如何将对象存储在数组中以备后用？

Question

I was curious how to store java objects in an array. I am confused, because my teacher does not know himself, and we have not gone over object arrays, only arrays and objects separately.

My code looks like such:

public class SongDriver
    {
    public static void main( String[] args )
        {
        Song a = new Song();
        a.title = "Freebird";
        a.artist = "Lynyrd Skynryd";

        Song b = new Song();
        b.title = "Sweet Home Alabama";
        b.artist = "Lynyrd Skynryd";

        Song c = new Song();
        c.title = "Black or White";
        c.artist = "Michael Jackson";

        Song d = new Song();
        d.title = "Smooth Criminal";
        d.artist = "Michael Jackson";

        }
    }

This is exactly how my objects are supposed to be according to the teacher.

My other class called song is:

public class Song
    {
    String title;
    String artist; 
    }

I was trying things such as String myPod = new Song[4]; .. But nothing is working, and my knowledge is basic.

Answer 1

An array is a defined container of a given type, for example...

{type}[] {variable};

So, based on the fact that you are trying to create an array of Song , you need to define the array as a type of Song , for example...

Song[] songs;

Of course, you should also initialise it, so based on your example, you could use something like...

Song[] songs = new Song[4];
songs[0] = new Song();
songs[0].title = "Freebird";
songs[0].artist = "Lynyrd Skynryd";

songs[1] = new Song();
songs[1].title = "Sweet Home Alabama";
songs[1].artist = "Lynyrd Skynryd";

songs[2] = new Song();
songs[2].title = "Black or White";
songs[2].artist = "Michael Jackson";

songs[3] = new Song();
songs[3].title = "Smooth Criminal";
songs[3].artist = "Michael Jackson";

Remember, an array is a fixed length container. It can only contain up to the number of elements you specify.

You can also assign values to an element, for example...

Song a = new Song();
a.title = "Freebird";
a.artist = "Lynyrd Skynryd";

songs[0] = a;

Which would be the same as using...

Song a = new Song();
songs[0] = a;
songs[0].title = "Freebird";
songs[0].artist = "Lynyrd Skynryd";

or

songs[0] = new Song();
songs[0].title = "Freebird";
songs[0].artist = "Lynyrd Skynryd";

The 0 element shares the same reference as a , so changing the properties of either a or songs[0] would effect the same object, until either reference was changed...

You can find out more by having a read through the Arrays trail...

Answer 2

you can use ArrayList to store objects in an array.

ArrayList<Song> arraySong = new ArrayList<Song>();

That will create an arraylist of Song object (but with no element currently)

Song a = new Song();
a.title = "Freebird"
a.artist = "Lynyrd Skynryd"
arraySong.add(a);

now the arraylist arraySong has one Song object. and you can do the same for the rest of the Song objects.

Answer 3

Don't use an array unless you can know or derive the number of elements at compile time.

A collection of album tracks is a collection, so I'd use a Collection. Because there is no natural association between an album track and an index, I would use a Set rather than an ArrayList (but you could also use an ArrayList).

If you wrote your Song class to honor the equals() contract and to have a usable hashcode() function (... write your Song class to be immutable, too), then it would be convenient to represent your data as:

Set<Song> mySongs = new HashSet<>();

Even better than this, though, is to create a normalized set of data tables and put this data into a database via JDBC (with Song as your data model object). That would also take care of persistence for you.