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Objects as parameters

Concept A reference to an object can be an actual parameter whose corresponding formal parameter is declared to be of the same class.As with all parameters, the value of actual parameter is used to initialize the formal parameter, but since it is a reference that ispassed, the method that is called can access fields and methods of the object. This is called reference semantics .

Program: Method07.java

// Learning Object Method07 //    objects as parametersclass Song {     int seconds;      Song(int s) {        seconds = s;     }      double computePrice(double pricePerSecond) {       return seconds * pricePerSecond;     }}  public class Method07 {      static double getPrice(Song s, double ppS) {         return s.computePrice(ppS);    }      public static void main(/*String[] args*/) {        Song song1 = new Song(164);         Song song2 = new Song(103);        double price1 = getPrice(song1, 0.01);         double price2 = getPrice(song2, 0.02);        System.out.println(price1);         System.out.println(price2);    } }

This program computes the cost of a song as the product of its length in seconds and the price per second. A class Song is defined to encapsulate the field seconds and the method computePrice . The method getPrice in the main method receives an object of class Song as a parameter and calls computePrice .

  • Two objects of class Song are instantiated and references to them are assigned to the variables song1 and song2 .
  • The method getPrice is called with two parameters: the first is a reference song1 to an object of class Song , while the second is a value of type double. The actual parameters are used to initialize the formalparameters; check that song1 and s reference the same object.
  • Since the formal parameter s receives a reference to an object of class Song (in this case song1 ), it can be used to call the method computePrice declared in the class.
  • The method returns a value that is assigned to price1 .
  • A second call to the method is executed exactly the same way, except that the actual parameter is the reference contained in song2 .
  • The values of price1 and price2 are printed.

Exercise Modify the program so that discount does not use the explicit parameter s .

Returning objects

Concept A return value can be a reference to an object.

Program: Method08.java

// Learning Object Method08 //    returning objectsclass Song {     int seconds;      Song(int s) {        seconds = s;     }      double computePrice(double pricePerSecond) {       return seconds * pricePerSecond;     }}  public class Method08 {      static Song longer(Song s1, Song s2) {         if (s1.seconds > s2.seconds)             return s1;        else             return s2;    }      public static void main(/*String[] args*/) {        Song song1 = new Song(164);         Song song2 = new Song(103);        Song longerSong  = longer(song1, song2);         double price2 = longerSong.computePrice(0.01);        System.out.println(price2);     }}

This program computes the cost of a song as the product of its length in seconds and the price per second. A class Song is defined to encapsulate the field seconds and the method computePrice . The method longer in the main method receives references to two objects of class Song as parameters and returns a reference to the one with the larger value of the field seconds .

  • Two objects of class Song are instantiated and references to them are assigned to the variables song1 and song2 .
  • The method longer is called with two parameters that are references to objects of class Song . The actual parameters are used to initialize the formalparameters; check that song1 and s1 reference the same object, as do song2 and s2 .
  • Since the formal parameters s1 and s2 receive references to objects of class Song , they can be used to access the fields seconds of each object.
  • The method returns the reference to the object whose field seconds has the larger value. The reference is assigned to the variable longerSong ; check that this reference is to the same object as the reference in song1 .
  • The reference in longerSong is used to call the method computePrice and the value returned is assigned to the variable price2 .
  • The value price2 is printed.

Exercise Modify the program so that discount does not use the explicit parameter s .

Exercise Replace the last declaration and statements of the program by one declaration.

Exercise Write a method to swap two integer values.

Returning locally instantiated objects

Concept When a method terminates, its activation record is deallocated. However, if an object has been instantiated within the method , a reference to the object can be returned to the calling method.

Program: Method09.java

// Learning Object Method09 //    returning locally instantiated objectsclass Song {     int seconds;      Song(int s) {        seconds = s;     }      double computePrice(double pricePerSecond) {       return seconds * pricePerSecond;     }}  public class Method09 {      static Song doubleSong(Song s1) {         Song d = new Song(s1.seconds*2);        return d;     }      public static void main(/*String[] args*/) {         Song song1 = new Song(164);        Song longSong  = doubleSong(song1);     }}

This program computes the cost of a song as the product of its length in seconds and the price per second. A class Song is defined to encapsulate the field seconds and the method computePrice . The method double in the main method receives a reference to an object of class Song as a parameter and returns a reference to an new object of class Song whose field seconds is twice as large.

  • Two objects of class Song are instantiated and references to them are assigned to the variables song1 and song2 .
  • The method doubleSong is called with an actual parameter that is a reference song1 to an object of class Song . The actual parameter is used to initialize the formalparameter; check that song1 and s1 reference the same object.
  • Within the method , the seconds field of the object referenced by s1 is used to instantiate a new object whose reference is assigned to the variable d of class Song .
  • The method returns the reference to the object contained in d ; although d disappears when the activation record is deallocated, the object still exists as does the reference that is returned.
  • The returned reference is assigned to the variable longSong ; check that song1 and longSong reference different objects!

Exercise Replace the last line of the program by: song1 = doubleSong(song1); and explain precisely what happens.

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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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