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    Classes revision Classes revision Presentation Transcript

    • Classes Revision CST200 – Week 4: Midterm revision Instructor: Andreea Molnar
    • Outline • Classes • Example • Constructor • Encapsulation
    • Classes In object oriented programming classes represent objects with different characteristics (attributes, data) and functionality (operations, methods).
    • Example Person Characteristics: name, social security number Functionality: sleeps, walks
    • Example Person Attributes: name, social security number Operations: sleeps, walks
    • Example Person Data: name, social security number Methods: sleeps, walks
    • Example Person Data: define the state of the object Methods: define the behavior of the object
    • Example public class Person { private String name; Data declaration private String socialSecurityNo; public void sleeps() { } public void walks() { } } Method declaration
    • Example public class Person { private String name; private String socialSecurityNo; Instance variables – variables created at the class level Each instance of a class (object) has its own instance variables/data space public void sleeps() { } public void walks() { } } Each instance of a class (object) share the method definitions
    • Constructor • • • Creates (and initializes) an object Similar to a method that has: • • the same name as the class no return type Each class has a default constructor that accepts no parameters (this is generated only if no explicit constructor is provided).
    • Constructor public class Test { public static void main(String[] args) { Person p = new Person(); } } The default constructor is called
    • Constructor public class Person { private String name; private String socialSecurityNo; public Person(String name, String socialSecurityNo) { this.name = name; Constructor this.socialSecurityNo = socialSecurityNo; } public void sleeps() { } public void walks() { } } this is a reference to the current object
    • Constructor public class Test { public static void main(String[] args) { Person p = new Person("Mary", "078-05-1120"); } }
    • Constructor public class Test { public static void main(String[] args) { Person p1 = new Person("Mary", "078-05-1120"); Person p2 = new Person(“John", "078-05-1121"); } } name name Mary John socialSecurityNo socialSecurityNo 078-05-1120 078-05-1121 Each instance of a class (object) has its own instance variables/data space.
    • Encapsulation • • Hides the data and implementation details of an object Protects the data integrity by making it difficult to have unauthorized access
    • Encapsulation • Uses visibility modifiers (e.g. private) to deny access private String name; private String socialSecurityNo; name and socialSecurityNo can be referenced only within Person class public class Test { public static void main(String[] args) { Person p = new Person("Mary", "078-05-1120"); p.name = "Alice"; } } error
    • Encapsulation Allows access to instance variables through methods: accessor method (getter) and mutator method (setter)
    • Encapsulation Accessor method (getter) – returns the variable value. private String name; public String getName() { return name; }
    • Encapsulation Accessor method (getter) – returns the variable value. public class Test { public static void main(String[] args) { Person p = new Person("Mary", "078-05-1120"); System.out.println(p.getName()); } } Will print Mary p name Mary socialSecurityNo 078-05-1120
    • Encapsulation Mutator method (setter) – changes the variable value private String name; public void setName(String name) { this.name = name; }
    • Encapsulation Mutator method (setter) – changes the variable value public class Test { public static void main(String[] args) { Person p = new Person("Mary", "078-05-1120"); System.out.println(p.getName()); //will print Mary p p.setName("Alice"); System.out.println(p.getName()); //will print Alice } } name Mary Alice socialSecurityNo 078-05-1120
    • Summary • Initialize instance variables in the constructor • Make the instance variables private unless there is a good reason to do otherwise • Allow access to instance variables through setter and getter methods