Pi j2.2 classes
- 1. Programming in Java 5-day workshop OOP Classes Matt Collison JP Morgan Chase 2021 PiJ2.2: OOP Classes
- 2. Four principles of OOP: 1.Encapsulation – defining a class •Hiding all but the required components •Providing modularity 2.Abstraction – modifying the objects 3.Inheritance – extending classes 4.Polymorphism – implementing interfaces
- 3. Classes and objects • Classes are made up of: • Attributes – fields to describe the class • Constructors – constrain how you can create the object • Methods – functions to describe it’s behaviour
- 4. Defining your first class public class Rectangle{ // Attributes public double width; public double height; final static int sides =4; // Constructors public Rectangle(double w, double h){ ... } // Methods public double getArea() { ... } }
- 5. Attributes/fields • For the Rectangle class, each rectangle object has different values for the following attributes. double width; double height; double originX=0.0; // need to be added double originY=0.0; • How about the number of sides? final int NUMBER OF SIDES = 4; • This attribute has the same value for all rectangles.
- 6. Attributes Attributes could be divided into: 1. Instance attributes: belongs to objects. • Individual value for each object even if the values are the same. 2. static attributes: belongs to the class. • common property of all objects (that is not unique for each object) • Through static keywords: e.g. static final int NUMBER OF SIDES = 4;
- 7. Methods • Methods are well-defined sets of instructions to perform a specific task. public void move(double dx, double dy) { originX += dx; originY += dy; } public double getArea() { return width*height; } Input data: known as method parameters or arguments Output data: through return statement. If the return type is void, no need to include a return statement. .
- 8. Methods • A method can access: • all of that class’s attributes (static and instance) • local variables • the parameters passed to it • variables declared within the method itself • Scope of local variables: • Exists only while the method is being invoked
- 9. Overloading methods • Java supports overloading methods: • i.e., two/more methods share the same name, but with different method arguments. public void zoom(double factor){...} public void zoom(double factorX, double factorY){...}
- 10. Static methods Methods could be divided into: • Instance methods: belongs to objects. • Static methods: belongs to the class. • Through static keywords. • The main() method is a static method. Q: Should the following methods instance or static methods? 1. void zoom(double factor) //instance method 2. double getArea() //instance method 3. boolean isOverlapped(Rectangle r1, Rectangle r2) //static method 4. boolean isOverlappedWith(Rectangle r2) //instance method
- 11. Constructors A Java constructor is invoked at the time of object creation, initialisation. It initializes data for the object within some rules: • Constructor name must be same as its class name. • Constructor must have no explicit return type. public class Rectangle{ public Rectangle(){ //constructor logic } … }
- 12. Default constructors • Constructors can be overloaded. • A class can have no constructor. • In this case, the compiler implicitly create a default no-arg constructor as below. public class Point { public double x; public double y; } public class Point { public double x; public double y; public Point(){ } }
- 13. Rectangle.java (file available on gh-pages) // A Rectangle class public class Rectangle { // 4 instance attributes public double width; public double height; public double originX = 0.0; public double originY = 0.0; // 1 static attributes public static final int NUMBER_OF_SIDES = 4; // method: move the rectangle public void move(double dx, double dy) { originX += dx; originY += dy;
- 14. Learning resources The workshop homepage https://mcollison.github.io/JPMC-java-intro-2021/ The course materials https://mcollison.github.io/java-programming-foundations/ • Session worksheets – updated each week
- 15. Additional resources • Think Java: How to think like a computer scientist • Allen B Downey (O’Reilly Press) • Available under Creative Commons license • https://greenteapress.com/wp/think-java-2e/ • Oracle central Java Documentation – https://docs.oracle.com/javase/8/docs/api/ • Other sources: • W3Schools Java - https://www.w3schools.com/java/ • stack overflow - https://stackoverflow.com/ • Coding bat - https://codingbat.com/java