Week 2

   Primitive types, operators, colors, random numbers, and conditionals

TYPES

All data in Java has either

- REFERENCE type: all objects, and NULL

- PRIMITIVE type: everything else (not objects, can't send them messages)


PRIMITIVE TYPES

Primitive numeric types: byte, char, short, int, float, long, double
These are 8, 16, 16, 32, 32, 64, and 64 bits long, respectively.

Use primarily int and double. 

float and double both called "floating point". All the other numeric
primitive types are "integer types"

One other primitive type: boolean


OPERATOR PRECEDENCE AND ASSOCIATIVITY

What does 2 + 4 / 2 mean? 
Ans: 2 + (4 / 2), not (2 + 4) / 2.

How about 8 / 4 / 2?
Ans: (8 / 4) / 2, since / associates left to right (L to R)

Operator precedence and associativity (Big Java: appendix A3 lists all
  operators in decreasing order of precedence, with all those in the same
  associativity group of equal precedence). Here are the ones we've seen so far:

  unary + and -, cast, R to L
  * / % (remainder) L to R
  (binary) + - L to R
  = assignment R to L

Associativity: Doesn't matter for associative operations like +.
  (Floating point + isn't exactly associative.)

arithmetic operators overloaded on integers and floating points.


  double diameter = 2.0;

is an example of a VARIABLE DECLARATION. In general, they are of the form

  <type> <name> = <expressions>;
or 
  <type> <name>;

In the latter case, without an INITIALIZER, the variable gets a default
value determined by its type if it is a field. If it is a local variable,
the type checker uses a somewhat crude algorithms to confirm that it is
assigned before it is used.

Generally, the type of the expression must be the same as the declared
type, but there are some IMPLICIT CONVERSIONS. E.g. we could have written 

   double diameter = 2;

instead of using 2.0, because there is an implicit WIDENING CONVERSION from
integer to double in an assignment (including declarations, as here, and
parameter passing, as we shall see).

Widening and narrowing conversions.
  - implicit: assignment conversion and (arithmetic) promotion (coercion), e.g.
      double twoPi = 2 * 3.14159; // promotion
      double zero = 1 / 2; // yikes!
      double half = 1.0 / 2;      
  - explicit: casting
      int intArea = (int)(diameter * twoPi);

Conversions between primitive types (other than boolean) that do not loose
any information are allowed and implicit. Conversions that loose
information are never implicit, but are allowed (other than those
involving boolean).

Exercise: what is the type and value (if well typed) of the expression
(int)-1.0/2*55.0 ? 


Increment and decrement operators ++ and --, either postfix or prefix.
Good style: use only as statements.

There are also assignment operators: +=, etc.


COLORS

The (ADDITIVE) PRIMARY COLORS are red, green, and blue, corresponding to
the three types of cones in our eyes and the three types of phosphors in
color monitors. This is often called the RGB COLOR SYSTEM.

RGB colors are represented in Java as three bytes, representing red, green, and
blue, with values from 0 to 255 (2^8 - 1). 

  new Color(int red, int green, int blue)

(0, 0, 0) is black, (255, 255, 255) is white.

How many possible Java colors are there?


RANDOM NUMBERS

  objectdraw.RandomIntGenerator randGen = new RandomIntGenerator(int first, int last)

returns a new generator of random numbers in the range [first, last].

****** SpiroGraph.java
import objectdraw.*;
import java.awt.*;

// mouse drags draw lines radiation from central points mouse down point
public class SpiroGraph extends FrameWindowController {
   
   // generator for RGB color values 
   private static final RandomIntGenerator COLOR_GEN = new RandomIntGenerator(0, 255);  
   
   private Location lineStart; // point from which lines radiate      
   private Color lineColor; // randomly generated color
      
   // create the random number generator
   public void begin() {
      canvas.setBackground(Color.BLACK);
      canvas.clear(); // necessary for the background color
   }
   
   // save the starting point
   public void onMousePress(Location point) {
      lineStart = point;
      lineColor = new Color(COLOR_GEN.nextValue(), 
                            COLOR_GEN.nextValue(), 
                            COLOR_GEN.nextValue());
      System.out.println(lineColor);
   }
   
   // draw lines as the user drags the mouse about
   public void onMouseDrag(Location point) {
      new Line(lineStart, point, canvas).setColor(lineColor);
   }   
}





******


CONDITIONALS

Dragging diagram with lastPoint and point for onMousePress, onMouseDrag
start, and onMouseDrag end.

