ITEC 120 2010fall ibarland

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lect12c
arrays of objects

 - discuss hw09-soln.java


   [Files before: lect12c-playlist/;]
 - An array of Songs: Show the pictures involved with:
new Song[10]
Song[] album;

album = new Song[10];
album[0] = new Song(...);
album[1] = new Song(...);
...

// Write a loop to find the total play-time of all Songs:
double timeSoFar = 0.0;
for (int i=0;  i < album.length;  ++i) {
  timeSoFar += album[i].length;
  }

// N.B. Don't confuse album.length with album[i].length -- 
//    entirely different beasts!  
// (And, it was dumb luck those fields have the same name.
//   Still, a common confusion is writing things like
//   `album.isCopyrighted` or `i.artist`.)

 - Aliasing:
   Consider a variable songOfTheYear.
   What happens when we `songOfTheYear.setIsCopyrighted(false)` --
   note that the array doesn't change, but the things it refers to *do* change.
   "When Barack Obama dyes his hair blue, so does The President of the U.S."

   **** Reached here, Nov.19 ****



 - Create a class which *contains* an array of Song,
     as well as has its own name, and a creator field.
   Task: write *Playlist* methods for total running time;
     write a method containsCopyrighted.

 - Another possible example:
   A Brood has a hideout(String), a name, and an array of all its Vampire members.
   We could have not only a variable "vampireOfTheMonth",
   but even an array of past winners.


 - swapping two elements of an array.
 - We can write a method swapArrayElements( double[], int, int )
   but not swapLocalVariables(int,int).
   Why not?
   The picture explains all!

   Assigning through a reference behaves *very differently*
   from assigning to a local variable.
   (Some languages have two different names for the assignment operator "=" --
   one which can be used for local variables, and one which can
   be used for a [reference-to-an-]objects's fields.
   

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©2010, Ian Barland, Radford University Last modified 2010.Nov.19 (Fri)

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