<< Chapter < Page Chapter >> Page >
Base myVar = new Base(); ((A)myVar).test();

Could call test directly on the reference

Having done this, the program could call the method named test directly on the reference variable using a statement such as the following,which is not part of this program.

myVar.test();

This statement would cause the version of the method named test defined in the class named Base to be called, causing the word Base to appear on the standard output device.

This downcast is not allowed

However, this program attempts to cause the version of the method named test defined in the class named A to be called, by downcasting the reference to type A before calling the method named test . This is shown in the following fragment.

((A)myVar).test();

A runtime error occurs

This program compiles successfully. However, the downcast shown above causes the following runtime error to occur under JDK 1.3:

Exception in thread "main" java.lang.ClassCastException: Base at Worker.doIt(Ap126.java:22)at Ap126.main(Ap126.java:15)

What you can do

You can store an object's reference in a reference variable whose type is a superclass of the class from which the object was originally instantiated.Later, you can downcast the reference back to the type (class) from which the object was instantiated.

What you cannot do

However, you cannot downcast an object's reference to a subclass of the class from which the object was originally instantiated.

Unfortunately, the compiler is unable to detect an error of this type. The error doesn't become apparent until the exception is thrown at runtime.

Back to Question 7

Answer 6

C. Base

Explanation 6

Totally straightforward code

This rather straightforward program instantiates an object of the class named Base and assigns that object's reference to a reference variable of the type Base as shown in the following fragment .

Base myVar = new Base(); myVar.test();

Then it calls the method named test on the reference variable.

Class Base defines the method named test

The class named Base contains a concrete definition of the method named test as shown in the following fragment. This is the method that is called by the code shown in the above fragment .

class Base{ public void test(){System.out.print("Base ");}; }//end class Base

Class A is just a smokescreen

The fact that the class named A extends the class named Base , and overrides the method named test , as shown in the following fragment, is of absolutely no consequence in the behavior of this program. Hopefully youunderstand why this is so. If not, then you still have a great deal of studying to do on Java inheritance.

class A extends Base{ public void test(){System.out.print("A "); }//end test()}//end class A

Back to Question 6

Answer 5

A. Compiler Error

Explanation 5

Cannot instantiate an abstract class

This program defines an abstract class named Base . Then it violates one of the rules regarding abstract classes, by attempting to instantiate an object of the abstract class as shown in the following code fragment.

Base myVar = new Base();

The program produces the following compiler error under JDK 1.3:

Questions & Answers

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
Commplementary angles
Idrissa Reply
hello
Sherica
im all ears I need to learn
Sherica
right! what he said ⤴⤴⤴
Tamia
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
im not good at math so would this help me
Rachael Reply
yes
Asali
I'm not good at math so would you help me
Samantha
what is the problem that i will help you to self with?
Asali
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Object-oriented programming (oop) with java' conversation and receive update notifications?

Ask