Question of the Day

1. Question of the Day • How do you add 5 matches to these 6 & make 9?

2. Question of the Day • How do you add 5 matches to these 6 & make 9?

3. Lecture 6:Static vs. Non-Static Members

4. Announcements • Weekly assignments problems due tomorrow • Helps get rid of rust; last week of reviewing material • Make sure that your coding skills are back up to speed • Next week JUnit tests provided for each problem

5. Calling Methods • Some methodsreally need an object • Integer.intValue()is unclear without an object • But other methods independent of an instance • main(String[]) lacks object to use to call it • Why require Integer to call parseInt(String)?

6. staticMethods • staticmethods do not need an instance • Behavior defined by class, not an instance • parseInt(String)based only on Integer • All methods similar in how written & executed • May or may not include parameters • Local variables can be declared and used • Must either be void or declare return type • Mix of methods possible for all classes & enums

7. Making Method static • Includestatickeyword in method declaration public static void main(String args[]) publicstatic intparseInt(String s) privatestatic intnextInt() • Behaves like any other method (almost) • Use locals & parameters like normal • As with other code, call(nearly all)other methods • Java operators can be used • Perform any I/O operations

8. staticv. Non-static • Methods that are non-statichave this • Aliased to instance on which method called • Can directly use fields & call all methods • No thisparameter in staticmethods • Code directlyusing non-staticmembers illegal… • … using staticfields & methods perfectly legal • As always, can use object to access its members

9. BadDuck public class BadDuck{private intquackVolume;public static void printVolume() {System.out.println(“Volume is ” + getVolume());BadDuckduck = new BadDuck();duck.quackVolume = 11;System.out.println(“Volume is ”+duck.getVolume());}public intgetVolume() { return quackVolume;} }

10. Compiler error. Without an object, whose volume are we getting? BadDuck public class BadDuck{private intquackVolume;public static void printVolume() {System.out.println(“Volume is ” + getVolume());BadDuckduck = new BadDuck();duck.quackVolume = 11;System.out.println(“Volume is ”+duck.getVolume());}public intgetVolume() { return quackVolume;} }

11. BadDuck public class BadDuck{private intquackVolume;public static void printVolume() {System.out.println(“Volume is ” + getVolume());BadDuckduck = new BadDuck();duck.quackVolume = 11;System.out.println(“Volume is ”+duck.getVolume());}public intgetVolume() { return quackVolume;} } This is good. There is a duck to get the volume.

12. BadDuck public class BadDuck{public staticString duckSpeak() { return “quack”;} } public class UseDuck{public void speak() {System.out.println(BadDuck.duckSpeak());BadDuckduck = new BadDuck();System.out.println(duck.duckSpeak());} }

13. This compiles. Static methods do not use an object, so this is preferred way to call them. BadDuck public class BadDuck{public staticString duckSpeak() { return “quack”;} } public class UseDuck{public void speak() {System.out.println(BadDuck.duckSpeak());BadDuckduck = new BadDuck();System.out.println(duck.duckSpeak());} }

14. Also fine, but since method is static,still cannot access instance. BadDuck public class BadDuck{public staticString duckSpeak() { return “quack”;} } public class UseDuck{public void speak() {System.out.println(BadDuck.duckSpeak());BadDuckduck = new BadDuck();System.out.println(duck.duckSpeak());} }

15. staticFields • Some data belongs to class, not instance Float.MAX_VALUECitizen.nationalPopulationInstructor.bestProfessor • staticfields used for any class-based data • All of the class instances see same value • Do not need an instance to access this value • Can be updated via any instance or via class

16. staticCode for Tracing public class HHL {staticintnumTeams;String name;HHL(String newName) {numTeams+= 1;name = newName;}public staticvoid main(String[] args){HHLme = new HHL(“MHz”);HHLyou = new HHL(“SFH”);HHLalias = you;alias.name = “Bob”;alias.numTeams = 10;} }

17. Instance v. Class Review • Class owns members declared as static • Accessing & using member possible without instance • Methods lack implicit thisparameter no matter what • To use a non-staticmember, instance required • Fields’ data instance-specific and not shared by class • Can access all members using implicit thisparameter • Other instances’ members accessible with reference

18. Your Turn • Get into your groups and complete activity

19. For Next Lecture • Reading about packages & visibility for Wed. • What are packages and what do they do? • What are the different visibility modifiers in Java? • What is javadoc and why is it so amazingly useful? • There is weekly assignment posted on Angel • Assignment due tomorrow; working week-to-week • While not due daily, problems still follow lectures