1. Week 9: Methods 1

2.  We have written lots of code so far  It has all been inside of the main() method  What about a big program?  The main() method is going to get really long and hard to read  Sometimes you need to do similar things several times:  Read some input and check that it falls within certain values  Draw the same image at several different locations  Find the square root of several different numbers 2

3.  Methods allow you to package some code to run over and over again  Methods usually take some input (like numbers or Strings ) so that they can be parameterized  Methods often give back an answer (like the square root of a number)  Methods are called functions in some other languages 3

4.  More modular programming  Break a program into separate tasks  Each task could be assigned to a different programmer  Code reusability  Use code over and over  Could be in a code library (like Math.sqrt() )  Less code duplication  Improved readability  Each method can do a few, clear tasks 4

5. public static type name(type arg1,…,type argn) { statement1; statement2; … statementm; } Required syntax Type of answer Name of last argument Name of 1 st argument Name of method Type of 1 st argument Type of last argument Code done by method 5

6.  Given two integers, find the smaller: public static int min(int a, int b) { if( a < b ) return a; else return b; } 6

7.  static methods are methods that are not connected to a particular type of object  They are just supposed to perform some simple task  We are going to focus on static methods until next week  For now, the definition of every method will include the static keyword 7

8.  It is possible to divide methods into two types:  Value returning methods  Void methods  Value returning methods return an answer: int small = min(x, y); 8

9.  Void methods are declared with void as their return type  Void methods just do something  If you try to save the value they give back, there will be an error public static void help(int times) { for( int i = 1; i <= times; i++ ) System.out.println(“Help!”); return; } 9

10.  Like most code in Java, the code inside a method executes line by line  Of course, you are allowed to put conditional statements and loops inside methods  You can also put in return statements  A method will stop executing and jump back to wherever it was called from when it hits a return  The return statement is also where you can put a value that will be given back to the caller 10

11.  Defining a method is only half the fun!  You have to call methods to use them  Calling a method means giving a method the parameters (or arguments) it needs and then waiting for its answer  By now, you have done many method calls  System.out.println()  Math.sqrt()  You can call your own methods the same way 11

12.  Proper syntax for calling a static method is first the name of the class that the method is in, a dot, the name of the method, then the arguments  If the method is in the same class as the code calling it, you can leave off the Class. part  If it is a value returning method, you can store that value into a variable of the proper type Class.name(arg1, arg2, arg3); 12

13.  Variables from outside of the method don’t exist unless they have been passed in as parameters  No matter how complex a program is, inside this method only x, y, and z variables exist public static int add(int x, int y) { int z = x + y; return z; } 13

14.  A magical process called binding happens which copies the values from the calling code into the parameters of the method  The calling code can use variables with the same names, with different names, or even just literals (like 3 or 2.6758)  The method does not change the values of the variables in the original code  Remember it only has copies of the values 14

15.  No connection between the two different x ’s and y ’s xy public static int add(int x, int y){ xy int z = x + y; //5 + 10 return z; } int a = 10; int x = 3; 5a int y = add( 5, a ); //y contains 15 now 15

16. 1. Start a method with the keywords public static 2. Next comes the return type 3. Then the name of the method 4. Then, in parentheses, the arguments you want to give to the method 5. Inside braces, put the body of the method 6. Include a return statement that gives back a value if it is that type of method 16

17.  Finding the Euclidean distance between two points is very important  There are applications in:  Route planning  Graphic design software  Video games  Let’s make a method  Formula: 17

18.  No matter how complex a program is, inside this method, only x, y, and z variables exist public static double euclid (double x1, double x2, double y1, double y2) { double xsq = (x1-x2)*(x1-x2); double ysq = (y1-y2)*(y1-y2); double d = Math.sqrt(xsq+ysq); return d; } 18

19.  A cell is represented by a block in a grid  Each cell has 8 neighbors  Simple rules for a cell “coming to life” or “dying”: 1. A live cell with fewer than 2 live neighbors dies from loneliness 2. A live cell with more the 3 live neighbors dies from overcrowding 3. A live cell with exactly 2 or 3 neighbors keeps living 4. A dead cell with exactly 3 neighbors comes to life 19

20.  No matter how complex a program is, inside this method, only x, y, and z variables exist public static void conway (int[][] cell, int m, int n) { int live = 0; for (int i=m-1; i<=m+1; i++) { for (int j=n-1; j<=n+1; j++) { if (cell(i,j)==1) live++; } 20

21.  No matter how complex a program is, inside this method, only x, y, and z variables exist if (cell(m,n)==1) { live--; if (live 3) cell(m,n) = 0; } else { if (live == 3) cell (m,n) = 1; } return; } 21