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

Transcription

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 Required syntax Type of answer Type of 1 st argument Type of last argument public static type name(type arg1,,type argn) { statement1; statement2; statementm; Code done by method Name of method Name of 1 st argument Name of last argument 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 public static void help(int times) { for( int i = 1; i <= times; i++ ) System.out.println( Help! ); return; Void methods just do something If you try to save the value they give back, there will be an error 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 Class.name(arg1, arg2, arg3); 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 12

13 Variables from outside of the method don t exist unless they have been passed in as parameters public static int add(int x, int y) { int z = x + y; return z; No matter how complex a program is, inside this method only x, y, and z variables exist 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 ) The method does not change the values of the variables in the original code Remember it only has copies of the values 14

15 int a = 10; int x = 3; int y = add( 5, a ); //y contains 15 now public static int add(int x, int y){ int z = x + y; // return z; No connection between the two different x s and y s 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: 2 2 d = ( x x y y ) + ( )

18 public static double euclid (double x1, double x2, double y1, double y2) { double xsq = (x1-x2)*(x1-x2); No matter how complex a program is, inside double ysq = (y1-y2)*(y1-y2); this method, only x, y, and z variables exist 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 public static void conway (int[][] cell, int m, int n) { int live = 0; for (int i=m-1; i<=m+1; i++) { No matter how complex a program is, inside for (int j=n-1; j<=n+1; j++) this method, only x, y, and z variables exist { if (cell(i,j)==1) live++; 20

21 if (cell(m,n)==1) { live--; if (live < 2 live > 3) cell(m,n) = 0; else { No matter how complex a program is, inside this method, only x, y, and z variables exist if (live == 3) cell (m,n) = 1; return; 21