<< Chapter < Page Chapter >> Page >
java.lang.NullPointerException at Worker.doArrays(Ap057.java:14)at Ap057.main(Ap057.java:6)

This is a common programming error, and most Java programmers have seen an error message involving a NullPointerException several (perhaps many) times during their programming careers.

Back to Question 9

Answer 8

C. OK

Explanation 8

Success at last

This program finally gets it all together and works properly. In particular, after accessing the reference values stored in each of the elements, the programdoes something legal with those values.

Call methods on the object's references

In this case, the code calls one of the public methods belonging to the objects referred to by the reference values stored in the array elements.

System.out.println(A[0].getText() + A[1].getText());

The getText method that is called, returns the contents of the Label object as type String . This makes it possible to perform String concatenation on the values returned by the method, so the program compiles and executes properly.

Back to Question 8

Answer 7

A. Compiler Error

Explanation 7

Java arrays may seem different to you

For all types other than the primitive types, you may find the use of arrays in Java to be considerably different from what you are accustomed to in otherprogramming languages. There are a few things that you should remember.

Array elements may contain default values

If the declared type of an array is one of the primitive types, the elements of the array contain values of the declared type. If you have not initializedthose elements or have not assigned specific values to the elements, they will contain default values.

The default values

You need to know that:

  • The default for numeric primitive types is the zero value for that type
  • The default for the boolean type is false
  • The default for the char type is a 16-bit unsigned integer, all of whose bits have a zero value (sometimes called a null character)
  • The default value for reference types is null , not to be confused with the null character above. (An array element that contains null doesn't refer to an object.)

Arrays of references

If the declared type for the array is not one of the primitive types, the elements in the array are actually reference variables. Objects are neverstored directly in a Java array. Only references to objects are stored in a Java array.

If the array type is the name of a class ...

If the declared type is the name of a class, references to objects of that class or any subclass of that class can be stored in the elements of the array.

If the array type is the name of an interface ...

If the declared type is the name of an interface, references to objects of any class that implements the interface, or references to objects of anysubclass of a class that implements the interface can be stored in the elements of the array.

Why did this program fail to compile?

Now back to the program at hand. Why did this program fail to compile? To begin with, this array was not designed to store any of the primitive types.Rather, this array was designed to store references to objects instantiated from the class named Label , as indicated in the following fragment.

Questions & Answers

how do they get the third part x = (32)5/4
kinnecy Reply
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
hii
Uday
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
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
I'm interested in nanotube
Uday
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
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
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