Lecture 17: More Arrays

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Lecture 17 More Arrays
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More on Casting

• Earlier in the semester, we learned how to
  convert doubles to ints
• This will make number equal to 4.
• This works for types other than int and double,
  as well

double d 4.5678 int number (int)d
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More on Casting

• On a JComboBox, the addItem() method is defined
  like this
• And getSelectedItem is defined like this
• This is flexible, because we can put anything
  into the JComboBoxs menu
• The trouble is we can only get an Object out

public void addItem(Object item)
public Object getSelectedItem()
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More on Casting

• In the case of JComboBox, we know the type of the
  objects we put in via addItem()
• Usually its a String
• Perhaps a Font
• You could do this for your TextPlay assignment
• We can then cast them back to what they were when
  we get them out
• We can now treat item like its a String, and
  were not restricted to only the methods of
  Object.

fontChoice.addItem(Courier New)
String item (String)fontChoice.getSelectedIt
em()
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More on Casting

• You can do the same thing with Font objects
• Adding
• Retrieving
• This will work for any type, so long as you get
  the type right
• If you cast something and its not actually that
  type, Java will give you a ClassCastException.

Font insertedFont . . .
fontChoice.addItem(insertedFont)
Font font (Font)fontChoice.getSelectedItem()

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Simple Drawing Program

• Weve got a really simple drawing program
• Can make new shapes
• Can select what shape and what color via
  JComboBoxes
• One new feature
• We can select and drag any shape on the canvas

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Simple Drawing Program

• To select and drag any shape, we need to be able
  to remember all the objects
• We need to call contains() on all of them to see
  which one the mouse was in
• Well store each object in an array when we draw
  it
• When we need an object again, we can go through
  the array and find it there.

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Arrays Finite size

• Arrays have to be constructed with a size, so
  when we build our program, we have to specify a
  maximum number of objects to create
• Well make this the size of our array

public class Drawing extends WindowController
// max number of objects to be displayed
private static final int MAX_OBJECTS 20
. . .
public class Drawing extends WindowController
// max number of objects to be displayed
private DrawableInterface geomObject
new DrawableInterfaceMAX_OBJECTS . .
.
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Arrays of Interface types

• Notice that we declared our array as an array of
  DrawableInterfaces
• Recall that this is an interface for all the
  different graphical objects in ObjectDraw.
• We can thus store any kind of shape in this array
• FilledRects, FilledOvals, VisibleImages, etc.
• Like any array of Objects, everything in this
  array starts out as null
• Well need to initialize the elements before we
  use them

public class Drawing extends WindowController
// max number of objects to be displayed
private DrawableInterface geomObject
new DrawableInterfaceMAX_OBJECTS . .
.
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Keeping track of elements

• Well need to keep track of how many elements
  there are in the array
• Well fill the array from the 0th slot up to the
  end
• If we keep track of this number, well know that
  exactly that many elements in the array are
  non-null
• Well increment this number whenever we add a new
  shape

public class Drawing extends WindowController
// max number of objects to be displayed
private DrawableInterface geomObject
new DrawableInterfaceMAX_OBJECTS
// number of objects added to the array
private int numObjects 0 . . .
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onMousePress

• In onMousePress, we just check to see what mode
  were in
• If the JComboBox says were adding, well add
• If the JComboBox says were selecting, well
  select
• addNew() and selectOld() are private methods to
  help us out
• Making private methods for these very different
  tasks simplifies things in onMousePress and makes
  it more readable.

// prepare to to drag it around the screen.
public void onMousePress(Location point)
selected null // indicate nothing currently
selected if(selectOrAdd.getSelectedItem().equa
ls("Add new item")) addNew(point)
else selectOld(point)
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addNew()

• How this works
• Make sure there is more space in the array
• Figure out which color the user chose
• Figure out which shape the user chose
• Create a shape and add it at the end of the array
• Set its color
• Center it
• Increment the number of elements in the array by 1

// Add new geometric shape where clicked. Type
and color of // object is determined by the
settings of the color and // shape choice
buttons. private void addNew(Location point)
if (numObjects lt MAX_OBJECTS) // only add if
still room // local variable - color of
object Color objectColor Object
colorString colorChoice.getSelectedItem()
if (colorString.equals("Red"))
objectColor Color.red else if
(colorString.equals("Green"))
objectColor Color.green else
objectColor Color.blue //
string from shapeChoice selection Object
shapeString shapeChoice.getSelectedItem()
// create new object to be shape chosen
if (shapeString.equals("Square"))
geomObjectnumObjects new
FilledRect(point, 100, 100, canvas)
else geomObjectnumObjects
new FilledOval(point, 100, 100, canvas)
geomObjectnumObjects.setColor(obj
ectColor) geomObjectnumObjects.move(-50,
-50) // of object numObjects
numObjects 1
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addNew()
0
MAX_OBJECTS - 1
1
2
3
4
. . .
numObjects is 3

• Suppose there are 3 elements in the array, and we
  add a red square.
• We check to see if theres room
• Then, if there is, we add it at the end of our
  shapes and increment numObjects.

