTH) / 2 + 1, 0, GraphicsTOP | GraphicsLEFT); gdrawString("T:" + (timer / 10) + "" + (timer % 10) + "s", getWidth() / 2 + 1, 0, GraphicsTOP | GraphicsLEFT); }
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Endgame: Losing or Winning
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A game isn't much of a game unless you either lose or win it In our Micro Racer game, you win if your car successfully reaches a track's finish line before losing all its energy If your energy reaches zero, however, the game is over To set the finish line's location, we add this bit of code to the initEnemies() method:
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private long line; public void initEnemies() { line = - FloatcreateFloat(length); }
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Recall that length is a global variable, which is set from the Tracks class Each track has a different length The finish line can then be painted in the paint method:
public void paint(Graphics gr) { gdrawLine((getWidth() - ROAD_WIDTH) / 2, (int)FloatgetInteger(line), (getWidth() + ROAD_WIDTH) / 2, (int)FloatgetInteger(line));
Finally, we'll create a method to detect whether the finish line has been crossed If it is successfully crossed, we'll set the finished global flag to true, and stop running the game loop:
private void checkFinishLine() { if (FloatgetInteger(line) > Cacheheight) { running = false; finished = true; } }