Class misc.ScrollableCanvas

public class ScrollableCanvas extends Panel {}

This is the main class that adds Scrollbars to the Canvas Mycanvas which is defined in the same file. The design of this class was inspired by an example in Sun's Java Tutorial.

Let me first define Myclass.

class Mycanvas extends Canvas {}

Field Index:

int tx
The x-offset value, used while repainting the Canvas.
int ty
The y-offset value, also used while repainting the Canvas.
int text_width
This keeps track of the text-width.
int text_height
This keeps track of the height of the text.
Dimension D
This sets the dimension of Mycanvas.
int numans
This has the current number of answers.
answer ans[]
This is the array that holds the answers( i.e., various trials. )

Constructor Index:

Mycanvas( int width, int height )

Method Index:

Dimension minimumSize()
This returns the minimum Size which is D.
Dimension preferredSize()
This returns the same as above.
void newanswer()
This increments numans by one and allocates a new answer in the array ans[].
void newline( entity ent )
This adds a line( the entity ent ) to the current answer by calling the algebra.answer.add_line() method.
void clear()
This clears the array ans[] by setting each member in the array to 'null'. This is the method
that is called when the "clear button" is pressed.
void paint( Graphics g )
• This loops through the answer array ans[].
• For each answer in the array, it again loops through the lines ( which are entitys, sentence or table ) and instantiates each entity as a Drawable( the interface that defines the draw() method and which is implemented by each entity. )
• It then call the draw() method of each entity by passing its Graphics object by reference.
• Updates the text_width, if necessary, using the integer object W which holds the width of the entity and which is also passed by reference. The y-coordinate( which is also passed by reference through the integer object Y ) is also incremented.
• At the end of each answer in the loop ans[] the text_height is incremented.
void paint( entity ent )
This is the first method the ScrollableCanvas calls after it gets an entity to be painted.
This calls the newline() method described above and repaints Mycanvas.

Here's how misc.ScrollableCanvas is defined..

This class has Mycanvas and adds the Scrollbars.

Field Index:

Scrollbar vert
This is the Vertical Scrollbar.
Scrollbar horz
This is the Horizontal Scrollbar.
Mycanvas canvas
This is the Canvas.

Constructor Index:

ScrollableCanvas( int w, int h )
This creates a Mycanvas with width w and height h. Creates the two scrollbars and positions
them using the Layout Manager BorderLayout.

Method Index:

void paint( entity ent )
This is the methods that gets called first from the application applet( which are individual
exercises that use these packages. This method just calls the paint() method of canvas(
which is of type Mycanvas described before. )
void newanswer()
Again, this calls the newanswer() method of canvas.
void clear()
When the "clear Button" is pressed, this method is called. This, in turn, calls the clear()
method of canvas.
boolean handleEvent( Event evt )
This handles the events like SCROLL_LINE_UP, SCROLL_LINE_DOWN,
SCROLL_PAGE_UP, SCROLL_PAGE_DOWN and
SCROLL_ABSOLUTE.
It sets the offset values of the canvas( tx and ty ) and resize the scrollbars using the
methods resizeHorz() and resizeVert() described below.
void resizeHorz()
This method first compares the sum of canvas's width and the current x-offset value tx with
the current text_width of the canvas. If the sum is greater than the textwidth, it updates the
x-offset value tx.
void resizeVert()
This does the same using text_height and ty.
void paint( Graphics g )
This just resizes the vertical and horizontal scrollbars.

Summary of the implementation of misc.ScrollableCanvas:

• From the method in the application applet( individual programs for each programming exercise ) where all the computations are performed, the paint() method of the misc.ScrollableCanvas is called when a line( an entity, sentence or table ) is complete.
• The paint( entity ent ) method of misc.ScrollableCanvas in turn calls the paint( entity ent ) method of Mycanvas and the rest is as described above.

Source.

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