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Label[] A = new Label[2];

Elements initialized to null

This is a two-element array. When first created, it contains two elements, each having a default value of null . What this really means is that the reference values stored in each of the two elements don't initially refer toany object.

Populate the array elements

The next fragment creates two instances (objects) of the Label class and assigns those object's references to the two elements in the arrayobject. This is perfectly valid.

A[0] = new Label("O");A[1] = new Label("K");

You cannot add reference values

The problem arises in the next fragment. Rather than dealing with the object's references in an appropriate manner, this fragment attempts toaccess the text values of the two reference variables and concatenate those values.

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

The compiler produces the following error message:

Ap055.java:14: error: bad operand types for binary operator '+' System.out.println(A[0]+ A[1]);^ first type: Labelsecond type: Label 1 error

This error message is simply telling us that it is not legal to add the values of reference variables.

Not peculiar to arrays

This problem is not peculiar to arrays. You would get a similar error if you attempted to add two reference variables even when they aren't stored in anarray. In this case, the code to access the values of the elements is good. The problem arises when we attempt to do something illegal with those values afterwe access them.

Usually two steps are required

Therefore, except in some special cases such as certain operations involving the wrapper classes, to use Java arrays with types other than the primitive types, whenyou access the value stored in an element of the array (a reference variable) you must perform only those operations on that reference variable that are legalfor an object of that type. That usually involves two steps. The first step accesses the reference to an object. The second step performs some operation onthe object.

Back to Question 7

Answer 6

C. 3

Explanation 6

Once you create an array object for a primitive type in Java, you can treat the elements of the array pretty much as you would treat the elements of anarray in other programming languages. In particular, a statement such the following can be used to assign a value to an indexed element in an arrayreferred to by a reference variable named A .

A[1] = 2;

Similarly, when you reference an indexed element in an expression such as the following, the value stored in the element is used to evaluate the expression.

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

For all Java arrays, you must remember to create the new array object and to store the array object's reference in a reference variable of the correct type. Then you can use the reference variable to gain access to the elements inthe array.

Back to Question 6

Answer 5

B. Runtime Error

Explanation 5

Good fences make good neighbors

One of the great things about an array object in Java is that it knows how to protect its boundaries.

Unlike some other currently popular programming languages, if your program code attempts to access a Java array element outside its boundaries, anexception will be thrown. If your program doesn't catch and handle the exception, the program will be terminated.

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket 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
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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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
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