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Random values
Note that several of the properties of each Sprite01 objects is initialized with random values.
The use of the nextFloat method of the object of the Random class may be new to you. If so, this method simply returns a random value between 0.0f and 1.0f each time it is called.
The conditional operator
If the use of the conditional operator involving the ? character and the : character is new to you, you will probably need to do some online researchin order to understand the use of this operator.
Otherwise, the code in Listing 4 is straightforward and shouldn't require an explanation beyond the embedded comments.
Listing 4 signals the end of the init method.
The update method is shown in its entirety in Listing 5 .
| Listing 5 . The update method. |
|---|
public void update(GameContainer gc, int delta)
throws SlickException{//Do the following for every sprite in the array
for(int cnt = 0;cnt<sprites.length;cnt++){
//Ask each sprite to move.sprites[cnt].move();//Ask each sprite to bounce off the edge if
// necessary.sprites[cnt].edgeBounce(backgroundWidth,backgroundHeight);
}//end for loop}//end update |
Move and bounce
Listing 5 uses a for loop to access each of the sprite objects, asking each object to move and to bounce off the edge of the game window if necessary.
Listing 5 could hardly be simpler. That is because the necessary complexity has been encapsulated in each object of the Sprite01 class.
The move method of the Sprite01 class
Listing 6 shows the move method from the class named Sprite01 .
| Listing 6 . The move method of the Sprite01 class. |
|---|
public void move(){
X += xDirection*xStep;Y += yDirection*yStep;
}//end move |
The code in Listing 6 is also simple. However, in this case, the simplicity is somewhat deceiving. The apparent simplicityderives from the fact that the four required property values are routinely maintained by the object and are readily available to the two statements inthe move method when needed.
The edgeBounce method of the Sprite01 class
The edgeBounce method of the Sprite01 class is shown in Listing 7 . It is not simple.
| Listing 7 . The edgeBounce method of the Sprite01 class. |
|---|
public void edgeBounce(float winWidth,float winHeight){
//Test for a collision with one of the edges and// cause to sprite to bounce off the edge if a
// collision has occurred.if(X + width*scale>= winWidth){
//A collision has occurred.xDirection = -1.0f;//reverse direction
//Set the position to the right edge less the// width of the sprite.
X = winWidth - width*scale;}//end if//Continue testing for collisions with the edges
// and take appropriate action.if(X<= 0){
xDirection = 1.0f;X = 0;
}//end ifif(Y + height*scale>= winHeight){
yDirection = -1.0f;Y = winHeight - height*scale;
}//end ifif(Y<= 0){
yDirection = 1.0f;Y = 0;
}//end if}//end edgeBounce |
Code that you have seen before
Listing 7 contains essentially the same code that was written into the update method of the earlier module mentioned above . In this case, however, all of the complexity has been encapsulated into the Sprite01 class and replaced by a single call to the edgeBounce method in the update method of the program named Slick0200 . Thus, the program's update method is now much simpler.
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