1. Methods (Functions) CSE 1310 – Introduction to Computers and Programming Vassilis Athitsos University of Texas at Arlington 1

2. Terminology: Methods/Functions In Java, the term methods is the one mainly used. In some other languages, the term functions is more common. In some languages (e.g., C++) methods and functions have minor differences in meaning. In this course, I will use the two terms (methods and functions) as synonyms. 2

3. A First Example Here we see our first example of a function. It is called square. – Its job is to compute the square of a number. 3 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); }

4. What Is a Function A function is a piece of code that does a specific job. Usually (but not always) the job is to compute something. Examples: – Computing the square of a number. – Determining if an number is prime. – Computing the number of digits of a number. – Counting the number of vowels in a string. 4 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); }

5. How to Write a Function Specify a name, like square. 5 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } function name

6. How to Write a Function Specify a name, like square. Specify the inputs (called arguments). – The arguments are the input that the function needs in order to compute an output. – For example, what does the square function take as input? 6 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); }

7. How to Write a Function Specify a name, like square. Specify the inputs (called arguments). – The arguments are the input that the function needs in order to compute an output. – For example, what does the square function take as input? A number. – We can have functions with 0, 1, or more arguments. We will see examples later. 7 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } argument(s)

8. How to Write a Function Specify a name, like square. Specify the inputs (called arguments). Specify the return type of the function. – What is the type of the value that the function computes? – In other words, what values are legal and illegal for the output of the function? 8 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } return type

9. How to Write a Function Specify a name, like square. Specify the inputs (called arguments). Specify the return type of the function. Specify what the function does. This is what we call the body of the function. 9 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } body

10. How to Write a Function Specify a name, like square. Specify the inputs (called arguments). Specify the return type of the function. Specify what the function does. This is what we call the body of the function. Specify what value the function returns, using one or more return statements. 10 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } return statement

11. Using a Function (Function Calls) Once you have written a function, you can use it anywhere in your code, by doing a function call. 11 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } function call

12. Using a Function (Function Calls) Once you have written a function, you can use it anywhere in your code, by doing a function call. When you call a function, you must: – Provide the appropriate argument(s). 12 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } argument(s)

13. Using a Function (Function Calls) Once you have written a function, you can use it anywhere in your code, by doing a function call. When you call a function, you must: – Provide the appropriate argument(s). – Use the return value appropriately. 13 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } storing the return value

14. Using the Return Value In the square function, the return value is a double. You can use this value in any way that you can use any double number. Examples: You can store the return value in a variable. 14 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); } storing the return value in a variable

15. Using the Return Value In the square function, the return value is a double. You can use this value in any way that you can use any double number. Examples: You can store the return value in a variable. You can print the return value directly. 15 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); System.out.printf("%.2f squared = %.2f\n", N, square(N)); } printing the return value directly

16. Using the Return Value In the square function, the return value is a double. You can use this value in any way that you can use any double number. Examples: You can store the return value in a variable. You can print the return value directly. You can use it as part of a more complicated expression. 16 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double my_var = 18 + square(N); System.out.printf("result = %.2f\n", my_var); } using the return value in an expression

17. Using the Return Value In the square function, the return value is a double. You can use this value in any way that you can use any double number. Examples: You can store the return value in a variable. You can print the return value directly. You can use it as part of a more complicated expression. You can use it as an argument for another function call. 17 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number: "); double N = in.nextDouble(); double my_var = 18 + square(square(N)); System.out.printf("result = %.2f\n", my_var); } the return value is used as argument for another function call

18. Function Calls You can call a function as many times as you like. On the next slide, we call the square function three times. 18

19. 19 import java.util.Scanner; public class example1 { public static double square(double number) { double result = number * number; return result; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Please enter a number M: "); double M = in.nextDouble(); System.out.printf("Please enter a number N: "); double N = in.nextDouble(); double M_square = square(M); System.out.printf("%.2f squared = %.2f\n", M, M_square); double N_square = square(N); System.out.printf("%.2f squared = %.2f\n", N, N_square); double MN_square = square(M*N); System.out.printf("%.2f squared = %.2f\n", M*N, M*N_square); } Example: Calling the square function three times.

