1	CS110 Lecture 9 February 24, 2004 Announcements hw4 due Thursday February 26 exam Tuesday March 2 Agenda questions scope classes vs objects how parameters really work
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
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)
4	import Import statement qualifies scope for class names No need to import String from java.lang java packages import java.util.Date import java.math.BigInteger … you could write your own BigInteger class
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
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
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.java line 21: HLine.main(args) sends message to HLine class to run main() there
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
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 s in method is value of screen in message x in method is (first) 2 in message y in method is (second) 2 in message
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
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12	In Box paintOn