CSC 212 – Data Structures Lecture 2: Primitives, References, & Classes

Announcements Still need a note-taker; please see me if you are interested in the $50 If you need more review of Java, talk to me and/or seek out additional resources

Tracing Code Vital skill for writing and debugging Still my favorite way to debug  Quickest way to find common, simple bugs Good way to understand how code works Execute line-by-line just like the computer  Update variables’ value  Record any output  Need to work slowly and methodically

Data Types Java has 7+1 primitive data types  boolean, char, int, long, float, double, String * Only types that work with Java operators  E.g., +, -, %, &&, ||, |, >=, <, … Computers also use these types natively  Simplifies how they work and are used

Primitive Types Primitive variables are simple to use Each variable is “box” holding its value  Assignments copy values  Updates only to the local variable But primitives of limited usefulness  Finite range of possible values  Each variable has 1 value at a time

Primitives Example int x = 5; int sum = 0; for (int i=0; i < x; i++) { sum = sum + x; }

Classes In real world, must describe more than just primitives Java classes define these additional types Classes begin with: public class ClassNameGoesHere {  Usually start with capital letter  Usually use interior capitals to highlight words ClassNameGoesHere is used as the type

Using Classes Normally use instances of a class Class instantiated using new command public class Kitty {... } Kitty kat = new Kitty(...); Each instantiated object is unique: Kitty cat = new Kitty(...); Kitty tiger = new Kitty(...); cat = new Kitty(...);

Reference Variables Variables of class type called references Must be assigned to instance before using  Special value, null, marks that reference is not currently assigned to any instance References similar to a remote control  Do not equal object, but refer to object TV kat = new TV(...); kat

Using References Assignments alias references  Makes variables refer to same instance  new is only way to create an instance Kitty cat, tiger; cat = new Kitty(...); tiger = new Kitty(...); tiger = cat; cat tiger

Aliasing Aliased variables refer to same instance  Can make change using either variable  Both variables will see changes that are made  Assignments only to one reference, however Kitty cat, tiger, kat; cat = new Kitty(...); tiger = new Kitty(...); kat = tiger tiger = cat; cat = kat; cat tiger kat

Arrays Arrays are special object type  Array variables are references in own right  Arrays must refer to instance before use int[] bob = new int[30]; Each entry in array is like its own variable  bob’s entries are primitive variables  Following array’s entries are references Car[] parkingLot = new Car[300];  Can’t use entry in Car until it refers to instance

Tracing Example int max = 0; Car[] parkingLot = new Car[4]; for (int i = 0; i < 2; i++) { parkingLot[i] = new Car(...); } Could we use:  parkingLot[0]?  parkingLot[3]?  parkingLot[4]?

Before Next Lecture… Review basic Java syntax and loops  Will not go over this in class (sorry!)  Book contains a good review of this Look over week #1 homework and see if you have questions Friday’s lecture will cover constructors, fields, & methods  May want to review any old notes & handouts