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Object-Oriented Programming:
Static Methods & Variables
Ewan Klein
Inf1 :: 2008/09
Ewan Klein Object-Oriented Programming: Static Methods & Variables
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Object-Oriented Programming: Static Methods & Variables, Schemes and Mind Maps of Object Oriented Programming
Cannot make a static reference to the non- static field loc. Ewan Klein. Object-Oriented Programming: Static Methods & Variables ...
Typology: Schemes and Mind Maps
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Object-Oriented Programming:
Static Methods & Variables
Ewan Klein
Inf1 :: 2008/
...... ..^1.^ Static Methods ..^2.^ Static Variables ..^3.^ Admin
......
.. What are Static Methods?
..^1.^ What are static methods?
..^2.^ You’ve already seen them.
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.. What are Static Methods?
..^1.^ What are static methods?
..^2.^ You’ve already seen them.
Calling a method from Maths
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int randomNum = (int)( Math.random() * 5)
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.. 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 .. .
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Math.max(7, 22);. . Calling a Non-Static Method .. .
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Cell c = new Cell(); c.move(6);
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.. Classes with Static Methods, 1
Classes with static methods often not meant to be instantiated. How to stop instantiation? ..^1.^ Make the class abstract. ..^2.^ Make the constructor private. .Private Constructor ..
public class Sim { private Sim() { } }
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.. 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(); ... }
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.. 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!”; } }
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.. Static Methods and Instance Variables, 1
A static method cannot use an instance variable. Think about it. .Mixed methods ..
public class Cell int loc = 0; public static void main(String[] args) System.out.println(”I’m at: ” + loc ); // Wrong! public int getLoc() return loc ; // OK
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.. Static Methods and Instance Variables, 2
Output ..
Exception in thread ”main” java.lang.Error: Unresolved com- pilation problem: Cannot make a static reference to the non- static field loc
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.. 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; } }
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.. Static Variables, 1
Static variable: one value per class, not one value per instance. Also called: static field, class variable
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.. Static Variables, 2
.How many prints?
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public class Lithograph { static int printsMade = 0; private int copyNo; public static void main(String[] args) { ArrayList
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.. 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