Session 23
Chapter 11: More on the
Implications of Inheritance
Stack-based Memory
Main:
ObjA a = new ObjA();
ObjB b = new ObjB();
a.do(5, b)
public class ObjA {
int x = 100;
public void do (int y, ObjB myB) {
int loc = 6;
int t = myB.doMore(loc);
...
}
public class ObjB {
int z = 30;
public int doMore(int i) {
z = z + i;
return z;
}
}
}
• 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
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;
}
}
// **
Assignment of Objects
• semantics for assignment simply copies
the pointer into the heap to the object
public class Box {
pubic class BoxTest {
public static void main (String [] args) {
private int value;
public Box() { value = 0; }
Box x = new Box();
public void setValue (int v) {
x.setValue ( 7 );
Box y = x;
value = v;
}
y.setValue( 11 );
public int getValue() {
System.out.println( “x’s value = “ + x.getValue());
System.out.println(“y’s value = “ + y.getValue());
return value;
} // end main
}
}
}
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()
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
}
}
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();
y.setValue( 11 );
// assigns y a copy of y
System.out.println( “x’s value = “ + x.getValue());
System.out.println(“y’s value = “ + y.getValue());
} // end main
}
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.
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;
}
}