Graphics Tools and Parameters Chris Nevison Barbara Wells

Two non-displayable classes Color –specifies a color –from java.awt library Location –specifies a location in two dimensions –part of objectdraw library

class Color constants –Color.white, Color.blue, Color.yellow, etc constructor: –takes red, green, blue parameters Color skyBlue = new Color(200, 200, 255); often used to set color of another object elevator.setColor(Color.blue); elevator.setColor(new Color(100, 150, 50));

class Location represents a position in 2 dimensional space –uses real value coordinates (double) has accessor methods to return coordinates –getX(), getY() has modifier or mutator methods to change loc.translate(dx, dy); constructor sets initial position Location loc = new Location(dx, dy);

Formal Parameters Specified in the definition of the method Represent information passed to the method when it is called Can be used to determine the behavior of the method or in calculations carried out by the method.

Formal Parameter Example public void onMouseClick(Location point) { elevator.moveTo(point); } Formal ParameterType of parameter Using the formal parameter Declaration of method

Actual Parameters the values given as parameters when a method is called –may be a literal value (number, string literal) –may be a variable in the current context –may be an expression –expression may be a newly created object new...

Actual Parameters Expression used for actual parameter must match the type specified in the declaration of the method Example: FilledRect documentation specifies that the moveTo method has a parameter of type Location –call to moveTo for an instance of FilledRect must supply an actual parameter of that type

Actual Parameters elevator.moveTo(point); instance of FilledRect call to method actual parameter

Actual Parameters elevator.moveTo(new Location(30,60)); instance of FilledRect call to method actual parameter, a newly created Location with no name

Communicating among methods using parameters objective: draw a line from location where mouse is pressed to location where mouse is released use WindowController methods –onMousePress –onMouseRelease –each has a formal parameter Location point create a Line with constructor that takes two Location parameters for the two ends

LineDraw, first attempt public class LineDraw1 extends WindowController { public void onMousePress(Location point) { Location startLine = point; } public void onMouseRelease(Location point) { Location endLine = point; new Line(startLine, endLine, canvas); }

Compile Error C:\apwork\Workshops\summer02\ObjDrawCode\LineDraw\Line Draw1.java:15: cannot resolve symbol symbol : variable startLine location: class LineDraw1 new Line(startLine, endLine, canvas); ^ 1 error error type offending word location of error

LineDraw, first attempt public class LineDraw1 extends WindowController { public void onMousePress(Location point) { Location startLine = point; } public void onMouseRelease(Location point) { Location endLine = point; new Line(startLine, endLine, canvas); } local variables parameters local to other method, not visible here

Scope variables declared within a method are only visible within that method formal parameters for a method are only visible within that method instance variables are visible within the class –an instance variable provides a means of communicating between methods within the same class

LineDraw, correct public class LineDraw2 extends WindowController{ Location startLine; public void onMousePress(Location point) { startLine = point;// assigns to instance variable } public void onMouseRelease(Location point) { Location endLine = point; new Line(startLine, endLine, canvas); } instance variable assign to the instance variable use the instance variable

Example: Scribble from Java, An Eventful Approach Uses these WindowController methods –onMousePress –onMouseDrag onMouseDrag is called repeatedly while the mouse is dragged

Scribble, attempt 1 public class Scribble1 extends WindowController{ Location startLine; public void onMousePress(Location point) { startLine = point; } public void onMouseDrag(Location point) { Location endLine = point; new Line(startLine, endLine, canvas); }

Scribble, attempt 1 result

Scribble, correct public class Scribble2 extends WindowController{ Location previousPoint; Location currentPoint; public void onMousePress(Location point) { previousPoint = point; } public void onMouseDrag(Location point) { currentPoint = point; new Line(previousPoint, currentPoint, canvas); previousPoint = currentPoint; }

State of an object Instance variables specify the attributes of an object State of an object is the set of attributes that determine its current configuration, that may change In Scribble class, state is given by –previousPoint –currentPoint Modifier or Mutator methods change state –affect the future behavior of the object