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Parameter Passing 7 January 2019 OSU CSE 1 Connecting Caller and Callee When you call a method, how are the arguments connected to the formal parameters ? When the called method body returns , how are results communicated back to the

  1. Parameter Passing 7 January 2019 OSU CSE 1

  2. Connecting Caller and Callee • When you call a method, how are the arguments connected to the formal parameters ? • When the called method body returns , how are results communicated back to the code that called the method? 7 January 2019 OSU CSE 2

  3. Example: GCD • Suppose we have a static method gcd that computes and returns the greatest common divisor (GCD) of two int s: public static int gcd( int i, int j) { ... } • For example: – GCD(24,80) = 8 – GCD(24,24) = 24 7 January 2019 OSU CSE 3

  4. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } ... int a, b; ... int c = gcd(a, b); 7 January 2019 OSU CSE 4

  5. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; This is the method gcd that is } being called; i and j are its formal parameters . ... int a, b; ... int c = gcd(a, b); 7 January 2019 OSU CSE 5

  6. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; This is a fragment of the calling } program; a and b are the arguments to this call of gcd . ... int a, b; ... int c = gcd(a, b); 7 January 2019 OSU CSE 6

  7. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; Suppose the solid red arrow } indicates where program flow-of- control has taken us so far. ... int a, b; ... int c = gcd(a, b); 7 January 2019 OSU CSE 7

  8. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } ... int a, b; ... int c = gcd(a, b); 7 January 2019 OSU CSE 8

  9. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } ... int a, b; ? ? ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 9

  10. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 10

  11. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } The call to gcd begins ... ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 11

  12. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ? ? ... i j return k; } ... so the formal parameters are effectively declared ... ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 12

  13. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 24 80 ... i j return k; } ... and the argument values are copied to initialize them. ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 13

  14. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 24 80 ... i j return k; } Execution of the calling program is “paused” at the point of the call… ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 14

  15. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 24 80 ... i j return k; } … and control is transferred to the beginning of the method body. ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 15

  16. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 24 80 ... i j return k; The scope of these variables } is the calling program where they are declared. ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 16

  17. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 24 80 ... i j return k; The scope of these variables } is the method body where they are declared. ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 17

  18. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 24 80 1 ... i j k return k; } ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 18

  19. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 0 8 8 ... i j k return k; } ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 19

  20. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 0 8 8 ... i j k return k; } The return statement immediately ends execution of method body… ... int a, b; 24 80 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 20

  21. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; 0 8 8 ... i j k return k; } ... so the returned value is copied back to the calling program … ... int a, b; 24 80 8 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 21

  22. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } ... and the method body has finished, so its variables go away. ... int a, b; 24 80 8 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 22

  23. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; Note that the values of the formal } parameters are not copied back to the arguments! ... int a, b; 24 80 8 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 23

  24. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } Execution of the calling program “resumes” in mid-statement ... ... int a, b; 24 80 8 ... a b int c = gcd(a, b); 7 January 2019 OSU CSE 24

  25. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } ... and the value that was returned by the call is assigned to c . ... int a, b; 24 80 8 ... a b c int c = gcd(a, b); 7 January 2019 OSU CSE 25

  26. How Calls Work In Java public static int gcd( int i, int j) { int k = 1; ... return k; } ... int a, b; 24 80 8 ... a b c int c = gcd(a, b); 7 January 2019 OSU CSE 26

  27. Connecting Caller and Callee • When you call a method, how are the arguments connected to the formal parameters ? – The argument values are copied into the formal parameters to initialize them • When the called method body returns , how are results communicated back to the code that called the method? – Only the returned value is copied back to the caller; the formal parameters are simply “lost” 7 January 2019 OSU CSE 27

  28. Names for This? • Parameter-passing mechanism of Java: – May be termed call-by-copying because argument values are copied into formal parameters – May be termed call-by-value because argument values are copied into formal parameters • There are other ways it might have been done (and is done in some languages) 7 January 2019 OSU CSE 28

  29. Tracing Over a Call Code State a = 24 b = 80 int c = gcd(a, b); a = 24 b = 80 c = 8 7 January 2019 OSU CSE 29

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