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addNew()
0
MAX_OBJECTS - 1
1
2
3
4
. . .
numObjects is now 4

• Suppose there are 3 elements in the array, and we
  add a red square.
• We check to see if theres room
• Then, if there is, we add it at the end of our
  shapes and increment numObjects

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selectOld()

• This routine finds the object we clicked on in
  the array
• This is called searching when we go through an
  array or list to get a particular element
• We test each shape to see if it containst the
  point where the user clicked
• If it does, we know the user clicked on it
• We then send that shape to the front

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selectOld()
private void selectOld(Coords point)
// see if point is in an object for (int
objNum 0 objNum lt numObjects objNum)
if (geomObjectobjNum.contains(point))
// remember top-most object
w/point selected
geomObjectobjNum
if (selected ! null) // if point is in one
of the objects lastPoint point
selected.sendToFront()

• The first for loop searches and tests each
  element by calling contains() on it
• If the element is found, then
• store it in the selected instance variable
• We also set lastPoint, in preparation for
  dragging the object
• selected remains null if the user didnt click
  anything.

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Were done, right?

• Not quite. The program doesnt do quite what a
  normal user might expect.
• Youll notice that the shapes are initially
  layered in the order they were created
• Some are on top of others
• When we click, we really want to pick the topmost
  shape under the mouse, but our program doesnt do
  that
• It picks the shape under the mouse that was last
  created, and sends it to the front
• Most users expect that when they click, theyre
  clicking on what they see

Clicking here could select the blue square!
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Layering

• Solution here is to store objects in order of
  their layering
• Whenever we send something to the front, we need
  to also move it to the end of our array.
• If we send the blue square to the front, we move
  it to the end, and shift all subsequent elements
  left

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New version of selectOld()
private void selectOld(Coords point)
// see if point is in an object int
selectIndex 0 for (int objNum 0
objNum lt numObjects objNum) if
(geomObjectobjNum.contains(point))
// remember top-most object w/point
selected geomObjectobjNum
selectIndex objNum
if (selected ! null) // if point is
in one of the objects lastPoint
point selected.sendToFront()
for (int objNum selectIndex objNum lt
numObjects-1 objNum)
geomObjectobjNum geomObjectobjNum1
geomObjectnumObjects-1
selected

• We now store the index of the frontmost item the
  user clicked on
• We also insert a loop to do the shifting of
  elements

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New version of selectOld()

• Take a look at that new loop
• What if I wrote it like this
• This is broken. Why? What happens to our array?

for (int objNum selectIndex objNum lt
numObjects-1 objNum)
geomObjectobjNum geomObjectobjNum1
geomObjectnumObjects-1 selected
for (int objNum numObjects objNum gt
selectIndex objNum--)
geomObjectobjNum-1 geomObjectobjNum
geomObjectnumObjects-1 selected
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Searching

• We used a loop to search for an object in an
  array
• This loop finds the last object in the array that
  contains the point
• Stores the object in selected
• Stores its index in selectIndex
• This code is not as clean as it could be!

int selectIndex 0 for (int objNum 0
objNum lt numObjects objNum) if
(geomObjectobjNum.contains(point))
// remember top-most object w/point
selected geomObjectobjNum
selectIndex objNum
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Searching
int selectIndex 0 for (int objNum 0
objNum lt numObjects objNum) if
(geomObjectobjNum.contains(point))
// remember top-most object w/point
selected geomObjectobjNum
selectIndex objNum

• We could make this better by starting at the
  right end of the list and finding the first
  object that contains the point
• Wouldnt have to go through the whole list that
  way
• We also dont really need that extra variable for
  selectIndex
• Could use the loop variable, objNum, for the same
  thing

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Better Search
int objNum numObjects - 1 while
(objNum gt 0 !geomObjectobjNum.contains(point
)) objNum-- if (objNum gt 0)
//if we found something
selected geomObjectobjNum //set
selected

• This loop will stop once it finds the topmost
  object that contains the point
• Doesnt have to go through all the objects all
  the time
• objNum is positive at the end if we found
  something
• Its the index of the object to select!
• Its -1 if we went through all the objects and
  didnt find anything
• We set selected only if objNum is nonnegative