AP COMPUTER SCIENCE A

Transcription

1 AP COMPUTER SCIENCE A METHODS Sep Week 3 1

2 Main Method? System.out.println( ); 2

3 Main Method? System.out.println( ); public static void anothermethod() { System.out.println( World ); 3

4 Main Method? System.out.println( ); anothermethod(); public static void anothermethod() { World System.out.println( World ); 4

5 Main Method? anothermethod(); public static void anothermethod() { for(int i=0; i<5; i++) { System.out.println( ); 5

6 Main Method? System.out.println( ); anothermethod(); theothermethod(); public static void anothermethod() { System.out.println( World ); theothermethod(); World Ook! Ook! public static void theothermethod() { System.out.println( Ook! ); 6

7 Definition and Call A method is defined just like main method : public static void methodnamehere() { //implementation A method is called with its name and (). And a semicolon, of course. methodnamehere(); 7

8 Definition and Call When a method is called, the lines are executed from top to bottom. They work just like main method, and yes, main WAS a method, too. Methods can call other methods, even itself. (Recursion, it will be covered later) Method name can contain alphabets, numbers and _. It cannot start with a number. It is conventional to start with lower case, but not necessary. 8

9 (a little taste of) Recursion System.out.println( ); itcallsitself(); public static void itcallsitself() { System.out.println( ); itcallsitself(); 9

10 Parameters printit( ); public static void printit(string s) { System.out.println(s); 10

11 Parameters printthem(, World ); public static void printthem(string s1, String s2) { World System.out.println(s1); System.out.println(s2); 11

12 Parameters printthem(); printthem( ); printthem(123,456); public static void printthem(string s1, String s2) { ERROR (for all three calls) System.out.println(s1); System.out.println(s2); 12

13 Parameters Methods can take parameters. (Parameters are what () is for). Parameters are declared in the parenthesis (but without a semicolon) Parameters are valid only inside the method TYPES and the NUMBER of parameters MUST match with method call 13

14 Parameters When a method is called with appropriate parameters, the value of the parameters are passed to the method. Now the parameters can be used like variables. 14

15 Your turn Define a method which takes an integer as parameter When the method is called, it should print as many as the input parameter For example, howmany(3); Prints 15

16 Your turn howmany(3); public static void howmany(int n) { for(int i=0; i<n; i++) { System.out.println( ); 16

17 Scope (Global and Local Variables) int number=1; printnumber(); printnumber(); printnumber(); public static void printnumber() { System.out.println(number); number++; 17

18 Scope (Global and Local Variables) printnumber(); printnumber(); printnumber(); public static void printnumber() { int number=1; System.out.println(number); number++; 18

19 Scope (Global and Local Variables) int number=1; printnumber(); printnumber(); printnumber(); public static void printnumber() { int number=1; System.out.println(number); number++; 19

20 Scope (Global and Local Variables) int number=1; printnumber(); printnumber(); printnumber(); ERROR public static void printnumber() { System.out.println(number); number++; 20

21 Scope (Global and Local Variables) A variable is valid only inside the scope it was declared When a variable is declared inside a class, it is valid throughout the class When a variable is declared inside a method, it is valid only inside the method In a loop, the same. Method parameters are considered to be declared inside the method Local variable (the inner one) has higher priority when called. 21

22 Return type and value int a=sum(3,4); System.out.println(a); public static int sum(int a, int b) { int c=a+b; return c; 7 public static int sum(int a, int b) { return a+b; 22

23 Return type and value Methods can return a value The type of return value (return type) should be specified in method definition Public static returntype methodname() { return thereturnvalue As shown in the examples When a method with a return value is called, the method call has the value, and can be assigned to a variable or become a parameter for another method 23

24 Your turn Define a method that returns the square of input value(double) For example, System.out.println(square(3.0)); prints

25 Your turn System.out.println(square(3.0)); public static void square(double d) { return d*d; 25