1. Chapter 11: More on the Implications of Inheritance
Session 23
Chapter 11: More on the Implications of Inheritance

2. Stack-based Memory Objects are stored on the heap
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 > 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( “x’s value = “ + x.getValue());
System.out.println(“y’s 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 Java’s 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( “x’s value = “ + x.getValue());
System.out.println(“y’s 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
return false; }