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Слайды и текст к этой презентации:
№4 слайд
Содержание слайда: 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
№10 слайд
Содержание слайда: 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;
}
№12 слайд
Содержание слайда: 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 ?
№13 слайд
Содержание слайда: 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.
№14 слайд
Содержание слайда: 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
№16 слайд
Содержание слайда: 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
№17 слайд
Содержание слайда: 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!
№21 слайд
Содержание слайда: 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
№22 слайд
Содержание слайда: 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.
№24 слайд
Содержание слайда: 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.
№25 слайд
Содержание слайда: 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
№31 слайд
Содержание слайда: 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
№32 слайд
Содержание слайда: 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
//...
}
№33 слайд
Содержание слайда: 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
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