Chapter 11: More on the Implications of Inheritance

1
Session 23

• Chapter 11 More on the Implications of
  Inheritance

2
Stack-based Memory
public class ObjB int z 30 public int
doMore(int i) z z i return z

public class ObjA int x 100 public
void do (int y, ObjB myB) int loc 6
int t myB.doMore(loc) ...
Main ObjA a new ObjA() ObjB b new
ObjB() a.do(5, b)

• Objects are stored on the heap
• When a method is called, an activation record is
  allocated on the stack to hold
• return address (where to return after execution)
• parameters
• local variables (stuff declared in the method)
• When a method returns, the activation record is
  popped

3
Consider Factorial Example

• class FacTest
• static public void main (String args)
• int f factorial(3) //
• System.out.println(Factorial of 3 is f)
• static public int factorial (int n)
• int c n 1
• int r
• if (c gt 0)
• r n factorial(c) //
• else
• r 1
• return r

4
Assignment of Objects

• semantics for assignment simply copies the
  pointer into the heap to the object

pubic class BoxTest public static void main
(String args) Box x new Box()
x.setValue ( 7 ) Box y x
y.setValue( 11 ) System.out.println( xs
value x.getValue())
System.out.println(ys value
y.getValue()) // end main
public class Box private int value
public Box() value 0 public void
setValue (int v) value v
public int getValue() return value
5
Cloneable Interface

• Java has no general mechanism to copy arbitrary
  objects
• But, the base class Object provides a clone()
  method that creates a bitwise copy
• The Cloneable interface represents objects that
  can be cloned
• Several methods in Javas library use clone()

6
Cloneable Box Class

• public class Box implements Clonable
• private int value
• public Box() value 0
• public void setValue (int v) value v
• public int getValue() return value
• public Object clone()
• Box b new Box()
• b.setValue ( getValue())
• return b
• // end clone

7
Using the Cloneable Box Class

• pubic class BoxTest
• public static void main (String args)
• Box x new Box()
• x.setValue ( 7 )
• Box y (Box) x.clone() // assigns y a
  copy of y
• y.setValue( 11 )
• System.out.println( xs value
  x.getValue())
• System.out.println(ys value
  y.getValue())
• // end main

8
Equality Test

• tests for pointer equality, so really tests
  for object identity and not equality.
• equals() is a method inherited from class Object.
  The Java run-time system uses equals() in a
  number of places and expects to be able to test
  any Object for equality with any other Object.

9
Equality Test Example

• class Circle extends Shape
• int radius
• ...
• int getRadius() return radius
• ...
• public boolean equals (Object arg)
• if (arg instanceof Circle)
• Circle argc (Circle) arg
• if (radius argc.getRadius())
• return true
• // end if
• // end if
• return false