Object oriented programming. (Lesson 6, part 2) презентация

Object Oriented Programming (part 2)

Agenda Inheritance Fields/Methods in Extended Classes Constructors in extended classes Inherited object construction Overloading and Overriding Methods Polymorphism Type compatibility

Agenda Type conversion protected members Object: the ultimate superclass

Inheritance Inheritance: you can create new classes that are built on existing classes. Through the way of inheritance, you can reuse the existing class’s methods and fields, and you can also add new methods and fields to adapt the new classes to new situations Subclass and superclass have a IsA relationship: an object of a subclass IsA(n) object of its superclass

Inheritance "is a" relationship Inheritance "has a" relationship Composition, having other objects as members Example Employee “is a” BirthDate; //Wrong! Employee “has a” Birthdate; //Composition

Definitions A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class).

Definitions Excepting Object, which has no superclass, every class has one and only one direct superclass (single inheritance).

Definitions Every class is an extended (inherited) class, whether or not it’s declared to be. If a class does not declared to explicitly extend any other class, then it implicitly extends the Object class

Inheritance modifier(s) class ClassName extends ExistingClassName {}

sample classes Superclass public class Person{ private String name; public Person ( ) { name = “no_name_yet”; } public Person ( String initialName ) { this.name = initialName; } public String getName ( ) { return name; } public void setName ( String newName ) { name = newName; }

Inheritance Class hierarchy of previous example Object Person Student

Fields/Methods in Extended Classes An object of an extended class contains two sets of variables and methods fields/methods which are defined locally in the extended class fields/methods which are inherited from the superclass What are the fields for a Student object in the previous example ?

Constructors in extended classes A constructor of the extended class can invoke one of the superclass’s constructors by using the super method. If no superclass constructor is invoked explicitly, then the superclass’s no-arg constructor super( ) is invoked automatically as the first statement of the extended class’s constructor. Constructors are not methods and are NOT inherited.

Three phases of an object’s construction When an object is created, memory is allocated for all its fields, which are initially set to be their default values. It is then followed by a three-phase construction: invoke a superclass’s constructor initialize the fields by using their initializers and initialization blocks execute the body of the constructor The invoked superclass’s constructor is executed using the same three-phase constructor. This process is executed recursively until the Object class is reached

To Illustrate the Construction Order. . .

Overloading and Overriding Methods Overloading: providing more than one method with the same name but different parameter list overloading an inherited method means simply adding new method with the same name and different signature Overriding: replacing the superclass’s implementation of a method with your own design. both the parameter lists and the return types must be exactly the same if an overriding method is invoked on an object of the subclass, then it’s the subclass’s version of this method that gets implemented an overriding method can have different access specifier from its superclass’s version, but only wider accessibility is allowed the overriding method’s throws clause can have fewer types listed than the method in the superclass, or more specific types

Accessibility and Overriding a method can be overridden only if it’s accessible in the subclass private methods in the superclass cannot be overridden if a subclass contains a method which has the same signature as one in its superclass, these methods are totally unrelated package methods in the superclass can be overridden if the subclass is in the same package as the superclass protected, public methods always will be Not as that simple as it seems!

Hiding fields Fields cannot be overridden, they can only be hidden If a field is declared in the subclass and it has the same name as one in the superclass, then the field belongs to the superclass cannot be accessed directly by its name any more

Polymorphism Java allows us to treat an object of a subclass as an object of its superclass. In other words, a reference variable of a superclass type can point to an object of its subclass. when you invoke a method through an object reference, the actual class of the object decides which implementation is used when you access a field, the declared type of the reference decides which implementation is used

Polymorphism Late binding or dynamic binding (run-time binding): Method to be executed is determined at execution time, not compile time. The term polymorphism means to assign multiple meanings to the same method name. In Java, polymorphism is implemented using late binding. These reference variables have many forms, that is, they are polymorphic reference variables. They can refer to objects of their own class or to objects of the classes inherited from their class.

Type compatibility Java is a strongly typed language. Compatibility when you assign the value of an expression to a variable, the type of the expression must be compatible with the declared type of the variable: it must be the same type as, or a subtype of, the declared type null object reference is compatible with all reference types.

Type conversion (1) The types higher up the type hierarchy are said to be wider, or less specific than the types lower down the hierarchy. Similarly, lower types are said to be narrower, or more specific. Widening conversion: assign a subtype to a supertype can be checked at compile time. No action needed Narrowing conversion: convert a reference of a supertype into a reference of a subtype must be explicitly converted by using the cast operator

Type conversion (2)

Type conversion (3) Type testing: you can test an object’s actual class by using the instanceof operactor e.g. if ( obj instanceof String) { String str2 = (String)obj; }

protected members To allow subclass methods to access a superclass field, define it protected. But be cautious! Making methods protected makes more sense, if the subclasses can be trusted to use the method correctly, but other classes cannot.

What protected really means

Protected Example

Object: the ultimate superclass The object class is the ultimate ancestor: every class in Java extends Object without mention Utility methods of Object class equals: returns whether two object references have the same value hashCode: return a hash code for the object, which is derived from the object’s memory address. Equal objects should return identical hash codes clone: returns a clone of the object getClass: return the run expression of the object’s class, which is a Class object finalize: finalize the object during garbage collection toString: return a string representation of the object

The class Object: Equivalent Definition of a Class public class Clock { //Declare instance variables as given in Chapter 8 //Definition of instance methods as given in Chapter 8 //... } public class Clock extends Object { //Declare instance variables as given in Chapter 8 //Definition of instance methods as given in Chapter 8 //... }

final Methods and Classes Declaring variables final Indicates they cannot be modified after declaration Must be initialized when declared Declaring methods final Cannot be overridden in a subclass static and private methods are implicitly final Program can inline final methods Actually inserts method code at method call locations Improves program performance Declaring classes final Cannot be a superclass (cannot inherit from it) All methods in class are implicitly final

This and super keywords