1. Recitation 09/12/2007 CS 180 Department of Computer Science, Purdue University

2. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2 Announcements & Reminders Project 1 grades out  Remember to collect them at the end of the recitation  Solution up on the web Project 2 was due last night Project 3 due on September 17 th, 10 PM Students must sign the academic integrity policy. CS 180 students will not be allowed to take Exam 1 if they have not "signed" this policy!

3. Template for class definition

4. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 Defining your own classes : What would a class look like? (Remember that a class is a blueprint for creating objects) class Terminator { private String name; public Terminator( ) { name = “T1000”; } public String getName( ) { return name; } public void setName(String newName) { name = newName; }

5. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5 //file: Terminator.java class Terminator { private String name; public Terminator( ) { name = “T1000”; } public String getName( ) { return name; } public void setName(String newName) { name = newName; } //file: RunTerminator.java class RunTerminator { public static void main( String [] arg ) { Terminator T1; T1 = new Terminator( ); T1.setName(“T101”); Terminator T2 = new Terminator(); Terminator T3 = new Terminator( ); T3.setName(“TX”); System.out.println(T1.getName()); System.out.println(T2.getName()); System.out.println(T3.getName()); } Using objects of your class

6. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6 class Terminator { //the beginning of our class’s definition private String name;//an instance variable, name, of type String public Terminator( ) {//the constructor for this class name = “T1000”;//(A constructor has no return value)‏ }//(constructors can take parameters, but this one doesn’t.)‏ public String getName( ) {//a method for this class that takes no parameters return name;//this method returns a String variable } public void setName(String newName) {//a method for that takes one String parameter name = newName;//this method doesn’t return a value. } }//the ending of the definition of our class Components of a class

7. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7 //file: Terminator2.java class Terminator2{ private String name; private String getAClue( ) { return “the 3 rd movie was weak”; } public Terminator2( ) { name = “T1000”; } public String getName( ) { return name + “:“ + getAClue( ); } public void setName(String newName) { name = newName; } //file: TestTerminator2.java class TestTerminator2 { public static void main( String [] arg ) { Terminator2 T = new Terminator2( ); T.setName(“Cyberdyne Systems”); System.out.println(T.getName()); System.out.println(T.getAClue()); } Public and Private members

8. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8 Constructor Specifies how the class should be initialized class Terminator { private String name; If not specified, default constructor will be used public Terminator( ) { name = “T1000”; } // A constructor that takes parameters public Terminator( int a) { System.out.prinln(a); } //other stuff }

9. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 Passing class objects as parameters: (Passing by reference vs. passing by value)‏ //file: RunTerminator.java class RunTerminator { public static void messup(Terminator t) { t.setName("defunct Terminator"); } public static void messup(String s) { s = "defunct String"; } public static void main( String [] arg ) { Terminator T1 = new Terminator( ); T1.setName("T1 Name"); messup(T1); System.out.println(T1.getName()); String S = "S Name"; messup(S); System.out.println(S); } class objects (except Strings) are transferred as references when they are passed as parameters to a method. In contrast, basic data types like int and double are passed by value. (*) Even though it is a class object, String objects behave similarly to basic data types and are passed by value Program output : defunct Terminator S Name

10. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10 Defining class constants: the static keyword class Terminator3{ private final int A = 10; private static final int B = 20; public Terminator3( ) { } public int getNumbers( ) { return A + B; } A is an instance constant, while B is a class constant. Here, each individual Terminator3 object will have its own copy of the int A. There will be only one copy of int B, that will be shared among all Terminator3 objects.

11. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 11 Calling methods class Terminator5 { public void didIt5( ) {System.exit(0);} public void doIt5( ) { didIt5( ); } Class Terminator6{ public void didIt6( ) {System.exit(0);} public void doIt6( ) { Terminator5 T = new Terminator5( ); T.didIt5( ); didIt6( ); } When you call a method that’s within the same class, you can call the method by just using its name. If you call a method that is in a different class, then you must refer to that method using a. (dot) notation that first references the separate class object.

12. ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12 The main method //file: TerminatorA.java class TerminatorA{ public int A; public static void main( String [ ] arg ) { TerminatorB T = new TerminatorB( ); } //file: TerminatorB.java Class TerminatorB{ public int B; public static void main( String [ ] arg ) { TerminatorA T = new TerminatorA( ); } A main( ) method must be defined in the file you run. Only the main( ) method of the file you call explicitly with “java” will be run. Eg:- See which command executes which main( ) method based on color coding: Java TerminatorA Java TerminatorB

13. Identifier types Identifiers can be declared almost anywhere in a program. There are three main types of declarations:  Data members of a class Declared outside any method Usually at the beginning of the class definition  As formal parameters of a method  Within a method -- local variables

15. Sample matching

16. Notice how one can hide data members by declaring a local variable with the same name

17. Things to remember A local variable can be declared just about anywhere!  Its scope (the area of code from where it is visible) is limited to the enclosing braces. Statements within a pair of braces are called a block. Local variables are destroyed when the block finishes execution. Data members of a class are declared outside any method. Their scope is determined by public and private modifiers.