Session 23Stack-based MemoryConsider Factorial ExampleAssignment of ObjectsCloneable InterfaceCloneable Box ClassUsing the Cloneable Box ClassEquality TestEquality Test ExampleSession 23Chapter 11: More on the Implications of InheritanceStack-based Memory•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 poppedMain: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; }}Consider Factorial Exampleclass 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 { private int value; public Box() { value = 0; } public void setValue (int v) { value = v; } public int getValue() { return value; }}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}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 Classpublic 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 Classpubic 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}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 Exampleclass 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;