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Class and Object Java Core

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Agenda Class and Object Access to data Fields of class Getters and Setters Constructors Methods of class Creating objects Examples

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Class and Object A class is a prototype (template) from which objects are created An object is a software bundle of related state and behavior

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$Class class ClassName { // fields variable1; ... variableN; // constructors ClassName(parameter_list1){ // method body } ......$

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Class class ClassName { // fields variable1; ... variableN; // constructors ClassName(parameter_list1){ // method body } ... ClassName(parameter_listN){ // method body } // methods method1(parameter_list){ // method body } ... methodN(parameter_list){ // method body }

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Class

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Access to data public class Student {...} private int age; public void print(){} Controlling Access to Members of a Class Class Package Subclass World private Y — — — (not) Y Y — — protected Y Y Y — public Y Y Y Y

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Special Requirements to source files a source code file (.java) can have only one public class name of this class should be exactly the same of file name before extension (including casing) source code file can have any number of non-public classes most code conventions require use only one top-level class per file

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Default values for fields

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Type casting Widening (implicit or automatic) type casting take place when, the two types are compatible the target type is larger than the source type

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Methods and overloading Methods are functions that are executed in context of object Always have full access to data of object Object can have multiple methods with same name but different signature (type and order of parameters) Signature doesn't include return type, methods can't be overloaded by return types

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Variable length arguments Methods in Java support arguments of variable length Should be last argument in method definition

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Access to fields The following class uses public access control: public class Student { public String name; public int age; ... } Student stud = new Student(); stud.name = “Krystyna”; stud.age = 22;

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Getters and Setters The following class uses private access control: public class Student { private String name; public String getName() { return this.name; } public void setName(String name) { this.name = name; } }

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Getters and Setters Student student = new Student(); student.setName(“Franko”); String nameStud = student.getName();

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Getters and Setters can be Complex public class Sum { private int a, b, c; void setA(int m) { this.a = m; c = a + b; } void setB(int n) { this.b = n; c = a + b; } int getA() { return this.a; } int getB() { return this.b; } int getC() { return this.c; } public void sum(int m, int n) { this.a = m; this.b = n; this.c = m + n; } }

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Keyword "this" this always points to current object can't lose context like JavaScript not required in most cases often needed to distinguish between parameters and fields:

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Keyword 'static' Keyword 'static' indicates that some class member (method or field) is not associated with any particular object Static members should be accessible by class name (good practice, not required by language itself)

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Keyword 'static'

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Constructors Constructors – special kind of methods called when instance created Name should be same as a class Class may have multiple overloaded constructors If not provided any constructor, Java provides default parameterless empty constructor

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Constructors

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Creating objects – new() Student stud1 = new Student(); stud1.setName(“Dmytro”); stud1.setAge(25); Student stud2 = new Student(“Olga”); stud2.setAge(24); Student stud3 = new Student(“Ivan”, 26); int n = Student.count;

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Private constructor Making constructor private will prevent creating instances of a class from other classes Still allows creating instances inside static methods of the class

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toString() System.out.println(student); com.edu.Student@659e0bfd @Override public String toString() { return "Student [lastNname=" + lastNname + ", firstName=" + firstName + ", age=" + age + "]"; } Student [lastNname=Ivanov, firstName=Vasiy, age=22]

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Example Create Console Application project in Java. Add class Student to the project. Class Student should consists of two private fields: name and rating; properties for access to these fields static field avgRating – average rating of all students default constructor and constructor with parameters methods: betterStudent - to definite the better student (between two, return true or false) toString - to output information about student changeRating - to change the rating of student In the method main() create 3 objects of Student type and input information about them. Display the average and total rating of all student.

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Practical task Create Console Application project in Java. Add class Employee to the project. Class Employee should consists of three private fields: name, rate and hours; static field totalSum properties for access to these fields; default constructor, constructor with 2 parameters (name and rate) and constructor with 3 parameters; methods: salary - to calculate the salary of person (rate * hours) toString - to output information about employee changeRate - to change the rate of employee bonuses – to calculate 10% from salary In the method main() create 3 objects of Employee type. Input information about them. Display the total hours of all workers to screen

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Homework Create Console Application project in Java. Add class Person to the project. Class Person should consists of two private fields: name and birthYear (the birthday year) properties for access to these fields default constructor and constructor with 2 parameters methods: age - to calculate the age of person input - to input information about person output - to output information about person changeName - to change the name of person In the method main() create 5 objects of Person type and input information about them.