****** WhatADrag1.java
import objectdraw.*;
import java.awt.*;

// croc.gif resized from Microsoft Design Galary wmf image
// http://dgl.microsoft.com/mgo1en/preview.asp?bInSelBasket=false
//   &nMedia=1&tName=j0215228&tExtension=wmf&nSize=21&nDuration=0&tCaption=CGL%20Preview

public class WhatADrag1 extends FrameWindowController {
   
   // starting position for image
   private static final int IMAGE_LEFT = 200;
   private static final int IMAGE_TOP = 100;
     
   private VisibleImage image; // visible image to be dragged      
   private Location lastPoint; // point where mouse was last seen
      
   // make the iamge
   public void begin() {
      image = new VisibleImage(this.getImage("croc.gif"), IMAGE_LEFT, IMAGE_TOP, canvas);
   }
   
   // save starting point and whether point was in image
   public void onMousePress(Location point) {
      lastPoint = point;
   }
   
   // if mouse is in image, then drag the image
   public void onMouseDrag(Location point) {      
      if (image.contains(lastPoint)) {
         double dx = point.getX() - lastPoint.getX();
         double dy = point.getY() - lastPoint.getY();
         image.move(dx, dy);
         lastPoint = point;
      }
   }   
}

******

The getImage method is sent to the CURRENT INSTANCE, the one executing the
current method, indicated by the keyword THIS. As a shorthand, if the
object of a method call is the current instance, "this." can be omitted, so
the call could be getImage("croc.gif") instead of this.getImage("croc.gif").

getImage returns an object of type java.awt.Image, but such images are not visible
by themselves. For this we create an instance of objectdraw.VisibleImage.

In onMouseDrag, there are LOCAL VARIABLE DECLARATIONS of dx and dy. Local
variable declarations are syntactically like field declarations, except
that they cannot have static or visibility (e.g. public, private)
modifiers. The SCOPE of local declarations is the block in which they are
enclosed (in this case the method body): they are not visible
elsewhere. They are created when the block is entered and their storage is
reclaimed when control leaves the block. 

contains(point) => contains(lastPoint) introduces a bug!

contains check is a bit time consuming: use grabbed boolean variable, 
set in onMousePress, tested in onMouseDrag

****** WhatADrag.java
import objectdraw.*;
import java.awt.*;

// croc.gif resized from Microsoft Design Galary wmf image
// http://dgl.microsoft.com/mgo1en/preview.asp?bInSelBasket=false
//   &nMedia=1&tName=j0215228&tExtension=wmf&nSize=21&nDuration=0&tCaption=CGL%20Preview

public class WhatADrag extends FrameWindowController {
   
   // starting position for image
   private static final Location IMAGE_LOCATION = new Location(200, 100);
     
   private VisibleImage image; // visible image to be dragged      
   private Location lastPoint; // point where mouse was last seen      
   private boolean grabbed = false; // whether the image has been dragged by the mouse
      
   // make the iamge
   public void begin() {
      image = new VisibleImage(getImage("croc.gif"), IMAGE_LOCATION, canvas);
   }
   
   // save starting point and whether point was in image
   public void onMousePress(Location point) {
      lastPoint = point;
      grabbed = image.contains(point);
   }
   
   // if mouse is in image, then drag the image
   public void onMouseDrag(Location point) {      
      if (grabbed) {
         double dx = point.getX() - lastPoint.getX();
         double dy = point.getY() - lastPoint.getY();
         image.move(dx, dy);
         lastPoint = point;
      }
   }   
}

******


Next we add a wall on the right. Want to stop if image runs into it. 

add    

COULD USE image.getWidth() INSTEAD:
   private static final int IMAGE_WIDTH = 70;
   
   private static final int WALL_X = 350;
   private static final int WALL_Y = 0;
   private static final int WALL_WIDTH = 20;
   private static final int WALL_HEIGHT = 400;

   in begin()
      new FilledRect(WALL_X, WALL_Y, WALL_WIDTH, WALL_HEIGHT, canvas);

   in onMouseDrag
         double dx = point.getX() - lastPoint.getX();
         double dy = point.getY() - lastPoint.getY();
         image.move(dx, dy);
         if (image.getX() + IMAGE_WIDTH > WALL_X) image.move(-dx, -dy);

Numeric comparison operators. ==, not =

Limits == for floats and strings. 
Use Math.abs(x - y) < epsilon
and string1.equals(string2)

If mouse is moved fast, stops short of wall.

Move it back just the right amount.

         double distancePastWall = image.getX() + IMAGE_WIDTH - WALL_X;
         // if image has collided with the wall, move it back
         if (distancePastWall > 0) image.move(-distancePastWall, -dy);


Strange behavior if moved fast past wall.
Ungrabb it if it hits the wall.

         if (distancePastWall > 0) {
            image.move(-distancePastWall, -dy);
            grabbed = false;
         }


Add OUCH! message.

   private Text ouch; // hurting text

in begin

      ouch = new Text("OUCH!", 0, 0, canvas); // initial position is ignored
      ouch.hide();

in onMousePress

      ouch.hide();

in onMouseDrag

         if (distancePastWall > 0) {
            image.move(-distancePastWall, -dy);
            grabbed = false;
            ouch.moveTo(image.getX(), image.getY() + OUCH_Y_OFFSET);
            ouch.show();
         }


Add a second wall.

