Methods and Parameters in Java Methods in Java

Methods and Parameters in Java

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

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

Basic anatomy of a method Return type 2. Name 3. Parameters (if any) 4. Instructions 1. … int find. Max(int[] scores) { // instructions }

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

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

Name of a Method �Same rules as naming a variable…

Parameters of a Method �Type and name E. g. int find. Max(int[] scores) �More than one parameter E. g. int find. Min(int x, int y)

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

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()

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

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

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

Defining Methods in Tic-tac-toe Display the Board 1. Return type ? Parameters ? 2. Make a move Return type ? Parameters ? 3. Decide Winner Return type ? Parameters ?

One Possibility Display the Board 1. Return type -- void Parameters -- board 2. Make a move Return type -- void Parameters -- board, player 3. Decide Winner Return type -- char ( X , O, space--no winner) Parameters – board

Main method—first level in decomposition … main(…) { char[][] board = new char[3][3]; char player = ’X’, winner = ’ ’; // initialize the board … display. Board(board); make. AMove(board, player); winner = decide. Winner(board); }

Main method—Refining it … main(…) { char[][] board = new char[3][3]; char player = ’X’, winner = ’ ’; // initialize the board … do { display. Board(board); make. AMove(board, player); player = switch. Player(player); winner = decide. Winner(board); } while (…); // no winner and board is not full }

Main method (high-level & readable) … main(…) { char[][] board = new char[3][3]; char player = ’X’, winner = ’ ’; // initialize the board … do { display. Board(board); make. AMove(board, player); player = switch. Player(player); winner = decide. Winner(board); } while (winner == ’ ’ && !is. Full(board)); // no winner and board is not full }

Two more methods 1. switch. Player(player) 2. is. Full(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