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lect06b
Objects: fields
Songs, Robots cont.

We continue our exploration of objects from the previous lecture (and lab).

Review: Where do you put fields, in the class-file?;
what is different about a constructor -- what name to use when calling constructor?;
when writing the signature, three differences from normal signatures?

To access an object's fields

We access fields using the . (“dot”) operator.

For example: If we have a variable s of type Song, then we can refer to s.length, s.title, etc..

The general syntax is:

field-access ::= expr_object.name_field

(The subscripts are just a reminder -- the technical syntax would be expr.name.)

We'll see later, how the expr_object doesn't need to be just a variable (or parameter), though often it is.

Functions taking in objects

Song.fitsOnDisk: Will a given song fit into a given amount of free-disk space? (Assume 1MB/min.)

starting code for lecture; finished code from lecture.
You try: Robot.currentForm(Robot)
starting code for lecture; finished code from lecture.

Functions returning objects

Song.longerOf(Song,Song): take in two Songs and return the one ¹ that's longer.
starting code for lecture; finished code from lecture.

You try: Robot.coolerOf(Robot)

    /** Which of two robots is cooler?
     * @param bot1 The first Robot.
     * @param bot2 The second Robot.
     * @return Either bot1 or bot2, depening on who is cooler.
     * A robot with a *shorter* transformed-shape description is cooler;
     * in case of ties, the evil one is cooler.
     */

starting code for lecture; finished code from lecture.

Song.mashup(Song,Song): make a new song which is a combination of the old:
the length is the sum of the two original lengths,
the artist is the original artists with “vs.”;
the title is perhaps a blend of the two original titles (the first half of the first title and the second half of the second title);
it is not copyrighted.

You try: Robot.evilCopy(Robot):

    /** Make a new, evil copy of a given Robot.
     * @param aBot The Robot to make an evil copy of.
     * @return a new, evil copy of `aBot`.
     * (Evil copies have the original's serial number but
     * with a '-E' concatenated; they are always evil;
     * they have the same disguise and transformation status.
     */

(See the links above.)

¹ Hopefully we get the question, after making test cases: What to do in case of a tie? Note that Java forces us to return a Song, and exactly one Song, by our signature. In light of that, we'll say longerOf can return either one in case of a tie. ↩

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©2010, Ian Barland, Radford University
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lect06b Objects: fieldsSongs, Robots cont.

To access an object's fields

Functions taking in objects

Functions returning objects

lect06b
Objects: fields
Songs, Robots cont.