Question

I have created a Vector object to store data in Table object as Vector<Table>. Vector<Table> contains components as below.

[Vector<Record> records, String tableName, String keyColumnName, int recordCount, int columnCount]

I need to sort tableName in above Vector to my own order and return Vector<Table> with sorted tableNames for other processes.

I have wrote method as below.

private Vector<Table> orderTables(Vector<Table> loadTables) {

    List<String> tableNames = new ArrayList<String>();

    for (Table table : loadTables) {

        String tblName = table.getTableName();
        tableNames.add(tblName);

    }
    Collections.sort(tableNames, new MyComparable());

    return null;
}

But I have no idea about how to write Comparator to this. My own sort order is stored in .properties file. I can read it and get value. But I have no idea about how to compare it.

How could I do it?

Answer 1

Before clarification

You need to write a Comparator for Table objects that delegates to the tableName‘s comparator:

new Comparator<Table>() {
    @Override public int compare(Table one, Table two) {
        return one.getTableName().compareTo(two.getTableName());
    }
}

Note that this will consider Tables that have the same name to be equal. This can mess things up if you put these tables in a HashMap or HashSet. To avoid this, you can detect this case and return one.hashCode() - two.hashCode() if the table names are the same.

Guava’s ComparisonChain is a convenient way to write such multi-stage comparisons:

new Comparator<Table>() {
    @Override public int compare(Table one, Table two) {
        return ComparisonChain.start()
                 .compare(one.getTableName(), two.getTableName())
                 .compare(one.hashCode(), two.hashCode())
                 .result();
    }
}

After clarification

Okay, the question is to impose a predefined sorting order rather than sorting the Tables by name. In that case, you need to make a Comparator that is aware of the ordering defined in the .properties file.

One way to achieve this is to initialize a mapping of table names to sorting order indices, and refer that mapping during the comparison. Given the property value:

SORT_ORDER = SALES,SALE_PRODUCTS,EXPENSES,EXPENSES_ITEMS

The mapping should look like:

{
    SALES: 0,
    SALE_PRODUCTS: 1,
    EXPENSES: 2,
    EXPENSES_ITEMS: 3
}

Here’s what the comparator would look like:

private static class PredefinedOrderComparator implements Comparator<Table> {

    public PredefinedOrderComparator() {

        // Initialize orderIndex here

    }

    private final Map<String, Integer> orderIndex;

    @Override public int compare(Table one, Table two) {
        return orderIndex.get(one.getTableName()) - orderIndex.get(two.getTableName());
    } 

}

To populate orderIndex from the property value, you need to:

1. Get the comma-separated list using getProperty() as you mentioned
2. Split that value on comma (I recommend using Guava’s Splitter, but String.split or others will work too)
3. Initialize a new HashMap<String, Integer> and an int index = 0
4. Iterate through the split tokens, map the current token to index and increment index

Note the implicit assumption that none of the table names have a comma in it.