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CSE115: Introduction to Computer Science I. Dr. Carl Alphonce 219 Bell Hall 645-4739 alphonce@buffalo.edu. Announcements. I did not lecture from slides. I showed a demo of ‘green’ for UML class diagrams. I showed a demo of the lab 4 solution. We talked about ‘null’, the null reference.
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CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall 645-4739 alphonce@buffalo.edu
Announcements • I did not lecture from slides. • I showed a demo of ‘green’ for UML class diagrams. • I showed a demo of the lab 4 solution. • We talked about ‘null’, the null reference. • We ended with a puzzle.
Attendance • I checked attendance today: • 72% of registered students attended • 28% of registered students did not attend • Lecture and lab attendance is closely linked with success in the course: • students who do NOT attend regularly tend to fail • students who do attend regularly tend to pass (with good grades)
‘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):
removeCollar rather than getCollar public class Dog { private Collar _collar; public Dog(Collar collar) { _collar = collar; } public void setCollar(Collar collar) { _collar = collar; } public Collar removeCollar() { Collar temp = _collar; _collar = null; return temp; } }
Can also use in constructor public class Dog { private Collar _collar; public Dog() { _collar = null; } . . . } Now a Dog can be created without a Collar
Consider this code(assume association via constructor) fido rover 1025 825 Dog fido = new Dog(new Collar()); Dog rover = new Dog(new Collar()); 800 1000 _collar • _collar 800 825 1000 1025
Consider this code(assume association via constructor) fido rover temp 1025 825 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; } 800 1000 _collar • _collar 800 825 1000 1025
Consider this code(assume association via constructor) fido rover temp 1025 825 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; } 800 1000 _collar • _collar 800 825 1000 1025 What happens here? Which _collar are we referring to here?