20. Example: Printing Prime Numbers Write a program that: – Asks the user to enter an integer N. – Prints all prime integers from 2 up to (and including) N. 20

21. Example: Printing Prime Numbers Write a program that: – Asks the user to enter an integer N. – Prints all prime integers from 2 up to (and including) N. First step: – Write the basic structure of the program. – Put placeholders where more detail is needed. 21 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); for (int i = 2; i <= N; i++) { if i is prime { System.out.printf("%d\n", i); }

22. Example: Printing Prime Numbers Write a program that: – Asks the user to enter an integer N. – Prints all prime integers from 2 up to (and including) N. Second step: – Identify functions that can be used to complete the program. 22 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); for (int i = 2; i <= N; i++) { if i is prime { System.out.printf("%d\n", i); }

23. Example: Printing Prime Numbers Write a program that: – Asks the user to enter an integer N. – Prints all prime integers from 2 up to (and including) N. We can use a function is_prime to check if i is prime. Arguments? Return type? 23 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); for (int i = 2; i <= N; i++) { if i is prime { System.out.printf("%d\n", i); }

24. Example: Printing Prime Numbers Write a program that: – Asks the user to enter an integer N. – Prints all prime integers from 2 up to (and including) N. We can use a function is_prime to check if i is prime. Arguments? an integer Return type? boolean 24 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); for (int i = 2; i <= N; i++) { if i is prime { System.out.printf("%d\n", i); }

25. Example: Printing Prime Numbers Write a program that: – Asks the user to enter an integer N. – Prints all prime integers from 2 up to (and including) N. Step 3: use the function. – We have not written the function yet, but that is OK. – Obviously, the program will not run until we write the function. 25 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); for (int i = 2; i <= N; i++) { if (is_prime(i)) { System.out.printf("%d\n", i); }

26. 26 import java.util.Scanner; public class example1 { public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } return true; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); for (int i = 2; i <= N; i++) { if (is_prime(i)) { System.out.printf("%d\n", i); }}}} Write a program that: – Asks the user to enter an integer N. – Prints all prime integers from 2 up to (and including) N. Step 4: write the function.

27. 27 Output: Enter an integer: 80 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 import java.util.Scanner; public class example1 { public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } return true; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); for (int i = 2; i <= N; i++) { if (is_prime(i)) { System.out.printf("%d\n", i); }}}}

28. Why Do We Need Functions To write better code: – More correct, easier to read/write/change. To write complicated code. Functions help us organize code. YOU CANNOT WRITE NON-TRIVIAL PROGRAMS IF YOU DO NOT USE FUNCTIONS 28

29. Example: Prime Numbers, Again Write a program that: – Asks the user to enter an integer N. – Prints out the smallest prime number greater than or equal to N. 29

30. Example: Prime Numbers, Again Write a program that: – Asks the user to enter an integer N. – Prints out the smallest prime number greater than or equal to N. First step: – Write the basic structure of the program. – Put placeholders where more detail is needed. 30 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); int i = N; while (i is not prime) { i++; } System.out.printf("%d\n", i); }

31. Example: Prime Numbers, Again Write a program that: – Asks the user to enter an integer N. – Prints out the smallest prime number greater than or equal to N. Second step: – Identify functions that can be used to complete the program. We can use the is_prime function again!!! 31 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); int i = N; while (is_prime(i) == false) { i++; } System.out.printf("%d\n", i); }

32. Write a program that: – Asks the user to enter an integer N. – Prints out the smallest prime number greater than or equal to N. To complete the program, we can just use the is_prime function we already have. Functions make it really easy to re-use code!!! 32 import java.util.Scanner; public class example1 { public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } return true; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); while (is_prime(N) == false) { N++; } System.out.printf("%d\n", N); }

