Understanding Method Parameters, Returns, and Encapsulation in Java

1. Chapter 4 • Method parameters • Formal and Actual parameters • Method returns • Encapsulation • ‘private’ access modifier • ‘public’ getters and setters • Instance vs Local variables

2. Parameters public class Library{ public static void main(String [] args){ Book book1 = new Book(“John Fish”, “Die Hard”, 25.69); } }

3. Parameters public class Library{ public static void main(String [] args){ Book book1 = new Book(“John Fish”, “Die Hard”, 25.69); } } Actual Parameter 1

4. Parameters public class Library{ public static void main(String [] args){ Book book1 = new Book(“John Fish”, “Die Hard”, 25.69); } } Actual Parameter 2

5. Parameters public class Library{ public static void main(String [] args){ Book book1 = new Book(“John Fish”, “Die Hard”, 25.69); } } Actual Parameter 3

6. Parameters public class Book{ String author, title; double price; Book( String a, String b, double c ){ author = a; title = b; price = c; } } Formal Parameter 1

7. Parameters public class Book{ String author, title; double price; Book( String a, String b, double c ){ author = a; title = b; price = c; } } Formal Parameter 2

8. Parameters public class Book{ String author, title; double price; Book( String a, String b, double c ){ author = a; title = b; price = c; } } Formal Parameter 3

9. Parameters public class Library{ public static void main(String [] args){ Book book2 = new Book(“J. Cox”, “Jaws”, 13); } }

10. Parameters public class Library{ public static void main(String [] args){ Book book2 = new Book(“J. Cox”, “Jaws”, 13); } } Is the 3rd actual parameter legal?

11. Parameters public class Library{ public static void main(String [] args){ Book book2 = new Book(“J. Cox”, “Jaws”, 13); } } Is the 3rd actual parameter legal? Yes… implicit widening conversion: int > double

12. Parameters public class Library{ public static void main(String [] args){ Book book2 = new Book(“J. Cox”, “Jaws”, 2 * 14.1f / 3 - 1); } } Is the 3rd actual parameter legal?

13. Parameters public class Library{ public static void main(String [] args){ Book book2 = new Book(“J. Cox”, “Jaws”, 2 * 14.1f / 3 - 1); } } Is the 3rd actual parameter legal? Yes… expression is evaluated before passing

14. Parameters public class Book{ String author, title; double price; Book( String a, String b, byte c ){ author = a; title = b; price = c; } } Change formal parameter to byte

15. Parameters public class Library{ public static void main(String [] args){ Book book2 = new Book(“J. Cox”, “Jaws”, 62); } } Is the 3rd actual parameter legal?

16. Parameters public class Library{ public static void main(String [] args){ Book book2 = new Book(“J. Cox”, “Jaws”, 62); } } Is the 3rd actual parameter legal? No… implicit narrowing conversion DOES NOT WORK in method invocations

17. Instance vs. Local variables public class Fun{ public static void main(String [] args){ inti = 0; addTwo( i++ ); System.out.println( i ); } static void addTwo( inti ){ i += 2; } } What is the output?

18. Instance vs. Local variables public class Fun{ public static void main(String [] args){ inti = 0; addTwo( i++ ); System.out.println( i ); } static void addTwo( inti ){ i += 2; } } i != i main의 i = addTwo의 i

19. Instance vs. Local variables public class Fun{ public static void main(String [] args){ inti = 0; addTwo( i++ ); System.out.println( i ); } static void addTwo( inti ){ i += 2; } } 1 ! Actual parameter and formal parameter are not the same variables

20. public class ChefTestDrive{ public static void main(String[] args){ Chef me = new Chef(); me.bakeCookies( 5 ); me.bakeCookies( 2 ); me.makeGimchi( “hot” ); me.makeGimchi( “sweet” ); } } Write this in eclipse and add the Chef class Mmm. 5 delicious cookies Mmm. 2 delicious cookies Oh! Gimchi is too hot! Oh! Gimchi is too sweet!

21. Method Returns • void go(){ • }

22. Method Returns • int go(){ • return 5; • }

23. Method Returns • int go(){ • int x = 5; • return x; • }

24. Method Returns • String go(){ • String x = “hello buddy”; • return x; • }

25. Method Returns • long go(){ • int x = 5; • return x; • }

26. Method Returns • byte go(){ • short x = 5; • return x; • } 불범! implicit narrowingconversion does not work for method returns

27. Method Returns • Dog go(){ • Dog x = new Dog(); • return x; • }

28. Encapsulation class Soju { int 술; }

29. Encapsulation class Soju { int 술; } class SojuTestDrive { public static void main(String[] args){ Soju ohMyGod = new Soju(); ohMyGod.술 = 90; } } What? 90% 술?

30. Encapsulation class Soju { int 술; }

31. Encapsulation class Soju { private int 술; }

32. Encapsulation class Soju { private int 술; void set술(int x){ 술 = x; } } setter method

33. Encapsulation class Soju { private int 술; void set술(int x){ if(x<45){ 술 = x; } else{ System.out.print(“안돼!”); } } }

34. Encapsulation class SojuTestDrive { public static void main(String[] args){ Soju ohMyGod = new Soju(); ohMyGod.set술(90); } } 안돼!

35. Encapsulation class Soju { private int 술; void set술(int x){ … } int get술(){ return 술; } } getter method

36. Encapsulation class SojuTestDrive { public static void main(String[] args){ Soju ok = new Soju(); ok.set술(20); System.out.print(ok.get술()); } }