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A type-generic array object

All classes in Java are subclasses of the class named Object . This program creates an array object with the declared type being type Object . An array of type Object can be used to store references to objects instantiated from any class.

After creating the array object, this program instantiates three objects of the class String and stores those object's references in the elements of the array. (As I pointed out in the comments, there is a simpler and better way to instantiate String objects, but it wouldn't illustrate my point as well as doing the way that I did.)

Sometimes you need to downcast

Although an array of type Object can be used to store references to objects of any type (including mixed types and references to other array objects) , you will sometimes need to downcast those references back to their true type once you extract them from the array andattempt to use them for some purpose.

Polymorphic behavior applies here

For this case, however, because the toString method is defined in the Object class and overridden in the String class, polymorphic behavior applies and it is not necessary to downcast the references to type String in order to be able to convert them to strings and display them.

Back to Question 11

Answer 10

C. 0 0 10 2 4

Explanation 10

Defer size specification for secondary arrays

It is not necessary to specify the sizes of the secondary arrays when you create a multi-dimensional array in Java. Rather, since the elements in theprimary array simply contain references to other array objects (or null by default) , you can defer the creation of those secondary array objects until later.

Independent array objects

When you do finally create the secondary arrays, they are essentially independent array objects (except for the requirement for type commonality among them) .

Ragged arrays

Each individual secondary array can be of any size, and this leads to the concept of a ragged array . (On a two-dimensional basis, a ragged array might be thought of as a two-dimensional array where each row can have a different numberof columns.)

This program creates, populates, and displays the contents of such a two-dimensional ragged array. Although this program creates a two-dimensionalarray that is triangular in shape, even that is not a requirement. The number of elements in each of the secondary arrays need have no relationship to the numberof elements in any of the other secondary arrays.

Back to Question 10

Answer 9

B. Runtime Error

Explanation 9

NullPointerException

The following code fragment shows that this program attempts to perform an illegal operation on the value accessed from the array object at index 1.

Integer[] A = new Integer[2]; A[0]= new Integer(1); System.out.println(A[1].intValue());

You can't call methods on null references

The reference value that was returned by accessing A[1] is the default value of null. This is the value that was deposited in the element when the array object was created (no other value was ever stored there) . When an attempt was made to call the intValue method on that reference value, the following runtime error occurred

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
what is titration
John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
what is inorganic
emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
hello friend how are you
Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir 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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