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So, what is the twist?

Unlike final member variables of a class, the Java compiler and runtime system do not require you to initialize a final local variable when you declare it. Rather, you can wait and assign a value to it later. (Some authors refer to this as a blank final.) However, once you have assigned a value to a final local variable, you cannot change that value later.

The bottom line

Whether you initialize the final local variable when you declare it, or assign a value to it later, the result is the same. It behaves as a constant.The difference is that if you don't initialize it when you declare it, you cannot access it until after you assign a value to it.

Back to Question 3

Answer 2

F. 20, 10

Explanation 2

Calling a parameterized constructor

This is a relatively straightforward implementation of the use of the super keyword in a subclass constructor to call a parameterized constructor in the superclass.

The interesting code in the program is highlighted in the following fragment. Note that quite a lot of code was deleted from the fragment for brevity.

class Superclass{ //...public Superclass(int x){//... }//end constructor//...}//end Superclass class Subclass extends Superclass{//...public Subclass(){ super(20);//... }//end constructor//...}//end Subclass

Using the super keyword

The code that is of interest is the use of super(20) as the first executable statement in the Subclass constructor to call the parameterized constructor in the superclass, passing a value of 20 as aparameter to the parameterized constructor.

Note that when the super keyword is used in this fashion in a constructor, it must be the first executable statement in the constructor.

As before, the program plays around a little with initial values for instance variables to see if you are alert, but the code that is really of interest ishighlighted in the above fragment.

Back to Question 2

Answer 1

D. 5, 10

Explanation 1

The execution of constructors

The purpose of this question and the associated answer is to illustrate explicitly what happens automatically by default regarding the execution ofconstructors.

The Subclass constructor

This program defines a class named Subclass , which extends a class named Superclass . A portion of the Subclass definition, including its noarg constructor is shown in the following code fragment. (The class also defines a getter method, which was omitted here for brevity.)

class Subclass extends Superclass{ private int y = 2;public Subclass(){super(); y = 10;}//end constructor//... }//end Subclass

The super keyword

The important thing to note in the above fragment is the statement containing the keyword super .

The super keyword has several uses in Java. As you might guess from the word, all of those uses have something to do with the superclassof the class in which the keyword is used.

Invoke the superclass constructor

When the super keyword (followed by a pair of matching parentheses) appears as the first executable statement in a constructor, this is an instruction tothe runtime system to first call the constructor for the superclass, and then come back and finish executing the code in the constructor for the class towhich the constructor belongs.

Call the noarg superclass constructor

If the parentheses following the super keyword are empty, this is an instruction to call the noarg constructor for the superclass.

Invoke a parameterized superclass constructor

If the parentheses are not empty, this is an instruction to find and call a parameterized constructor in the superclass whose formal arguments match the parameters in the parentheses.

Invoke the noarg superclass constructor by default

Here is an important point that is not illustrated above. If the first executable statement in your constructor is not an instruction to call the constructor for the superclass, an instruction to call the noarg constructor for the superclass will effectively be inserted into yourconstructor code before it is compiled.

Therefore, a constructor for the superclass is always called before the code in the constructor for your new class is executed.

You can choose the superclass constructor

The superclass constructor that is called may be the noarg constructor for the superclass, or you can force it to be a parameterizedconstructor by inserting something like


as the first instruction in your constructor definition.

Always have a noarg constructor ...

Now you should understand why I told you in an earlier module that the classes you define should almost always have a noarg constructor, either the default noarg version, or a noarg version of your own design.

If your classes don't have a noarg constructor, then anyone who extends your classes will be required to put code in the constructor for theirnew class to call a parameterized constructor in your class.

In this program, the super(); statement in the Subclass constructor causes the noarg constructor for the Superclass to be called. That noarg constructor is shown in the following code fragment.

class Superclass{ private int x = 1;public Superclass(){x = 5; }//end constructor//...}//end Superclass

Additional code

Beyond an exposure and explanation of the use of the super keyword to call the superclass constructor, this program plays a few games with initial valuesof instance variables just to see if you are alert to that sort of thing. However, none of that should be new to you, so I won't discuss it further here.

Back to Question 1


Questions & Answers

how can chip be made from sand
Eke Reply
is this allso about nanoscale material
are nano particles real
Missy Reply
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
where is the latest information on a no technology how can I find it
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
has a lot of application modern world
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Berger describes sociologists as concerned with
Mueller Reply
what is hormones?
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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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