08-objects3

08-objects3 - COMP 202 COMP 202 Introduction to Computing 1...

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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 1 COMP 202 More on OO CONTENTS : • static revisited • this reference • class dependencies • method parameters • variable scope • method overloading
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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 2 Static member variables So far: Member variables were instance variables • Every object instance has its own set of these variables Member variables can also be static – static variables are also called class variables – Shared by all objects/instances of a class – i.e., there is only one copy of this variable If one object changes the value of a static variable, all other objects see and access the new value private static int count = 0; – Example use: • Increase count whenever the Constructor is called • Keeps track of the number of objects created;
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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 3 Static methods • Static methods can be executed without an object of the class instantiated • Static methods – can not access member variables because they always belong to a specific object/instance – can access static variables and local variables • All methods of Math class are static – Math.abs(-5); main method is static
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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 4 Calling Methods • A method calls a method m1 of the same class: Only indicate method name: m1(…); • A method calls a static method m1 of another class AnotherClass: AnotherClass.m1(); • A method calls a non-static m1 of another object which is referred to by the variable objVariable objVariable.m1();
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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 5 The calculator as a class with only static methods import java.util.Scanner; public class CalcMain { public static void main(String[] args) { int op; double v1,v2,result; scan = new Scanner(System.in); System.out.println(“Enter 0 to exit, 1 to add, 2 to subtract”); op=scan.nextInt(); if (op==1 || op==2) { System.out.print(“Enter number: ”); v1=scan.nextDouble(); System.out.print(“Enter number: ”); v2=scan.nextDouble(); if (op==1) result = CalcMain.add(v1,v2); else result = subtract(v1,v2); System.out.println(“Result: ”+result); } }
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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 6 The calculator as a class with only static methods public static double add(double d1,double d2) { return d1+d2; } public static double subtract(double d1,double d2) { return d1-d2; } // etc }
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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 7 Class dependencies • Stand-alone – Classes that do not depend or need other classes to run • Dependencies – When a program calls methods of other classes and/or instantiates objects of other classes. – eg. Bank uses Client class – eg. use of String and Scanner classes • Aggregate Classes – A class that is composed from multiple other classes
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COMP 202 – Introduction to Computing 1 COMP 202 - Week 7 8 Aggregate Classes • A class that defines an object as one of its class variables has created a dependency between the class they are building and the
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08-objects3 - COMP 202 COMP 202 Introduction to Computing 1...

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