Rectangle Example

logger.java

public interface Logger {
  public void debug(String msg);
  public void info(String msg);
  public void warn(String msg);
  public void error(String msg);
  public void fatal(String msg);
}

Rectangle.java

public class Rectangle {
  public static int COUNTER = 0;

  public int width;
  public int height;
  private int myX;
  private int myY;

  public Rectangle(int x, int y, int w, int h) {
    // Position and dimensions were given, so we construct a rectangle at (x,y)
    // with a width of w, and a height of h.
    myX = x;
    myY = y;
    width = w;
    height = h;
    Rectangle.COUNTER += 1;
  }

  public Rectangle(int w, int h) {
    // Width and height were given, so we construct a rectangle at (0,0)
    // with a width of w, and a height of h.
    this(0, 0, w, h);
    // The previous line tells the JVM to use a constructor that takes
    // 4 integral arguments and uses the given width and height.
  }

  public Rectangle() {
    // No arguments were given, so we construct a rectangle at (0,0)
    // with a width of 0, and a height of 0. (The zero's are the default
    // values.)
    this(0, 0, 0, 0);
    // The previous line tells the JVM to use a constructor that takes
    // 4 integral arguments.
  }

  public int getX() {
    return myX;
  }

  public int getY() {
    return myY;
  }

  public int getArea() {
    return width * height;
  }

  public void setX(int x) {
    myX = x;
  }

  public void setY(int y) {
    myY = y;
  }

  public void move(int x, int y) {
    myX = x;
    myY = y;
    // or, we could replace the previous 2 lines with:
    // setX(x);
    // setY(y);
  }
}

slightly_less_shit.java

public void anExampleOfCasting() {
  Object arr[] = new Object[3];

  arr[0] = "Hello!";
  arr[1] = new Integer(5);
  arr[2] = "World!";

  String myString = arr[0];
  // That line will generate a compile time error, because Java sees
  // every element in the array `arr` as an instance of Object. So, even
  // though `arr[0]` really is an instance of String, we have to typecast
  // it to use it as one:

  String myString = (String)arr[0];
}

wrecked_em.java

public void someMethod() {
  Rectangle rect = new Rectangle(0, 0, 5, 10); // centered at (0,0), width is 5, height is 10
  rect.move(3, 4);
  int xPos = rect.getX();
  int yPos = rect.getY();
  // 1) xPos == ?
  // 2) yPos == ?
  rect.move(-10, 8);
  int xPos2 = rect.getX();
  int yPos2 = rect.getY();
  // 3) xPos2 == ?
  // 4) yPos2 == ?
}