Java object-oriented chapter III

One, polymorphic "Subclass is the parent class"

1. Why Use polymorphic

Increase program extensibility, maintainability, and code reusability

2, what is polymorphic "P71"

Features that exhibit a variety of morphological capabilities the same implementation interface, using different instances to perform different operations

3. How to use polymorphic

The parameter type of the L method is the parent class

Access modifier return type method name (parent class name Object name) {

.......................................................

}

When calling a method, you can pass the object of the child class

3.1 Parent (Child) class Name object = new subclass name ();

3.2 Method Name (subclass object);

The return type of the 3.3 method is the parent class

Access modifier parent class name Method name ([parameter list]) {

Return parent class object or subclass object;

}

//Subclass, Round Public classCircular extends Graph {PrivateFinalDoublex=3.14;//radius                 PublicCircular (DoubleR)        {super (R); }                 Public voidGetarea () {DoubleM= (Super.getlen () *x*y); System. out. println ("the radius of the circle is"+super.getlen () +"\ n area of"+m); }

//sub-class, rectangular Public classRectangle extends Graph {Private DoubleWeight//width         PublicRectangle (DoubleLenDoubleweight)        {super (LEN);  This. weight=weight; }         Public Doublegetweight () {returnweight; }         Public voidGetarea () {DoubleM= (Super.getlen () * This. Getweight ()); System. out. println ("the length of the rectangle is"+super.getlen () +", wide for"+ This. weight+"\ n area of"+m); }

//Parent Class Public Abstract classGraph {Private DoubleLen//side length of graphs         PublicGraph (DoubleLen) {         This. len=Len; }        //Get length     Public DoubleGetlen () {returnLen; }        //abstract Approach-oriented     Public Abstract voidGetarea ();}

4. Conversion between polymorphic Neutron class and parent class

a) conversion of subclasses to parent class (up conversion)

Parent class Object name = new subclass ();//Automatic conversion

The name of the object. Method name ([value]);

Use rule "P72" for upward conversion:

The parent class refers to a subclass object when referred to as up conversion or transformation (upcasting), and is automatically type converted

L refer to a variable through the parent class, called by a subclass overriding (overriding) or inheriting the parent class method "Choice 1, 2"

L reference variable through parent class, cannot call subclass-specific method "Select Question 3"

b) Conversion of the parent class to a subclass (down conversion)--coercion of type conversions, restoring the true face of the object (which seed class is specific)

Parent Class object name = new Subclass ();

Object name. Subclass method ();//Compile Error

should be modified to:

Parent class Object name = new Subclass ();

Subclass sub-Class object = (subclass) object name;

Subclass object. Subclass Method ();

//Parent Class Public Abstract classZhu {PrivateString name;//the names of the two    PrivateString Food; PrivateString Nation; /** * @return the name*/     PublicString GetName () {returnname; }     PublicString getnation () {returnNation; }     PublicZhu (String name,string food,string Nation) { This. name=name;  This. food=Food ;  This. nation=Nation; }     PublicString Getfood () {returnFood ; }     Public Abstract voidprint ();} Parent class

Parent Class

//sub-class Public classYuehan extends zhu{PrivateString habit;  PublicYuehan (String name,string habit,string food,string Nation) {super (name,food,nation);  This. habit=habit; }         PublicString gethabit () {returnhabit; }         Public voidprint () {System. out. println ("My name"+super.getname () +", from"+super.getnation () +", like to eat"+ This. Getfood () +", hobbies are playing"+ This. Gethabit ()); }}

sub-class

Java object-oriented chapter III