33. 33 import java.util.Scanner; public class example1 { public static boolean is_prime(int N) { for (int i = 2; i < N; i++) { if (N % i == 0) { return false; } return true; } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter an integer: "); int N = in.nextInt(); while (is_prime(N) == false) { N++; } System.out.printf("%d\n", N); } Example Output: Enter an integer: 1000000 1000003 Example Output: Enter an integer: 123456789 123456791 Example Output: Enter an integer: 7500 7507

34. Making Code Easier to Read In lots of programs, we need to perform a task many times. Copying and pasting code can work, but has disadvantages: The resulting code can look long and ugly. If we make a mistake, we end up copying and pasting the mistake many times. – Fixing such mistakes, that have been copied and pasted many times, can be a pain. 34

35. Example: Input Validation Write a program that: – Asks the user to enter integers A, B, C. if the user enters an invalid input, the program should keep asking the user to enter a valid integer, – Prints out the sum of the three integers. 35

36. Example: Input Validation Write a program that: – Asks the user to enter integers A, B, C. if the user enters an invalid input, the program should keep asking the user to enter a valid integer, – Prints out the sum of the three integers. A first version of the solution can be seen on the next slide. 36

37. import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int A; while (true) { System.out.printf("Enter integer A: "); String s = in.next(); try { A = Integer.parseInt(s); } catch (Exception e) { System.out.printf("Invalid input\n"); continue; } break; } int B; while (true) { System.out.printf("Enter integer B: "); String s = in.next(); try { B = Integer.parseInt(s); } catch (Exception e) { System.out.printf("Invalid input\n"); continue; } break; } int C; while (true) { System.out.printf("Enter integer C: "); String s = in.next(); try { C = Integer.parseInt(s); } catch (Exception e) { System.out.printf("Invalid input\n"); continue; } break; } System.out.printf("A + B + C = %d\n", A + B + C); }

38. Example: Input Validation Write a program that: – Asks the user to enter integers A, B, C. if the user enters an invalid input, the program should keep asking the user to enter a valid integer, – Prints out the sum of the three integers. A first version of the solution can be seen on the previous slide. What is wrong with that code? 38

39. Example: Input Validation Solution: 39

40. Example: Input Validation Solution: write a function for getting an integer from the user. Name: user_integer Arguments: Return value: 40

41. Example: Input Validation Solution: write a function for getting an integer from the user. Name: user_integer Arguments: – One argument - what we print to the user. – Type of the argument: String Return value: – int 41

42. import java.util.Scanner; public class example1 { public static int user_integer(String message) { Scanner in = new Scanner(System.in); int result; while (true) { System.out.printf(message); String s = in.next(); try { result = Integer.parseInt(s); } catch (Exception e) { System.out.printf("%s is not a valid integer.\n\n", s); continue; } return result; } public static void main(String[] args) { int A = user_integer("Enter integer A: "); int B = user_integer("Enter integer B: "); int C = user_integer("Enter integer C: "); System.out.printf("A + B + C = %d\n", A + B + C); }} Here we see a solution that defines and uses the user_integer function. The code is shorter. Unlike the previous version, code is not duplicated. The main function is short and easy to read.

43. Example: Repeated Printing Write a program that: – Asks the user to enter a string S. – Asks the user to enter a number N. – Prints N times string S. 43 Enter string S: hello Enter integer N: 5 hello Example Output:

44. Example: Repeated Printing Write a program that: – Asks the user to enter a string S. – Asks the user to enter a number N. – Prints N times string S. Let's write a function, that: – takes as input a string S and an integer N. – prints the string N times. 44 import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter string S: "); String S = in.next(); System.out.printf("Enter integer N: "); int N = in.nextInt(); print String S repeatedly, N times. }

45. Example: Repeated Printing The complete program is shown on the right. Function repeat_print has two features we have not seen before: It takes two arguments. It returns nothing. 45 import java.util.Scanner; public class example1 { public static void repeat_print(String message, int times) { for (int i = 0; i < times; i++) { System.out.printf("%s\n", message); } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter string S: "); String S = in.next(); System.out.printf("Enter integer N: "); int N = in.nextInt(); repeat_print(S, N); }

