Previously:
How to make a (true) copy of an object
- Steps used to
make a copy of
an
object:
public static void main(String[] args)
{
Circle circle1 = new Circle(4);
// Make a COPY of circle1
Circle circle2 = new Circle(); // (1) Create a new Circle obj
circle2.radius = circle1.radius; // (2) Copy the properties over
}
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Unfortunately...
this
only
works
when we
access
the
member variable(s)...
What is the problem ?
This solution
will
only work when
all instance variables in the
class are
public:
public class Circle
{
public double radius = 1; /** The radius of this circle */
public Circle() { } /** Constructor 1 for a circle object */
public Circle(double newRadius) /** Constructor 2 for a circle object */
{
radius = newRadius;
}
public double getArea() /** Return the area of this circle */
{
return 3.14159 * radius * radius;
}
public void setRadius(double newRadius) /** Set new radius for this circle */
{
radius = newRadius;
}
}
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What is the problem ?
Instance variables in
classes are
often
private because
we want
data field
encapsulation
(discussed
later):
public class Circle
{
private double radius = 1; /** The radius of this circle */
public Circle() { } /** Constructor 1 for a circle object */
public Circle(double newRadius) /** Constructor 2 for a circle object */
{
radius = newRadius;
}
public double getArea() /** Return the area of this circle */
{
return 3.14159 * radius * radius;
}
public void setRadius(double newRadius) /** Set new radius for this circle */
{
radius = newRadius;
}
}
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DEMO:
demo/03-classes/11-copy
---- compile error...
What is the
solution ???
Solution:
define a
"copy" constructor in the class
The "copy" constructor is
a constructor that has
a object variable of the
classname as
parameter:
public class Circle
{
private double radius = 1; /** The radius of this circle */
public Circle() { } /** Constructor 1 for a circle object */
public Circle(double newRadius) /** Constructor 2 for a circle object */
{
radius = newRadius;
}
public Circle(Circle c) /** Constructor 3: copies Circle c */
{
radius = ????????; // What should we do here ?
}
public double getArea() /** Return the area of this circle */
..
public void setRadius(double newRadius) /** Set new radius for this circle */
..
}
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The "copy" constructor will
copy all
the instance variables from its
input parameter to the
new object... how ??
Solution:
define a
"copy" constructor in the class
To copy the
instance variables, use
assignment:
public class Circle
{
private double radius = 1; /** The radius of this circle */
public Circle() { } /** Constructor 1 for a circle object */
public Circle(double newRadius) /** Constructor 2 for a circle object */
{
radius = newRadius;
}
public Circle(Circle c) /** Constructor 3: copies circle c */
{
radius = c.radius; // Copies c.radius to radius of new Circle object
}
public double getArea() /** Return the area of this circle */
..
public void setRadius(double newRadius) /** Set new radius for this circle */
..
}
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DEMO:
demo/03-classes/11-copy
How to use the copy constructor
Before: (fails when radius is private)
public static void main(String[] args)
{
Circle circle1 = new Circle(4);
// Make a copy of circle1
Circle circle2 = new Circle();
circle2.radius = circle1.radius; // Fail due to private access
}
Using "copy" constructor:
public static void main(String[] args)
{
Circle circle1 = new Circle(4);
Circle circle2;
circle2 = new Circle(circle1); // Use "copy" constructor to make Circle
System.out.println( circle2.getArea() );
circle2.setRadius(10); // Proof that we made a copy
System.out.println( circle1.getArea() ); // Unchanged
System.out.println( circle2.getArea() );
}
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DEMO:
demo/03-classes/11-copy
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