Beginners Question: Typesafe collection with 1.5

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scherer_mike@web.de - 18 Jun 2006 10:51 GMT

dear all,
i have a little problem of understanding and i am not able to get out
of it:
et voilà the snippets:

with java 1.4 i wrote:
// -------------------------------------------------------
myclass cc = ...;
...
List myList;
myList = new ArrayList();
...
myList.add(cc);

same with 1.5 to make it typesafe:
// -------------------------------------------------------
myclass cc = ...;
...
List < myclass > myList;
myList = new < myclass > ArrayList();
...
myList.add(cc);

so far, so good ... but now the snippet with 2 dimensional array of
ArrayList with 1.4
// -------------------------------------------------------
myclass cc = ...;
...
List myTable[][] ;
mytable = new ArrayList[n][n];
...
myTable[a][b].add(cc);

this works well with 1.4. but this last snippet typesafe with 1.5 ... i
cannot solve! Whereever i place the < type > thingy in the source, i
get an error message.

could you advise me, please?

thank you very much,
Michael

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Thomas Hawtin - 18 Jun 2006 11:36 GMT

> myclass cc = ...;
> ....
[quoted text clipped - 6 lines]
> cannot solve! Whereever i place the < type > thingy in the source, i
> get an error message.

List<List<List<MyClass>>> myTable =
new ArrayList<List<List<MyClass>>>();

Prefer collections to arrays of references.

You are probably better off introducing some sort of table class, rather
than working with little abstraction.

Tom Hawtin

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Unemployed English Java programmer
http://jroller.com/page/tackline/

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scherer_mike@web.de - 18 Jun 2006 16:05 GMT

Thomas Hawtin schrieb:

> > myclass cc = ...;
> > ....
[quoted text clipped - 9 lines]
> List<List<List<MyClass>>> myTable =
> new ArrayList<List<List<MyClass>>>();

hi tom,

thank you very much. but it does not work this way.
in my example i would have better written
mytable = new ArrayList[N][M] for a fix N, M.
with your hint i get the compiler-message: "The type of the expression
must be an array type but it resolved to List<List<List<mmCode>>>."

but in general i agree: the data abstraction is poor. i am also a
novice in this kind of object-modelling.

thank you very much

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Oliver Wong - 19 Jun 2006 22:17 GMT

> Thomas Hawtin schrieb:
>
[quoted text clipped - 22 lines]
> but in general i agree: the data abstraction is poor. i am also a
> novice in this kind of object-modelling.

Have you considered creating a new class to represent your tables?

class Table<T> {
final private FixedLengthList<FixedLengthList<T>> data;
final private int width, height;

public Table(int width, int height) {
this.data = new FixedLengthList<List<T>>(width);
this.width = width;
this.height = height;
}

public T get(int x, int y) {
this.data.get(x).get(y);
}
/*put more code here*/
}

class FixedLengthList<T> extends AbstractList<T> {
private final int length;

public FixedLengthList(int length) {
this.length = length;
}

/*put more code here*/
}

- Oliver

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Beginners Question: Typesafe collection with 1.5

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