CS 177

1. Week 9: Methods CS 177

2. main() method • 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

3. Idea of a method • 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

4. Advantages of methods • 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

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

6. Simple method example • Given two integers, find the smaller: • public static intmin(int a, int b) • { • if( a < b ) • return a; • else • return b; • }

7. Why static? • 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

8. Value returning methods • 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);

9. Void methods • 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( inti = 1; i<= times; i++ ) System.out.println(“Help!”); return; }

10. return statements • Like most code in Java, the code inside a method executes line by line • Of course, you are allowed to put conditionalstatements 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

11. Calling methods • 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

12. Calling syntax • 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);

13. Scope • 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 intadd(int x, int y) • { • int z = x + y; • return z; • }

14. Binding • 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

15. Binding example inta = 10; int x = 3; int y = add( 5, a ); //y contains 15 now • No connection between the two different x’s and y’s • public static intadd(intx, inty){ • int z = x + y; //5 + 10 • return z; • }

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

17. Distance • 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:

18. method euclid() • 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, doubley1, doubley2) • { • doublexsq= (x1-x2)*(x1-x2); • doubleysq= (y1-y2)*(y1-y2); • doubled = Math.sqrt(xsq+ysq); • returnd; • }

19. Conway’s Game of Life • 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”: • A live cell with fewer than 2 live neighbors dies from loneliness • A live cell with more the 3 live neighbors dies from overcrowding • A live cell with exactly 2 or 3 neighbors keeps living • A dead cell with exactly 3 neighbors comes to life

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

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