46. A Function That Returns Nothing Function repeat_print does something useful. However, we do not want any value returned from the function. In that case, we specify the return type of the function as void. 46 import java.util.Scanner; public class example1 { public static void repeat_print(String message, int times) { for (int i = 0; i < times; i++) { System.out.printf("%s\n", message); } public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Enter string S: "); String S = in.next(); System.out.printf("Enter integer N: "); int N = in.nextInt(); repeat_print(S, N); }

47. The main Function When Java executes a program, how does Java know where to start? Every program must have a function called main, such that: – It takes one argument, of type String []. We will understand this type in our next topic, when we do arrays. – the return type is void. Until we saw how to write functions, all our code used to go to main. Now that we have started using functions, the main code will be a relatively small part of the program. Functions will do the bulk of the work. 47

48. Function Arguments Functions have arguments. To call a function XYZ, you write something like : XYZ(argument_1, …, argument_N) How would you know how many arguments to use? 48

49. Function Arguments Functions have arguments. To call a function XYZ, you write something like: XYZ(argument_1, …, argument_N) How would you know how many arguments to use? – From the function definition, which (among other things) defines EXACTLY: how many arguments to use. the type of each argument. the order of the arguments. public static type XYZ(type 1 arg_1, …, type_N arg_N) 49

50. Executing a Function Call When the body of the function starts executing, the only variables visible to the function are: – the arguments. – variables defined in the body of the function. 50

51. What Will This Program Do? 51 public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }

52. Step-by-step Execution 52 Current line Main Namespace: public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }

53. Step-by-step Execution 53 First line to execute: variables in main: public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }

54. Step-by-step Execution 54 variables in main: var1 = "hello" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } First line creates variable var1

55. Step-by-step Execution 55 Next line to execute: variables in main: var1 = "hello" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }

56. Step-by-step Execution 56 Next line to execute: variables in main: var1 = "hello" var2 = goodbye public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }

57. Step-by-step Execution 57 Next line to execute: variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }

58. Step-by-step Execution 58 Next line to execute: function call variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }

59. Step-by-step Execution 59 Next line to execute: function call. First, assign values to arguments. variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } variables in foo: var1 = ??? var2 = ???

60. Step-by-step Execution 60 Next line to execute: function call. First, assign values to arguments. variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } variables in foo: var1 = " earth " var2 = " moon "

61. Step-by-step Execution 61 Next line to execute: How does Java know which var1 to print? variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } variables in foo: var1 = " earth " var2 = " moon "

62. Step-by-step Execution 62 Next line to execute: variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } variables in foo: var1 = " earth " var2 = " moon " Currently executing the body of foo. Only the variables of foo are visible. "hello" is printed.

63. Step-by-step Execution 63 Next line to execute: variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } variables in foo: var1 = " earth " var2 = " moon " "moon" is printed.

64. Step-by-step Execution 64 Next line to execute: variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } variables in foo: var1 = " earth " var2 = " moon " Which line comes next?

65. Step-by-step Execution 65 Next line to execute: variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } Done with the function call. The variables of foo disappear.

66. Step-by-step Execution 66 Next line to execute: variables in main: var1 = "hello" var2 = "goodbye" var3 = "earth" var4 = "moon" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); } "goodbye" is printed.

67. Summary of Program Output 67 Output: var1 = "earth" var2 = "moon" var2 = "goodbye" public class example1 { public static void foo(String var1, String var2) { System.out.printf("var1 = %s\n", var1); System.out.printf("var1 = %s\n", var2); } public static void main(String[] args) { String var1 = "hello"; String var2 = "goodbye"; String var3 = "earth"; String var4 = "moon"; foo(var3, var4); System.out.printf("var2 = %s\n", var2); }