Data Structures and Algorithms revision Objects An Object

Data Structures and Algorithms revision

Objects • An Object is “some data/attributes”, • and a set of ways (methods) of changing/accessing the data.

Classes vs Objects • You have a single class e. g. Person. • You can make many objects of the type Person p 1 = Person(); // the default object Person p 2 = Person(“John”, 99); // makes a person called p 2 with the values John with age 99 Person p 3 = p 2; //makes a person p 3 …. • All 3 Person objects belong to the Person class CSCU 9 A 3 - Autumn 2015 3

Analogy: In an Office (before computers) 1. 2. 3. 4. Imagine you work in an office (bank). You have a paper form with blanks in (e. g. name, address, salary) When somebody opens a bank account – you photocopy the form and fill in their individual details. 5. The blank form is like a Class – there is only one blank form (THE MASTER COPY). 6. Each photocopied form is like an Object. CSCU 9 A 3 - Autumn 2015 4

One class, many objects • A class is a template for making objects. • E. g. PERSON CLASS Int Age String Name P 2 P 1 PERSON OBJECT Int Age String Name PERSON OBJECT 55 Int Age 98 dave String Name james

DANGER OF PUBLIC • If your data is public – it can be changed to anything PERSON CLASS public int age public string Name Person p 1 = new Person(“dave”, 55); PERSON OBJECT int age -100 string Name “dave” P 1. age = -100; //SHOULD NOT HAPPEN

Private data • If your data is private– you are forces to use the set methods to change it. PERSON CLASS private Int age private string Name Person p 1 = new Person(“dave”, 55); p 1. age = -100; //SHOULD CANNOT HAPPEN PERSON OBJECT int Age 55 String Name “dave” We use a set. Age method

set. Age method public void set. Age(int a) { if (a > 0) { this. age = a; } else { System. out. println("age not set, cannot be negative"); } }

Overloaded Methods • A class can have a number of methods with the same name, but different parameters • Example: println methods, one for each different type of thing you might want to print: • public void println(int output) • public void println(double output) • public void println(String output) CSCU 9 A 3 - Autumn 2015 9

Overloaded Methods • This solve the problem of having to explicitly tell the program which method: • public void println. INT(int output) • public void println. DOUBLE(double output) • public void println. STRING(String output) CSCU 9 A 3 - Autumn 2015 10

Encapsulation 1. Encapsulation means we do not need to know the details of how the data is stored. 2. You just need to know how to use it. 3. This is sometimes called “data hiding” 4. We do not want to expose the internal workings of out class – but have an agreed set of methods by which we communicate with other classes.

e. g. counter class • • A counter class with method Increment (adds one to the count) Reset (sets the count to zero) get. Count (give the current number) • (DO AN EXAMPLE WITH THE CLASS) • This is the interface of the class – it is just the methods you can see (and the constructor)

Encapsulation - example • A counter class with methods • Increment and get. Count. • Could store an integer, which is incremented by one each time the counter is pressed. • Could store a string “ 1111111”, and append “ 1” each time. The length represents the count. • Could store a number 1000, and multiply by 10 each time we press counter. The count would just be log of the number.

Syntax 3. 1 Instance Variable Declaration Big Java by Cay Horstmann Copyright © 2009

Accessing Instance Variables • The count method advances the counter value by 1: public void count() { value = value + 1; } • The get. Value method returns the current value: public int get. Value() { return value; } • Private instance variables can only be accessed by methods of the same class Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Instance Variables 1. Encapsulation is the process of hiding object data and providing methods for data access 2. To encapsulate data, declare instance variables as private and declare public methods that access the variables 3. Encapsulation allows a programmer to use a class without having to know its implementation 4. Information hiding makes it simpler for the implementor of a class to locate errors and change implementations 5. Protects your data (get/set methods accessor/mutator methods) from others and from yourself!!! 6. How else could the class counter be implemented instead of using int ? ? (float, string…) Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Equals (==) for primitive data types • When comparing primitive data types – int – boolean – float – double – char, … • USE x == y • NOT x=y (means x becomes the value y)

Equals with objects • Person p 1 = new person(“John”, 55); • Person p 2 = new person(“John”, 55); • == compares memory address!!! • Need to use. equals • • String name 1 = “John”; String name 2 = “John”; name 1==name 2; // give False name 1. equals(name 2); // gives True

Steps to check equality • 1/ is the object null • 2/ is the object the same type as you want e. g. Person, or Percent or Thermometer. . • 3/ now check each of the relevant fields for equality. (usually all fields)

Example Percent public boolean equals(Object object) { if (object == null) { return false; } else if (!(object instanceof Percent )) { return false; } else { Percent percent = (Percent ) object; return this. percent == percent. get. Percent(); } }

Static vs non-Static 1. Static variables are the same for every instance of a class 2. Non-Static variables are different for each instance of a class 3. E. g. we all have an individual blood pressure (e. g. 140 over 100 mm. Hg) non-static 4. But the normal for all of us is 120 over 80 mm. Hg. 5. What about height, weight and indicators of obesity as body mass index. CSCU 9 A 3 - Autumn 2015 21

Object References • Object reference: describes the location of an object • The new operator returns a reference to a new object: Rectangle box = new Rectangle(); • Multiple object variables can refer to the same object: Rectangle box = new Rectangle(5, 10, 20, 30); Rectangle box 2 = box; box 2. translate(15, 25); • Primitive type variables ≠ object variables Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Object Variables and Number Variables Big Java by Cay Horstmann Copyright © 2009 by

Inheritance • Means you can use the methods of the super class. • This saves you time writing more java which is common to a set of classes. • The common methods and data attributes can abstracted out as a super class.

Inheritance e. g. 1 • E. g. in the lecture/tutorial we looked at vehicles. • All vehicles (car, coach, train) have a weight/color/passenger. Capacity/fuel. Type. • Things specific to the subclass can be stored only in that class e. g. for coach • double. Decker = True • has. Toilet = False

Inheritance e. g. 2 • If you are storing information about people, there are commonalities that all people have • E. g. name, nationality, height, date of birth. • For different people you store different information • E. g. Professor class extends Person • String area. Of. Expertize = “blackholes”; • String Chair = “Lucasian ”; • String university = “Cambridge”; • Int number. Books. Published = 23;

Syntax 9. 1 Inheritance Big Java by Cay Horstmann Copyright © 2009 by John

Overriding Methods • Example: deposit and withdraw methods of the Checking. Account class override the deposit and withdraw methods of the Bank. Account class to handle transaction fees: public class Bank. Account {. . . public void deposit(double amount) {. . . } public void withdraw(double amount) {. . . } public double get. Balance() {. . . } } public class Checking. Account extends Bank. Account {. . . public void deposit(double amount) {. . . } public void withdraw(double amount) {. . . } public void deduct. Fees() {. . . } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Overriding Methods • A subclass method overrides a superclass method if it has the same name and parameter types as a superclass method • When such a method is applied to a subclass object, the overriding method is executed Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Overriding Methods • Use the super reserved word to call a method of the superclass: public class Checking. Account extends Bank. Account { public void deposit(double amount) { transaction. Count++; // Now add amount to balance super. deposit } } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Syntax 9. 2 Calling a Superclass Method Big Java by Cay Horstmann Copyright ©

Overriding methods. • • The subclass may override superclass methods. We do not want this to happen in some cases E. g. get. Age To stop overriding say final get. Age To force overriding say abstact get. Age