Introduction to Programming David Goldschmidt, Ph.D. Computer Science The College of Saint Rose Java Methods (a.k.a. Functions)

Methods (a.k.a. Functions) We’ve seen and used methods already.... These are all examples of method calls r = Math.sqrt( 9.0 ); q = Math.pow( r, 3.0 ); answer = Math.sqrt(Math.pow(x, 2.0) + Math.pow(y, 2.0)); z = Math.abs( y – x ); p = Math.exp( n );

Methods Method calls change a program’s flow of control: The flow of control jumps to the method Method code is executed An output is returned and flow of control returns to the calling method r = Math.sqrt(9.0); execute the square root code (algorithm) 2 capture the returned value 3 1 call the sqrt() method input to method 3.0 output of method 2 1 3

Writing our own Methods We can build our own methods in Java by identifying the three required elements : Input values (and data types)... Name of method... Output data type... public static double cube(double x) a Java method can have no more than 1 output (to have zero outputs, use void instead)

Writing our own Methods public class CubeProgram { public static void main(String args[]) { double v = 12.5; double vCubed = cube(v); // CALL THE METHOD System.out.println(v + " cubed is: " + vCubed); } public static double cube(double x) { double y = x * x * x; // METHOD CODE return y; } Add a quadruple() method and call it from main()

Writing our own Methods public class CubeProgram { public static void main(String args[]) { double v = 12.5; double vCubed = cube(v); // CALL THE METHOD System.out.println(v + " cubed is: " + vCubed); } public static double cube(double x) { double y = x * x * x; // METHOD CODE return y; } Add a quadruple() method and call it from main() Write a program that asks the user to enter four numbers in main() ; write a method called calcAverage() that calculates and returns the average of the four numbers Write a program to simulate the rolling of two dice, displaying the results; use a method called rollDice()

Why Create Methods? Methods are reusable program units Write once, use repeatedly Algorithm details are hidden, increasing readability Methods are compact modules that serve a specific purpose for which they were designed It’s easier to read, write, and maintain individual modules

Decision-Making Methods Aside from calculations, methods are often used to make decisions Return value is of type boolean (i.e. true or false ) public static boolean isPassingGrade( int grade ) { if ( grade >= 60 ) { return true; } else { return false; }

Decision-Making Methods public static void main( String[] args ) {... if ( isPassingGrade( grade ) ) { System.out.println( "You passed!" );... }... } public static boolean isPassingGrade( int grade ) { if ( grade >= 60 ) { return true; } else { return false; }

Input Parameters When we call a method, we pass in values int max = findMax( num1, num2 ); double c = cube( 9.0 ); double r = power( x, 4 ); int fact = factorial( n ); int total = rollDice();...or sometimes we don’t pass in any values at all

Input Parameters public class FindMaxProgram { public static void main( String[] args ) { int num1 = 19, num2 = 24; int max = findMax( num1, num2 ); // CALL METHOD System.out.println( max + " is the maximum." ); } public static int findMax( int x, int y ) { // METHOD CODE GOES HERE... } Write the findMax() method code ( HINT : use an if-else statement)

Input Parameters public static void main( String[] args ) {... int max = findMax( num1, num2 ); // CALL METHOD... } public static int findMax( int x, int y ) { // METHOD CODE GOES HERE... } Pass values to the method

Input Parameters storage in memory Variables x and y are usable only within the findMax() method public static void main( String[] args ) {... int max = findMax( num1, num2 ); // CALL METHOD... } public static int findMax( int x, int y ) { // METHOD CODE GOES HERE... }

Variable Scope We know that a variable must be declared before it can be used public static void main( String[] args ) { int x = 5; System.out.println( x ); System.out.println( y ); } Variable y is not declared! Declaring variable x creates storage for it in memory 5 x

Variable Scope Variables are usable (in scope) only within the section of code in which they are declared public static void main( String[] args ) { int x = 5; int r; r = cube( x ); } 5 x r public static int cube( int w ) {... 5 w x and r only usable within main() method w only usable within cube() method

public static int cube( int w ) {... public static void main( String[] args ) { int x = 5; int r; r = cube( x ); } Variable Scope Variables are usable (in scope) only within the section of code in which they are declared 5 xr main() 5 w cube() w only usable within cube() method x and r only usable within main() method

public static int cube( int w ) { int z = w * w * w; public static void main( String[] args ) { int x = 5; int r; r = cube( x ); } Variable Scope Variables are usable (in scope) only within the section of code in which they are declared 5 xr 5 w main() cube() w and z only usable within cube() method x and r only usable within main() method 125 z

public static int cube( int w ) { int z = w * w * w; return z; } public static void main( String[] args ) { int x = 5; int r; r = cube( x ); } Variable Scope Variables are usable (in scope) only within the section of code in which they are declared 5 xr 5 w main() cube() w and z only usable within cube() method x and r only usable within main() method 125 z

public static int cube( int w ) { int z = w * w * w; return z; } public static void main( String[] args ) { int x = 5; int r; r = cube( x ); } Variable Scope Variables are usable (in scope) only within the section of code in which they are declared 5 x 125 r 5 w main() cube() w and z only usable within cube() method x and r only usable within main() method 125 z