CS51 - Spring 2010 - Lecture 5
Mixing doubles and ints
- What is the type of?
- What do you think will happen if I do the following?
- private double d = 2;
- java automatically type casts (changes) the int 2 to a double
- private int i = 2.5;
- java will complain. Why?
- we can force it by manually type casting it to an int
- private int i = (int)2.5
- what do you think the value of i will be?
- 1 + 1.0
- Java will try to convert both to the other type
- if one doesn't result in an error, then it will do that one
- Math.round(double num)
- Math.sqrt(double num)
- Math.sin(double num)
- Math.abs(double num)
- What is the type and answer to?
- 2 + 2
- 2 + 2.0
- 50 * 2.0
what is the difference between a built-in data type (like int, double, boolean) and a class?
- built-in data types have operators
- +, -, *, ...
- classes have methods
- FramedRect rect = new FramedRect(...)
- classes represent objects and therefore have underlying information
- how we view variables of these types are slightly differently
- a class variable is a reference to an object
- a built-in data type variable actually holds that information directly
- for built in types use "=="
int num1 = 10;
int num2 = 10;
int num3 = 20;
- what would the the value of the following expressions?
- num1 == num2
- num1 == num3
- for classes, use the equals method
Location loc1 = new Location(10, 10);
Location loc2 = new Location(10, 10);
Location loc3 = new Location(20, 20);
- What would be the value of the following expressions?
- loc1 == loc2
- loc1 and loc2 are two different variables and are there for not '=='
- loc1 == loc3
- I've said everything in java has a type
- What is the type of "Score = "?
- We've seen that we can use the '+' operator with Strings
- "Score = " + score
- String s = "this is a ";
- s = s + "String";
- String is actually a class
- it's the only class in java that uses operators and literals
- "this is a String literal"
- similar to
except now we're dragging a shirt
- do have the added functionality that it resets the location when we enter the window
- how are we going to do this?
- design a Tshirt class
- size and location of the various objects
- instance variables
- body, neck, sleeves
- double x, y locations
- canvas (very often we'll need the canvas variable)
- moves everything
- need to check if the point is contained in any of the t-shirt parts
- again, we can leverage our move method
look at Tshirt class in
- note the contains methods checks the three different shirt parts
look at DragAShirt class in
- again, we can just create an instance variable of type Tshirt and then use it like we would other objects, because we have implemented that functionality
- what do we need to do to get two shirts?
- one of the biggest benefits of creating your own new classes is that we can reuse the functionality. Just need to created a second Tshirt object. If we'd done this all in one class, we'd have to rewrite a second set of variables, etc.
- we have added some functionality to the Tshirt class. We can just go in and add some new methods:
- added setColor method
- return type?
- void (it's a mutator method)
- how are we going to handle dragging two different shirts? Before we had just used a boolean to check if we're dragging it or not.
- could have a boolean for each shirt, i.e. draggingShirt1 and draggingShirt2
- the if-else logic gets complicated if you do it this way
- a better way is to have two Tshirt variables, selectedShirt and otherShirt and then one dragging variable whether or not we're dragging the "selected" shirt
- the major change is in the onMousePress method
- if it's the selected shirt, just set dragging to true
- if it's the other shirt, what do we need to do?
- swap selected and other
- set dragging to true
- we'll bring the selected on to the front, so that the one being dragged is always on top
- if it's neither, then dragging = false
- note though that this keeps our onMouseDrag method simple
- note that we can define our own methods (in this case reset) for the Tshirt class
- it's important not only to lay things out, but to think about how things will interact
- read the handout very carefully
- look at the Pole class and make sure you understand it
- let's walk through the basic design process
- how many classes will we have?
- Magnet class (representing the magnets), like our Tshirt class above
- MagnetGame class, that extends WindowController to handle the mouse events
- Pole class, to properly handle attracting and repelling (we give this to you)
- design of the Magnet class
- locations, sizes, distances
- variables: what makes up a magnet?
- outer rectangle
- two poles
- constructor: what do we need to know to create a new magnet?
public Magnet(Location upperLeft, DrawingCanvas canvas)
- design of the MagnetController class
- NO constructor: the class that extends the WindowController class does NOT have a constructor
- notice that you create a Pole as follows:
Pole myPole = new Pole (this, xLoc, yLoc, "N", canvas);
- what does "this" refer to?
- remember "this" refers to the current object
- if we're in the magnet class, then this will be a magnet
- what would the constructor to Pole look like (note, we've implemented the Pole class for you):
public Pole(Magnet magnet, double x, double y, String label, DrawingCanvas canvas)
- look at grading guidelines
- only 8/20 for functionality!
- design (will become increasingly more points)
- style/organization are half of the points