Question

I have a list of objects that contain row data for a table. There are multiple columns in that table but I don’t think that’s an issue. The logic I need to apply would require only two columns. These two columns are known as Chemical Name and Batch Number . Now the interesting part here is that a chemical can occur many times for a particular batch number. For example,

CHEMICAL | BATCH | OTHER COLUMNS

A—————-100———ABC

A—————-100———GHF

A—————-100———GHL

Now what I need is the batch number and its associated rows. I realized that the best way to do this would be HashMap where K is Batch number and V is a List. Now I am not sure how to assign a Key to a List containing row objects.

I wish but I cannot do this at the database level so that is out of question.

Answer 1

Or you could just use guava library. There is Multimap class(technically interface) to fit your need: One key mapping to multiple values.

You might first get all your Row objects in a list, then use the Multimaps.index method to create a Multimap instance. Multimaps is a util class which provide a lot of methods to create a Multimap instance.

Your code maybe looks like:

List<Row> rows = ... // all the row objects.
Multimap<String, Row> rowsMap = Multimaps.index(rows, new Function<Row, String>() {
    @Override
    public String apply(Row row) {
        return row.getRowKey(); // assume that getRowKey() return the key of one row.
    }
});

See more details from guava api.