CSE115: Introduction to Computer Science I

1. CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall 645-4739 alphonce@buffalo.edu

2. Phones off Signs out

3. Announcements • Exam 2 – 2 weeks away • covers material from exam 1 up to & including 10/21 • review on Monday 10/24 • exam on Wednesday 10/26

4. Agenda • null • this • interfaces

5. ‘null’ • ‘null’ denotes the null reference, a reference which does not refer to any object. • We can use ‘null’ to solve the two dogs, one collar problem (see code on next slide):

6. removeCollar rather than getCollar public class Dog { private Collar _collar; public Dog(Collar collar) { this._collar= collar; } public void setCollar(Collarcollar) { this._collar= collar; } public Collar removeCollar() { Collar temp =this._collar; this._collar= null; return temp; } }

7. Can also use in constructor public class Dog { private Collar _collar; public Dog() { _collar = null; } . . . } Now a Dog can be created without a Collar

8. Consider this code(assume association via constructor) fido rover 3500 4000 Dog fido = new Dog(new Collar()); Dog rover = new Dog(new Collar()); fido – 3500 rover – 4000 fido’s _collar – 3600 rover’s _collar – 4100 two collars are at 4850 and 4925 • 4850 _collar • _collar 3500 3600 4000 4100 • 4925 4850 4925

9. Consider this code(assume association via constructor) fido rover 3500 4000 temp Dog fido = new Dog(new Collar()); Dog rover = new Dog(new Collar()); fido.setCollar(rover.removeCollar()); public Collar removeCollar() { Collar temp = _collar; _collar = null; return temp; } • 4850 _collar • _collar 3500 3600 4000 4100 • 4925 4850 4925

10. Consider this code(assume association via constructor) fido rover 3500 4000 temp Dog fido = new Dog(new Collar()); Dog rover = new Dog(new Collar()); fido.setCollar(rover.removeCollar()); public Collar removeCollar() { Collar temp = _collar; _collar = null; return temp; } • 4850 _collar • _collar 3500 3600 4000 4100 What happens here? Which _collar are we referring to here? • 4925 4850 4925

11. thisthe object on which a method is invoked fido rover this 3500 4000 temp fido.setCollar(rover.removeCollar()); public Collar removeCollar() { Collar temp = _collar; _collar = null; return temp; } • 4000 • 4925 • 4850 _collar • _collar 3500 3600 4000 4100 What happens here? Which _collar are we referring to here? • 4925 4850 4925

12. thisimplicitly in code fido rover this 3500 4000 temp fido.setCollar(rover.removeCollar()); public Collar removeCollar() { Collar temp = this._collar; this._collar = null; return temp; } • 4000 • 4925 • 4850 _collar • _collar 3500 3600 4000 4100 What happens here? Which _collar are we referring to here? • 4925 4850 4925

13. this fido rover this 3500 4000 temp rover.setCollar(fido.removeCollar()); public Collar removeCollar() { Collar temp = this._collar; this._collar = null; return temp; } • 3500 • 4850 • 4850 _collar • _collar 3500 3600 4000 4100 What happens here? Which _collar are we referring to here? • 4925 4850 4925