Blue Box Diagram

1. Blue Box Diagram Definition: A method is a block of instructions in a class or object that is called by name Method contains a block of instructions Instance Variables (side effects) We pass (give) The method information (expressions) as arguments to use Out can come a value that is returned Processing is done in the blue box

2. Parameters and Arguments First Call x = triple(10); public int triple(int x) { return x*3; } 30 • Parameter: name a function gives an expression used to call a method • Argument: expression used to call a method • Why the difference? • We might want to use themethod more than once. • We might want to call a function using literals Second Call public int triple(int x) { return x*3; } x = triple(20); 60 Third Call int z = 20; x = triple(z); 60 public int triple(int x) { return x*3; } Fourth Call x = triple(triple(10)); public int triple(int x) { return x*3; } 90

3. General Class Syntax <public, private, protected> class <className> { … Your instance variables here … <public, private, protected> <mods><type><methodName>(<parameters>) { …. Instructions here … } ∙ ∙ ∙ <public, private, protected> <mods><type><methodName>(<parameters>) { …. Instructions here … } } Example that we did in labs public class Grader { public final static int LABMAX = 90;public static void main(String[] args) { … instructions …} } Note: protected is something that can be inherited by a child class (This is a CS 257 topic – memorize the definition for now.

4. Methods Definition: Block of instructions called (executed) by name • Static method • It is using the class name • Call round() method in Math class : System.out.println(Math.round(4.5) ; • Non static method • It is called using an instantiated object • String str = “abc”; • Call the length() method in str, an instantiated object of type StringSystem.out.println(str.length(); • Java has a huge class library . • We can use this library to access methods that do all sorts of useful things. • See: http://java.sun.com/j2se/1.5.0/docs/api/ Dot notation: The syntax to access methods inside classes and objects

5. More on static and non-static public class MyClass { public static void main(String[] args) { MyClass data = new MyClass(); // Make instance of MyClass(). String s1 = data.name(); // The name method executes. String s2 = name(); // Error – name() is not static . String s3 = name2(); // OK – name2() is a static method. String s4 = MyClass.name2(); // OK – name2() is a static method. System.out.println(s1); } public String name() { return “Dan ” + doIt(); } public String doIt() { return “Harvey”; } public static String name2() { return “Static Dan Harvey”; } } Note: when we call methods in the same class, dot notation is not needed.

6. A method returning a value public class ReturnValue { private int y = 10; public static void main(String[] args) { ReturnValue myObject = new ReturnValue(); int x = myObject.doReturn(); System.out.println(x); } private int doReturn() { int x = 33 + y; return x; } } What prints? Where is x visible? Where is y visible? Is x in doReturn the same variable as in main? Which are the instance variables?

7. A method with parameters public class LotsOfParams { protected static int x = 3; public static void main(String[] args) { LotsOfParams params = new LotsOfParams(); int y = x + 4; int x = params.doSomething (1,2,x + y -3,4,params.doSomething(x, y, 3, 4, 5)); } private int doSomething(int a, int b, int c, int d, int e) { System.out.println(“” + c + b + d + a + e ); return a + b + c + d + e; } } What prints? Why private? Which are the arguments? Which are the parameters (formal arguments)? Which are instance variables? What are the methods?

8. Pass by value • A copy of arguments gets passed to a method, not the variable itself • Example • Call: int x = 5; tryToChange(x); • Method: private void tryToChange(int x){ x = 3; } • Result: x in the call does not change! The x in the method only lives between the method’s braces. Question: Why doesn’t x change?

9. Method to search an array public class FindMe { String[] list = {“hello”, “this”, “is”, “a”, “good”, “day”}; public static void main(String[] args) { FindMe find = new FindMe(); System.out.println(find.isItThere(“is”)); System.out.println(find.isItThere(“cat”)); } private int isItThere(String which) { for (int i=0; i<list.length; i++) { if (list[i].equals(which)) return i; } return -1; } } Question: Why do we need find in find.isItThere(“is”));

10. Method to remove and add public class RemoveMe { String[] list = {“hello”, “this”, “is”, “a”, “good”, “day”};int howMany = list.length; public static void main(String[] args) { removeIt(3); addIt(“greetings”); } private static void removeIt(int which) { for (int i=which; i<howMany-1; i++) { list[i] = list[i+1]; } howMany--; } public static boolean addIt(String newOne) { if (howMany == list.length) return false; list[howMany++] = newOne; } }

11. Review • What is a method? • What is an instance variable? • What is the difference between an argument an a parameter? • What does it mean to limit scope? • What is an argument? • What is a side effect? • What does private and public mean? • What is the definition of protected? • How do you return a value from a method? • What does pass by value mean? • What is the difference between an object variable and primitive variable?