Introduce starfield classes

Star.java

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+public class Star
+{
+    private Ball b;
+    private double speedX;
+    private double speedY;
+
+    private double startX;
+    private double startY;
+
+    private GameArena arena;    
+
+    public Star(double x, double y, double size, String colour, double speedX, double speedY, GameArena a)
+    {
+        this.startX = x;
+        this.startY = y;
+        this.speedX = speedX;
+        this.speedY = speedY;
+        this.arena = a;
+
+        b = new Ball(x, y, size, colour);
+    }
+
+    public void addTo(GameArena a)
+    {
+        a.addBall(b);
+    }
+
+    public void move()
+    {
+        b.move(speedX, speedY);
+
+        // Check bounds within the given GameArena
+        if (b.getXPosition() > arena.getArenaWidth() || b.getXPosition() < 0 || b.getYPosition() > arena.getArenaHeight() || b.getYPosition() < 0)
+        {
+            b.setXPosition(startX);
+            b.setYPosition(startY);
+        }
+    }
+}

StarField.java

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+/**
+ * A class to represent a simple starfield effect.
+ * A cnfigurable number of stars appear in the centre of the screen,
+ * and move outward to the edges in a randome direction.
+ */
+
+public class Starfield
+{
+    private Star[] stars;
+    private Ball horizon;
+    private int numberOfStars;
+    private double effectDepth = 1.0;
+    private double effectSpeed = 1.0;
+    private double starSize = 3;
+    private String starColour = "#DDDDDD";
+
+    private GameArena arena;
+
+    /**
+	 * Constructor. Creates a Ball with the given parameters.
+	 * @param numberOfStars The number of stars that should be on screen at any point in time
+	 * @param speed The spparent speed of the effect - higher numbers are faster.
+	 */
+    public Starfield(int numberOfStars, double speed, GameArena a)
+    {
+        stars = new Star[numberOfStars];
+        this.effectSpeed = speed;
+        this.arena = a;
+
+        for (int i=0; i<stars.length; i++)
+        {
+            stars[i] = new Star(arena.getWidth()/2, arena.getHeight()/2, starSize, starColour, speed * (0.5 - Math.random()), speed * (0.5 - Math.random()), arena);
+            stars[i].addTo(a);
+        }
+
+        horizon = new Ball(arena.getWidth()/2, arena.getHeight()/2, 20, "BLACK");
+        arena.addBall(horizon);
+    }
+
+    public void move()
+    {
+        for (int i=0; i<stars.length; i++)
+            stars[i].move();
+    }
+}