Methods and Parameters in Java

1. Methods and Parametersin Java

2. Methods in Java • Also known as • functions, procedures, subroutines • For example • System.out.println() • Math.sqrt() • Provide a name for a sequence of instructions • Provide abstraction and hide the underlying details • Can be used by many programs • One of the ways to help implement decomposition

3. User-defined methods • Defined by Java • System.out.println() • Math.sqrt() • Defined by users/programmers

4. Basic anatomy of a method Return type Name Parameters (if any) Instructions … intfindMax(int[] scores) { // instructions }

5. Basic syntax of methods … returnTypename(parameters) { // instructions } For simplicity for this workshop, … is public static

6. Return Type of a Method • Any data type • int, double, boolean, char, … • The method returns a value of that type • ie, a function • No return value • void • ie, a procedure

7. Name of a Method Same rules as naming a variable…

8. Parameters of a Method • Type and name • E.g. intfindMax(int[] scores) • More than one parameter • E.g. intfindMin(int x, int y)

9. Instructions (Body) of a Method Instructions as before in the main method You have been mostly writing instructions for the main method

10. Using/Calling a Method • Once a method is defined • It can be used or “called” • For example, • You have been calling the method System.out.println()

11. Using/Calling a Method—Parameter Passing • If a method has parameters • The parameter types have to match when the method is called • Definition • … intfindMin(int x, int y) • Call • int a, b. m; • m = findMin(a, b); • m = findMin(a, 10); • m = findMin(a + 10, b);

12. Summary • Defining a method • Return type • Name • Parameters • Instructions (body) • Calling a method • Parameter types need to match

13. Problem decomposition for Tic-tac-toe • Display the board • Make a move • Ask for a move • Update the board • Decide winner • Check 3 rows • Check 3 columns • Check 2 diagonals Tasks 1 to 3 can be implemented as methods Subtasks 2a-2b, 3a-2c can be as well

14. Defining Methods in Tic-tac-toe • Display the Board • Return type ? • Parameters ? • Make a move • Return type ? • Parameters ? • Decide Winner • Return type ? • Parameters ?

15. One Possibility • Display the Board • Return type -- void • Parameters -- board • Make a move • Return type -- void • Parameters -- board, player • Decide Winner • Return type -- char (X , O, space--no winner) • Parameters – board

16. Main method—first level in decomposition … main(…) { char[][] board = new char[3][3]; char player = ’X’, winner = ’ ’; // initialize the board … displayBoard(board); makeAMove(board, player); winner = decideWinner(board); }

17. Main method—Refining it … main(…) { char[][] board = new char[3][3]; char player = ’X’, winner = ’ ’; // initialize the board … do { displayBoard(board); makeAMove(board, player); player = switchPlayer(player); winner = decideWinner(board); } while (…); // no winner and board is not full }

18. Main method (high-level & readable) … main(…) { char[][] board = new char[3][3]; char player = ’X’, winner = ’ ’; // initialize the board … do { displayBoard(board); makeAMove(board, player); player = switchPlayer(player); winner = decideWinner(board); } while (winner == ’ ’ && !isFull(board)); // no winner and board is not full }

19. Two more methods • switchPlayer(player) • isFull(board) Note that: • We don’t have to know exactly how to implement the methods • we just need to know we need them • we can further decompose them to solve them • It’s ok to add methods • we might not cover everything the first try