1. Lecture 12: Using Classes Yoni Fridman 7/19/01 7/19/01

2. OutlineOutline ä How objects are stored  The null keyword ä Assigning objects ä Garbage ä Examples ä How objects are stored  The null keyword ä Assigning objects ä Garbage ä Examples

3. How Objects Are Stored  Objects are stored differently from int ’s, double ’s, and boolean ’s  An int variable, for example, stores a value. ä An object variable, on the other hand, stores a reference, not the object itself.  Objects are stored differently from int ’s, double ’s, and boolean ’s  An int variable, for example, stores a value. ä An object variable, on the other hand, stores a reference, not the object itself. int number = 10; number 10 Telephone myPhone = new Telephone(); myPhone on color volume number on color volume number false “Black” 5 942... Telephone object vs.

4. The null Keyword  If we want to declare an object variable, but not have it point to an object yet, we can use the null keyword. ä Now the object variable myPhone exists, but we can’t do anything with it until it points to an object.  If we want to declare an object variable, but not have it point to an object yet, we can use the null keyword. ä Now the object variable myPhone exists, but we can’t do anything with it until it points to an object. Telephone myPhone = null; myPhone

5. Assigning Objects  Since objects are stored differently from int ’s (or double ’s or boolean ’s), they also act differently.  Suppose we assign an int i to an int j :  Now, let’s change the value of i :  Since objects are stored differently from int ’s (or double ’s or boolean ’s), they also act differently.  Suppose we assign an int i to an int j :  Now, let’s change the value of i : int i = 10; int j = i; int i = 10; int j = i; i j i j 10 i = 20; i j i j 20 10

6. Assigning Objects  This time, suppose we assign an object myPhone to an object otherPhone :  Now, what happens if we call the method myPhone.turnOn() ?  This time, suppose we assign an object myPhone to an object otherPhone :  Now, what happens if we call the method myPhone.turnOn() ? Telephone myPhone = new Telephone(); Telephone otherPhone = myPhone; Telephone myPhone = new Telephone(); Telephone otherPhone = myPhone; myPhone otherPhone myPhone otherPhone on … on … false Telephone object

7. GarbageGarbage ä In this example, the first Tele- phone object is now garbage – it can never be accessed again. ä This can lead to memory leaks. ä To avoid memory leaks, Java performs garbage collection while your program is running. ä In this example, the first Tele- phone object is now garbage – it can never be accessed again. ä This can lead to memory leaks. ä To avoid memory leaks, Java performs garbage collection while your program is running. Telephone myPhone = new Telephone(); Telephone otherPhone = new Telephone(); myPhone = otherPhone; Telephone myPhone = new Telephone(); Telephone otherPhone = new Telephone(); myPhone = otherPhone; myPhone otherPhone myPhone otherPhone on … on … false on … on … false

8. HomeworkHomework ä NONE!!! (The reading in the textbook on input and text files bites.)