CS51 - Spring 2010 - Lecture 17

  • exercise 14.2.2
       - provide a declaration for diamond, an array of four Lines needed to make the drawing. In your declaration, include an array construction.
       - write the four assignment statements needed to create the diamond drawing and to associate these with the appropriate array entries. Assume the diamond has to just fit within a 200 by 200 pixel window.
       - add additional lines to change the top two lines of the drawing red.

  • Write a function that given an array of ints, returns the largest number in the array
       public int largestNumber(int[] numbers){
          int largest = numbers[0];

          for( int number: numbers ){
             if( number > largest ){
                largest = number;

  • show TextStat demo
       - String member method: split
          - public String[] split(String splitString);
       - We're going to write the four supporting methods to make this class
          - private String getLongestWord(String[] words)
          - private String getShortestWord(String[] words)
          - private int getNumberOfWords(String[] words)
          - private int getNumberOfCharacters(String[] words)

  • Show DrawingPanelWithShapeArray demo
       - Note how we can move things around and different objects are "in front" of other objects
       - Hard to get this behavior with recursion
       - We'll do this using arrays:
          - we'll construct an array with some maximum size, call it MAX_OBJECTS
             - private DrawableInterface[] shapes = new DrawableInterface[MAX_OBJECTS];
          - initially, the array will be empty
          - as we add things, add the objects to the array
          - how can we do this?
             - keep an integer (int numShapes) that tells us how many things are in the array so far   
       - How do we add a new item?
          - construct the shap
             - figure out the shape from the menu and create a new shape
             - figure out the color from the menu and set color
          - how do we add it to the shapes array?
             - remember, we're using numShapes to keep track of how many items are already in the array
             - set shapes[numShapes] to the new object
             - increment numShapes
       - how do we delete an item?
          - figure out which object we clicked on
          - remove it from the canvas
          - remove it from our shapes array
          - let's look at each of these in more detail
             - how do we find out which object we clicked on?
                // returns the index into shapes of the object that contains point
                // or -1 if none contain that point
                public int getIndexOf(Location point)
             - how do we deal with overlapping items, i.e. what if we click and two items contain the point?
                - where are we adding more recent items?
                   - to the end of the array
                - therefore, items towards the end of the array are more recent
                - look at getIndexOf method in DrawingPanelWithShapeArray code
                   - start from the back not the front
                   - return the first one we find
             - how do we remove something at a particular index from the shapes array?
                - starting at that index, shift everything down
                - set the last thing to null
                - decrement numShapes
                - look at removeEltWithIndex method
       - How do we implement move?
          - how do we normally move an object?
             - onMousePress
                - check if we clicked on the object
                - set a boolean variable that we're dragging
                - remember where the mouse was pressed
             - onMouseDrag
                - move the object the difference between where the mouse was previously and currently
                - remember where the mouse was pressed
          - how is this going to change?
             - onMousePress
                - iterate through all of the things in our array to see if any of them were clicked on
                - rather than a boolean, we'll use a variable of type DrawableInterface to keep track of whether or not something is selected
                   - if null, then nothing was clicked on
                   - if non-null, then the variable has the object that was clicked on
             - onMouseDrag
                - doesn't change
             - how does moving affect overlapping items?
                - the challenge is that when we move something, we bring it to the front
                - if we don't change our shapes array ordering, will it work properly?
                   - we're assuming things later on in the array are on top of those previous
                   - if we don't change the ordering, then this will not be true
                - so, what do we need to do when we move an item?
                   - we need to move it to the end of the array!
                   - how?
                      - delete it using removeEltWithIndex
                      - add it back on to the end of the array
                      - look at selectShapeAt method
       - How do we implement Recolor? (should be easy now)
          - getIndexOf(point)
          - recolor that objects

  • Other misc. array things
       - what happens here?
          int[] numbers = new array[10];
          int currentIndex = 0;
          currentIndex = 10;
          ** ArrayOutOfBounds exception
       - what happens here?
          FramedRect[] rects = new FramedRect[10];
          rect[0].move(10, 10);

          ** null pointer exception
          - we've just created an array of FramedRect variables
          - as like any other FramedRect variable, their initial value is null until you assign something to them