Posts Tagged ‘functional’

Java anonymous classes are too verbose

2010-11-11

Java doesn’t have first-class functions or closures, but you can emulate some of that with anonymous classes. Alas, they are just too cumbersome and verbose—it’s not elegant anymore if you need more LoC than with the iterative loop.

Recently at work, I wanted to execute some code for each member of a List, and I needed to know which iteration step I was at. A straightforward solution is, obviously, the classic for loop:


for (int i = 0; i < keyword.getSynonyms(); i++) {
    String synonym = keyword.getSynonyms().get(i);
    // do something
}

That’s fine in many cases, but it has two problems. The extra line to get at the List element is annoying. More importantly, depending on the List implementation the get(i) operation might be in O(n), requiring another scan of the list each time.

So we could iterate normally and count ourselves:


int order = 0;
for (String synonym : keyword.getSynonyms())
{
    // do something
    order++;
}

It’s better, but I needed several such loops, and I wondered if I couldn’t write all that plumbing just once. I came up with this:


public abstract class Counting<T, E extends Throwable>
{
    public void loop(Iterable<T> things) throws E
    {
        int step = 0;
        for (T t : things)
        {
            iteration(t, step);
            step++;
        }
    }

    public abstract void iteration(T thing, int step) throws E;
}

As the loop body can throw exceptions, and we might want to declare the specific kind of exception, this needs to be an additional generic type. This breaks if you need more than one exception type.

The above loop then becomes


new Counting<String, SQLException>()
{
    @Override public void iteration(String synonym, int step)
        throws SQLException
    {
        // do something
    }
}.loop( keyword.getSynonyms() );

Hmmm. Even after writing an abstract class to extract the repeated parts, and not counting my preference for opening-brace-on-new-line, I still haven’t saved a single line. Can I have map and first-class functions, please? Time for Scala?


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