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These learning objects explain constructors, which are used to create and initialize objects in Java.

Learning objects for constructors

Concept The process of creating an object involves allocating memory for the object and assigning the reference to this block of memory to a variable. Constructors enable arbitrary initialization of the object during its creation.

These source code of these learning objects can be found in constructor.zip .

LO Topic Java Files (.java) Prerequisites
"What are constructors for?" What are constructors for? Constructor01A, B, C
"Computation within constructors" Computation within constructors Constructor02 1
"Overloading constructors" Overloading constructors Constructor03 2
"Invoking an overloaded constructor from within a constructor" Invoking another constructor Constructor04 3
"Explicit default constructors" Explicit default constructors Constructor05 3
"Constructors for subclasses" Constructors for subclasses Constructor06A, B, C 3
"Constructors with object parameters" Constructors with object parameters Constructor07 3
"Constructors with subclass object parameters" Constructors with subclass
object parameters Constructor08 6, 7

Program The example used in these LOs is class Song with three fields: the name of the song, the length of the song in seconds and the pricePerSecond . The class is to be used to implement a website which charges for downloading the song; theprice is the product of the length of the song in second and the price per second. To focus the discussion on constructors, the fields are notdeclared private.

What are constructors for?

Concept An object is created by allocating memory for its fields. The fields are given the default values for their types. A reference to the object is returned and assigned to a variable; the reference can be used to access the fields and methods of the object.

Program: Constructor01B.java

// Learning Object Constructor01A //    what are constructors for?class Song {     String name;    int seconds;     double pricePerSecond;      public double computePrice() {        return seconds * pricePerSecond;     }}  public class Constructor01A {     public static void main(/*String[] args*/) {         Song song1 = new Song();        song1.name = "Waterloo";         song1.seconds = 164;        song1.pricePerSecond = 0.01;         double price = song1.computePrice();    } }

If a constructor is not explicitly declared a default constructor is called.

  • The variable song1 is allocated and contains the null value.
  • Memory is allocated for the fields of the object; this is displayed in the Instance and Array Area. Default values are assigned to the three fields.
  • The default constructor is called but does nothing except return a reference to the object.
  • The reference is stored in the variable song1 .
  • The reference in song1 is used to assign values to the fields of the object.
  • The reference in song1 is used to call the method computePrice on the object; the method computes and returns the price, which is assigned to the variable price .

Concept An explicit constructor method can be declared and used to initialize each object.The constructor method is identified by a special syntax: the name of the method is the same as the name of the class and there is no return type (because the value returned is of the type of the class itself).

Questions & Answers

explain and give four Example hyperbolic function
Lukman Reply
The denominator of a certain fraction is 9 more than the numerator. If 6 is added to both terms of the fraction, the value of the fraction becomes 2/3. Find the original fraction. 2. The sum of the least and greatest of 3 consecutive integers is 60. What are the valu
1. x + 6 2 -------------- = _ x + 9 + 6 3 x + 6 3 ----------- x -- (cross multiply) x + 15 2 3(x + 6) = 2(x + 15) 3x + 18 = 2x + 30 (-2x from both) x + 18 = 30 (-18 from both) x = 12 Test: 12 + 6 18 2 -------------- = --- = --- 12 + 9 + 6 27 3
2. (x) + (x + 2) = 60 2x + 2 = 60 2x = 58 x = 29 29, 30, & 31
on number 2 question How did you got 2x +2
combine like terms. x + x + 2 is same as 2x + 2
Mark and Don are planning to sell each of their marble collections at a garage sale. If Don has 1 more than 3 times the number of marbles Mark has, how many does each boy have to sell if the total number of marbles is 113?
mariel Reply
Mark = x,. Don = 3x + 1 x + 3x + 1 = 113 4x = 112, x = 28 Mark = 28, Don = 85, 28 + 85 = 113
how do I set up the problem?
Harshika Reply
what is a solution set?
find the subring of gaussian integers?
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Shirley Reply
please can go further on polynomials quadratic
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I need quadratic equation link to Alpa Beta
Abdullahi Reply
find the value of 2x=32
Felix Reply
divide by 2 on each side of the equal sign to solve for x
Want to review on complex number 1.What are complex number 2.How to solve complex number problems.
yes i wantt to review
use the y -intercept and slope to sketch the graph of the equation y=6x
Only Reply
how do we prove the quadratic formular
Seidu Reply
please help me prove quadratic formula
hello, if you have a question about Algebra 2. I may be able to help. I am an Algebra 2 Teacher
Shirley Reply
thank you help me with how to prove the quadratic equation
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what is math number
Tric Reply
x-2y+3z=-3 2x-y+z=7 -x+3y-z=6
Sidiki Reply
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Solve for the first variable in one of the equations, then substitute the result into the other equation. Point For: (6111,4111,−411)(6111,4111,-411) Equation Form: x=6111,y=4111,z=−411x=6111,y=4111,z=-411
x=61/11 y=41/11 z=−4/11 x=61/11 y=41/11 z=-4/11
Need help solving this problem (2/7)^-2
Simone Reply
what is the coefficient of -4×
Mehri Reply
the operation * is x * y =x + y/ 1+(x × y) show if the operation is commutative if x × y is not equal to -1
Alfred Reply
A soccer field is a rectangle 130 meters wide and 110 meters long. The coach asks players to run from one corner to the other corner diagonally across. What is that distance, to the nearest tenths place.
Kimberly Reply
Jeannette has $5 and $10 bills in her wallet. The number of fives is three more than six times the number of tens. Let t represent the number of tens. Write an expression for the number of fives.
August Reply
What is the expressiin for seven less than four times the number of nickels
Leonardo Reply
How do i figure this problem out.
how do you translate this in Algebraic Expressions
linda Reply
why surface tension is zero at critical temperature
I think if critical temperature denote high temperature then a liquid stats boils that time the water stats to evaporate so some moles of h2o to up and due to high temp the bonding break they have low density so it can be a reason
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?
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Source:  OpenStax, Learning objects for java (with jeliot). OpenStax CNX. Dec 28, 2009 Download for free at http://cnx.org/content/col10915/1.2
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