Pre-defined class JFrame. Object & Class an analogy

Transcription

1 CS1M Lecture 17 Mar 29, 25 1 Announcements: Project 4 due Sunda 4/3 at 6pm Use Keboard class or reading input Section in classrooms this week Previous Lecture: Selection statement Reading input using Keboard class Iteration with while loop Toda s Lecture: Object and class Creating objects and calling their methods Reading: JV: Sec 3.8, Sec 4.1, 4.2 March 29, 25 Lecture 17 1 March 29, 25 Lecture 17 2 Pre-deined class Deals with windows (rames) on the monitor All the predeined classes are collectivel called the Java API Classes are grouped into packages. E.g., java.io, java.net, java.swing Use the import statement: import java.swing.*; To ind out what the classes do, read the API speciications: Object & Class an analog Object: a older that stores inormation (data and instructions) Class: a drawer in a iling cabinet that holds olders o the same tpe March 29, 25 Lecture 17 3 March 29, 25 Lecture 17 4 What is in an object? (What is in a older?) Fields to store data Instructions or dealing with the object Fields, Instance variables Instance Methods What is in an object? (What is in a older?) Fields to store data Instructions or dealing with the object Reerence name (a unique ID o the older) Class name (Drawer name) March 29, 25 Lecture 17 6 March 29, 25 Lecture 17 7

2 CS1M Lecture 17 Mar 29, 25 2 Creating an object The epression new () Creates a object (older) and gives it a reerence name Calls method () to set initial values or the object Yields the reerence o the object Reerence variable Use a reerence variable to hold on to an object: = new (); Use the class name as a tpe March 29, 25 Lecture 17 8 March 29, 25 Lecture 17 9 ; = new () = new (); = new ();

3 CS1M Lecture 17 Mar 29, 25 3 Object & Class Object: contains variables (ields, instance variables) and methods Variables: state or characteristics e.g., name, age Methods: behavior or action e.g., ell, bounce Calling instance methods = new ();.;.setsize(6,2); int w =.getwidth(); Class: blueprint (deinition) o an object No memor space is reserved or object data An object is an instance o a class Snta : reerencevariablename. methodname (arguments ) March 29, 25 Lecture Accessing a ield Snta: reerencevariablename. ieldname import java.swing.*; public class MakeFrame { public static void main(string[] args) { = new ();.;.setsize(6,2); int w=.getwidth(); Sstem.out.println( Width is " + w); import java.swing.*; We have used dierent classes alread: Sstem, Math Keboard, Above classes provide various services (related services are grouped in same class) Implementation details o the class are hidden rom the client (user) March 29, 25 Lecture public class MakeFrame { public static void main(string[] args) { = new ();.;.setsize(6,2); int w=.getwidth(); Sstem.out.println( Width is " + w); Local variables: Live and die inside a block (in a method)

4 CS1M Lecture 17 Mar 29, 25 4 Instance methods are accessed through the instance settitle( ); Instance methods are accessed through the instance.settitle( ); Whose settitle method???? March 29, 25 Lecture March 29, 25 Lecture Reerence Object Reerence Object 3; //local variable //no deault value 3? 3; 3.setTitle( ); 3? March 29, 25 Lecture March 29, 25 Lecture null 4= null; 4.setTitle( ); 4 null null means the reerence variable does not reer to an object. int = 2; int = 2; Epression == gives true March 29, 25 Lecture March 29, 25 Lecture 17 31

5 CS1M Lecture 17 Mar 29, 25 5 int = 2; int = 2; Epression == gives alse int = 2; int = 2; 3= ; alias March 29, 25 Lecture March 29, 25 Lecture Class deinition vs. object instantiation I ou want make a whole lot o cookies, ou ma want to Make a cookie cutter deine the class Stamp out the cookie instantiate an object Making a cookie cutter Getting a cookie Prelim 2 Q1: Tracing code (2-d, 1-d, unction ) Q2: short ragments (vector inde epression, calling with 2 return values, vectorized code) Q3: vector, loop, algorithm to ind min Q4: nested loop Q5: algorithm to sum triangle areas March 29, 25 Lecture 17 4 March 29, 25 Lecture Prelim 2 Median 9 Mean 84.1; Standard Deviation 16. A rough mapping o grades: Q (A-like) 9-1 R (B-like) S (C-like) 6-7 March 29, 25 Lecture 17 53