CS110 Lecture 9 February 24, 2004

1. CS110 Lecture 9February 24, 2004 • Announcements • hw3 due Thursday February 26 • exam Tuesday March 2 • Agenda • questions • scope • classes vs objects • how parameters really work Lecture 9

2. Scope • scope of a variable or method: where its unadorned name is visible to the program • Usually: the block { … } where it’s declared • Examples from HLine.java • scope of all fields: lines 15-114 • scope of screen (line 100) : lines 100-113 • scope of length (line 25): lines 25-29 • scope of i (line 41): lines 41-43 • Scope of a method is the class it’s declared in • public declaration does not change scope Lecture 9

3. Scope • To see a method or field outside its scope, qualify the name of the method or field: • account.getBalance() • System.out.println() • this.contents • Compare 7-6440, 287-6440 and (617)287-6440 • But • account.balance will fail because balance is private (an unlisted phone number) Lecture 9

4. import • Import statement qualifies scope for class names • No need to import String, … • java packages • import java.lang.Date • import …BigInteger • … you could write your own BigInteger class Lecture 9

5. static • Java keyword for belongs to whole class rather than to an instance of the class • Static things are rare, objects are common: too much static is bad design • public static void main( ) • main() is a static method - it can run before any objects are created with new • TestShapes (like many testing programs) is all static: there is a TestShapes class, but never a TestShapes object (although main uses objects Lecture 9

6. What can main() see? • HLine is meant to be a client class • private fields • public getters, setters, other methods as appropriate • HLine has a static main method, for unit testing • main in HLine • can’t refer to length field or paintOn method, since those belong only to HLine objects • can instantiate an HLine object, and then send it a paintOn message Lecture 9

7. Static tools in the library • To invoke a static method, send a message to the class (there is no object) - syntax ClassName.methodName( args ) • Math.sqrt( x ) • Calendar.getInstance() a factory method -Java designers chose this rather than new Calendar • UnitTest.javaline 21: HLine.main(args) sends message to HLine class to run main() there Lecture 9

8. static fields • Syntax for accessing static field: ClassName.fieldName (e.g. System.out ) (no System constructor, no System object) • Like global variables (in other languages) • In Integer class (part of Java API) public static final int MAX_VALUE= 2147483647; • final: Java keyword for “can’t be changed” int big = Integer.MAX_VALUE; // OK Integer.MAX_VALUE = 255; // error • Naming convention for final fields: ALL_CAPS Lecture 9

9. How parameters really work • Box.java line 143 sends a message: box2.paintOn( screen, 2, 2 ); • Execution shifts to method at Box.java line 52: public void paintOn( Screen s, int x, int y) • Value of parameter • sin method is value of screenin message • xin method is (first) 2in message • yin method is (second) 2in message Lecture 9

10. How parameters really work • Name of parameter (s) in method declaration need not match the name of the value in the message (screen) • You can’t even think they should match: • The value in the message might not even have a name (the 2 in the example) • The method can be written before the client (in some other class) has even been imagined - and the client programmer does not have access to the source code with the method declaration • The type of the value in the message must match the type in the method declaration Lecture 9

11. In Box main Screen Screen screen: Box Box box1: Box Box int box2: width: 3 int width: 4 char pntCh: 'G' Lecture 9

12. In Box main In Box paintOn Screen Screen screen: Box Box box1: Box Box Box int this: box2: width: 3 Screen out of scope int s: width: 4 int char 2 x: pntCh: 'G' int 2 y: Lecture 9