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Session 23

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

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Session 23

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  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 > 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 anyother 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; } }

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