Object-Oriented Programming: Static Methods & Variables, Schemes and Mind Maps of Object Oriented Programming

Download Object-Oriented Programming: Static Methods & Variables and more Schemes and Mind Maps Object Oriented Programming in PDF only on Docsity! . . . . . . Static Methods Static Variables Admin . . . .. . . Object-Oriented Programming: Static Methods & Variables Ewan Klein Inf1 :: 2008/09 Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . . .1 Static Methods . . .2 Static Variables . . .3 Admin Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . What are Static Methods? ...1 What are static methods? ...2 You’ve already seen them. . Calling a method from Maths .. . . .. . . int randomNum = (int)( Math.random() * 5) Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . What are Static Methods? Methods from Math not dependent on the state of instances of the Math class. . Wrong .. . . .. . . Math m = new Math(); m.random(); . Output .. . . .. . . Exception in thread ”main” java.lang.Error: Unresolved com- pilation problem: The constructor Math() is not visible Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Static vs. Non-Static All ‘regular’ methods are non-static. A static method can be run without instantiating the class. Also called class methods. . Calling a Static Method .. . . .. . .Math.max(7, 22); . Calling a Non-Static Method .. . . .. . . Cell c = new Cell(); c.move(6); Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Classes with Static Methods, 2 . Static Method in Sim .. . . .. . . public static Sim getInstance() { if (instance == null) { instance = new Sim(); } return instance; } . Calling the Method .. . . .. . . public class AnimalCell extends BaseCell { Sim world = Sim.getInstance(); ... } Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Classes with Static Methods, 3 Classes with static methods may be instantiated. And static and non-static methods can be mixed. . Mixed methods .. . . .. . . public class Cell1 { public static void main(String[] args) { Cell1 c = new Cell1(); c.hello(); } public String hello() { return ”I’m a cell!”; } } Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Classes with Static Methods, 4 main() is a static method. You can do the following (though it’s a bit pointless!): . Calling a main() method .. . . .. . . public class CellLauncher { public static void main(String[] args) { String[] arglist = ; // Make an empty array Cell1.main(arglist); // prints ”I’m a cell!” } } Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Static Methods calling Non-Static Methods, 1 . Doesn’t Work .. . . .. . . public class Cell3 { int loc = 0; public static void main(String[] args) { System.out.println(”I’m at: ” + getLoc()); } public int getLoc() { return loc; } } . Output .. . . .. . . Exception in thread ”main” java.lang.Error: Unresolved com- pilation problem: Cannot make a static reference to the non- static method getLoc() from the type Cell3 Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Static Methods calling Non-Static Methods, 2 . Does Work .. . . .. . . public class Cell3 { int loc = 0; public static void main(String[] args) { Cell3 c = new Cell3(); System.out.println(”I’m at: ” + c.getLoc()); } public int getLoc() { return loc; } } Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Static Variables, 1 Static variable: one value per class, not one value per instance. Also called: static field, class variable Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Static Variables, 3 . Output .. . . .. . . Copy 1 of 6 Copy 2 of 6 Copy 3 of 6 Copy 4 of 6 Copy 5 of 6 Copy 6 of 6 Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Static Final Variables, 1 Final variable: Once initialized, value cannot be changed. Value stays the same as long the class stays loaded. This is how constants are created. Convention: write the variable all in capitals . Some constants .. . . .. . . public static final double PI = 3.141592653589793; public static final int DAYS_PER_WEEK = 7; public static final String BULGARIAN_NATNL_DAY = ”3 March”; Ewan Klein Object-Oriented Programming: Static Methods & Variables . . . . . . Static Methods Static Variables Admin . Finally, … Instance and local variables can be declared final — value cannot be subsequently changed. Methods can be final — cannot be overriden. Classes can be final — cannot be extended. Ewan Klein Object-Oriented Programming: Static Methods & Variables