OOP_2

• Objects

  • Software components that are created and destroyed dynamically during program execution
  • Instances of their class which defines their type
  • Have a set of variables (fields) representing their state
  • Behaviour determined by a set of instance methods which can be procedures that change their state or functions that return information about their state
  • Interact with each other by sending messages (instance method invocation)
• Class

  • A compilation unit defining the state and behaviour of its objects
  • Contains declarations of fields (or instance variables), constructors, instance methods
  • Allows control of visibility of class members (private or public)
  • Can contain static variables, static methods, static or non-static local classes, static or non-static initialization blocks
• Primitives of the Class Duration

  • Constructors with various arguments like number of seconds, minutes and seconds, hours, minutes, and seconds
  • Methods to add or subtract durations, multiply a duration (by a scalar), compare two durations (equality, inequalities), express a duration in days
• Constructors for class Duration

  • Various arguments for different time units
• Methods for class Duration

  • Adding or subtracting durations
  • Multiplying a duration
  • Comparing two durations
  • Expressing a duration in days, hours, minutes, and seconds
  • Converting a duration to days, hours, minutes
  • Getting a daily time offset from another moment
  • Representing a duration as a string
• Primitives of the Class DailyTime

  • Two distinct constructors for setting hours, minutes, and seconds
  • Functions for testing equality, preceding or following another moment, and comparison
  • Addition or subtraction of a duration
  • Determining time elapsed between two instants
  • Representing a daily time as a string
• The access to a field of an object is written object.fieldName
• Creation of an object is expressed by an expression of the form new ClassName(actualArguments)
• Access to a field of an object

  Written as object.fieldName
• Creation of an object

  Expressed by an expression of the form new ClassName(actualArguments) where the actualArguments must match the formalArguments of an existing creator of the class
• In Java, in most cases, destruction of objects is done implicitly
• Object Organisation in Memory
• Illustration of Aliases


  • Canvas canvas = new Canvas(300, 300);
  • Point point1 = new Point(100, 100);
  • Point point2 = new Point(100, 200);
  • Point point3 = point1;
  • Point point4 = point2.clone();
  • Circle circle1 = new Circle(90, point1, Color.RED);
  • Circle circle2 = new Circle(90, point2, Color.BLUE);
  • Circle circle3 = new Circle(60, point3, Color.YELLOW);
  • Circle circle4 = new Circle(60, point4, Color.GREEN);
  • point3.translate(100, 0);
  • point4.translate(100, 0);
  • circle1.draw(canvas);
  • circle2.draw(canvas);
  • circle3.draw(canvas);
  • circle4.draw(canvas);
• Object types in Java

  • Represented as references to objects, which has several important consequences
  • The comparison operator == only compares the references
  • The assignment only makes a copy of the reference
  • Two variables can refer to the same object (alias)
• Reminder: in Java, all variables (local, static, or instance), as well as all formal parameters (of methods and constructors), are typed
• Sending a message to an object (or invoking an instance method)


  Written as object.methodName(arguments);
• Java distinguishes

  • Primitive types (boolean, byte, char, double, float, int, long, short)
  • Object types
• The garbage collector in Java is responsible for detecting objects that are no longer accessible and freeing up memory space occupied by these objects
• The keyword null is used to represent a reference that doesn’t point to any object
• To avoid NullPointerException, it is recommended to protect the access to a field or the invocation of a method by a conditional statement
• According to Java documentation, when redefining equals(), hashCode() should also return the same values
• To compare the equality of two objects based on field values, you must implement a function equals(other)
• The == operator in Java compares two objects for identity and is not based on their field values
• Using "this" as an argument of a method

  It can be used to return a new object
• According to the Java documentation, when you redefine equals(), you have to make sure that hashCode() also returns the same values; otherwise, you have to redefine it too
• Static Methods

  Methods not associated with an object, declared with the keyword static
• The java.lang.Math class only defines static methods
• Using "this" in a constructor


  Invoking another constructor of the same class
• Using "this" to remove ambiguities

  Distinguishing a field from an argument
• Static Variables (or Class Variables)

  Fields have values specific to each object, while static variables are shared between all objects of a class
• Factory Methods

  Invoking a static method that returns a new object
• Static Methods

  • myMethod
• The keyword "this" designates the current object in a method
• Factory Methods

  • inSeconds
  • inMinutes
  • inHours
• Static Initialization Blocks can be defined with the form static {...} and are executed only once when the class is loaded into memory
• Execution Order of Initialization Blocks

  Static initialization block, Instance initialization block, Constructor
• Static Initialization Blocks are executed before the creation of each object
• To create an application, the main difficulty is to design the types of objects (i.e. the classes)
• Initialization block

  1. Static initialization: System.out.println("Static initialization."); staticVar = 5;

  2. Instance initialization: System.out.println("Instance initialization."); instanceVar = 7;
• Constructor


  System.out.println("Constructor.");