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Computation within constructors

Concept Constructors are often used simply for assigning initial values to fields of an object; however, an arbitrary initializing computation can be carried out within the constructor.

Program: Constructor02.java

// Learning Object Constructor02 //    computation within constructorsclass Song {     String name;    int seconds;     double pricePerSecond;    double price;      Song(String n, int s, double p) {         name = n;        seconds = s;         pricePerSecond = p;        price = computePrice();     }      private double computePrice() {        return seconds * pricePerSecond;     }}  public class Constructor02 {     public static void main(/*String[] args*/) {         Song song1 = new Song("Waterloo", 164, 0.01);    } }

The price of a song will not change as long as the fields second and pricePerSecond do not change; to avoid recomputing the price each time it is needed, the class contains a field price whose value is computed within the constructor. The method computePrice is declared to be private because it is needed only by the constructor.

  • The variable song1 is allocated and contains the null value.
  • Memory is allocated for the fields of the object and default values are assigned to the three fields.
  • The constructor is called with three actual parameters; these values are assigned to the formal parameters of the constructor method and the values of the formal parameters are assigned to the three fields.
  • The method computePrice is called; it returns a value which stored in the field price .
  • The constructor returns a reference to the object, which is stored in the variable song1 . The field price can be accessed to obtain the price of a song.

Exercise Modify the class so that no song has a price greater than two currency units.

Overloading constructors

Concept Constructors can be overloaded like other methods. A method is overloaded when there is more than one method with the same name; the parameter signature is used to decide which method to call. For constructors, overloading is usually done when some of the fields of an object can be initialized with default values, although we want to retain the possibility of explicitly supplying all the initial values.

Program: Constructor03.java

// Learning Object Constructor03 //    overloading constructorsclass Song {     String name;    int seconds;     double pricePerSecond;    double price;     final static double DEFAULT_PRICE = 0.005;      Song(String n, int s, double p) {        name = n;         seconds = s;        pricePerSecond = p;         price = computePrice();    }      Song(String n, int s) {         name = n;        seconds = s;         pricePerSecond = DEFAULT_PRICE;        price = computePrice();     }      private double computePrice() {        return seconds * pricePerSecond;     }}  public class Constructor03 {     public static void main(/*String[] args*/) {         Song song1 = new Song("Waterloo", 164, 0.01);        Song song2 = new Song("Fernando", 253);     }}

The website charges a uniform price per second for all songs, except for special offers. We define two constructors, one that specifies a price for special offers and another that uses a default price for ordinary songs.

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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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