Classes and Objects

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Classes and Objects

• A class is a collection of data and methods that
  operate on that data.
• A class models some real world object.
• Example Circle - how would you define it?

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Circle class
class Circle double x,y //coordinates of
center double r //radius public double
circumference()return 23.14159r public
double area()return 3.14159rr
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Objects are instances of a class
Circle c c new Circle() c.x2.0 c.y2.0
c.r1.0 double a a c.area()
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Constructor
public Circle(double x, double y, double
r) this.xx this.yy this.rr Circle c
new Circle(1.414, -1.0, .25)
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Multiple Constructors
Public Circle(double r)x0.0 y0.0
this.rr //special use of this to call
constructor public Circle(double
r)this(0.0,0.0,r)
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Class variable

• Use static keyword
• static int num_circles 0
• num_circles //in the constructor

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Accessing class variables

• System.out.println(Number of circles created
  Circle.num_circles)

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class methods

• Declared with the static keyword
• it is not passed a this reference
• it is invoked using the class name instead of an
  object

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Class method example
Public boolean isInside(double a, double
b) double dx a-x double dy b-y double
dist Math.sqrt(dxdx dydy) if(dist lt r)
return true return false