Use boolean || operator and nested if.

         double distancePastRightWall = image.getX() + IMAGE_WIDTH - RIGHT_WALL_X;
         double distancePastLeftWall = image.getX() - (LEFT_WALL_X + LEFT_WALL_WIDTH);
         // if image has collided with the wall, move it back
         if (distancePastRightWall > 0 || distancePastLeftWall < 0) {
            if (distancePastRightWall > 0) image.move(-distancePastRightWall, -dy);
            else image.move(-distancePastLeftWall, -dy); // distancePastLeftWall < 0

****** WhatADragWalls.java
import objectdraw.*;
import java.awt.*;

// croc.gif resized from Microsoft Design Galary wmf image
// http://dgl.microsoft.com/mgo1en/preview.asp?bInSelBasket=false
//   &nMedia=1&tName=j0215228&tExtension=wmf&nSize=21&nDuration=0&tCaption=CGL%20Preview

public class WhatADragWalls extends FrameWindowController {
   
   private static final int IMAGE_LEFT = 200;
   private static final int IMAGE_TOP = 100;
   private static final int IMAGE_WIDTH = 70;
   
   private static final int LEFT_WALL_X = 0;
   private static final int LEFT_WALL_Y = 0;
   private static final int LEFT_WALL_WIDTH = 20;
   private static final int LEFT_WALL_HEIGHT = 400;   
   
   private static final int RIGHT_WALL_X = 350;
   private static final int RIGHT_WALL_Y = 0;
   private static final int RIGHT_WALL_WIDTH = 20;
   private static final int RIGHT_WALL_HEIGHT = 400;
   
   private static final int OUCH_Y_OFFSET = -20;
     
   private VisibleImage image; // visible image to be dragged 
   private Text ouch; // hurting text
   private Location lastPoint; // point where mouse was last seen   
   private boolean grabbed = false; // whether the image has been grabbed by the mouse   
   
   // make the image and walls
   public void begin() {
      new FilledRect(LEFT_WALL_X, LEFT_WALL_Y, LEFT_WALL_WIDTH, 
                            LEFT_WALL_HEIGHT, canvas);
      new FilledRect(RIGHT_WALL_X, RIGHT_WALL_Y, RIGHT_WALL_WIDTH, 
                            RIGHT_WALL_HEIGHT, canvas);
      image = new VisibleImage(getImage("croc.gif"), IMAGE_LEFT, IMAGE_TOP, canvas);
      ouch = new Text("OUCH!", 0, 0, canvas); // initial position is ignored
      ouch.hide();
   }
   
   // save starting point and whether point was in image
   public void onMousePress(Location point) {
      lastPoint = point;
      grabbed = image.contains(point);
      ouch.hide();
   }
   
   // if mouse is in image, then drag the image
   public void onMouseDrag(Location point) {      
      if (grabbed) { 
         double dx = point.getX() - lastPoint.getX();
         double dy = point.getY() - lastPoint.getY();
         image.move(dx, dy);
         double distancePastRightWall = image.getX() + IMAGE_WIDTH - RIGHT_WALL_X;
         double distancePastLeftWall = image.getX() - (LEFT_WALL_X + LEFT_WALL_WIDTH);
         // if image has collided with the wall, move it back
         if (distancePastRightWall > 0 || distancePastLeftWall < 0) {
            if (distancePastRightWall > 0) image.move(-distancePastRightWall, -dy);
            else image.move(-distancePastLeftWall, -dy); // distancePastLeftWall < 0
            grabbed = false;
            ouch.moveTo(image.getX(), image.getY() + OUCH_Y_OFFSET);
            ouch.show();
         }
         else lastPoint = point;
      }
   }   
}

******


If statement 

- Syntax.

- Test expression must have boolean type.

- Then and else statements are often blocks of statements.

- Flow charts with and w/o else clause.

- Indent then and else parts.

- Exception: else if doesn't introduce new nesting level.

- Flow chart of else if.


There is also a conditional operator

- example: min of x and y: x < Y ? x : y same as Math.min(x,y) 


SWITCH STATEMENT

Use only the following restricted form:

switch (EXPRESSION) {
   case CONSTANT_EXPRESSION : STATEMENT ... break;
   ...
   default : STATEMENT ...
} 

- Can only switch on ints or types that can be widened to int (e.g. chars). 

- End every clause (except maybe the last: a DEFAULT clause) with a break statement.

- End every switch statement with a default clause.

- It is easy to forget the break statements. On the other hand, switch is
faster for many-way branches.


BOOLEAN OPERATORS

Boolean expressions comparison operators: ==, !=, <, >, <=, >=

- logical operators: !, &&, and || (in decreasing precedence order)

- && and || operators associate to the left and don't evaluate
  right-hand arguments if result is known.

- usually used in control constructs, but can appear as arguments, return
  values (boolean valued methods), etc.

Common mistake, things like: i < j < k
Instead use: i < j && j < k


|| (or) is inclusive: it means one or the other or both.


Be careful about the difference between == and =.

> boolean a = false;
> a == true
false
> a = true
true
> a == true
true

& and | are bitwise operations on integer values as well as strict
boolean operator.

XOR is the name of the exclusive-or operation: one or the other but not
both.

EXERCISE: write boolean xor(boolean a, boolean b) [same as a ^ b]

~~~~ Text omitted from pre-lecture notes ~~~~


EXERCISE: write boolean inOrder(double x, double y, double z), where the
order may be ascending or descending.

~~~~ Text omitted from pre-lecture notes ~~~~