Data transformation

Question

I have data composed of a list of employers and a list of workers. Each has a many-to-many relationship with the other (so an employer can have many workers, and a worker can have many employers).

The way the data is retrieved (and given to me) is as follows: each employer has an array of workers. In other words:

employer n has:
  worker x, worker y etc.

So I have a bunch of employer objects each containing an array of workers.

I need to transform this data (and basically invert the relationship). I need to have a bunch of worker objects, each containing an array of employers. In other words:

worker x has:
  employer n1, employer n2 etc.

The context is hypothetical so please don't comment on why I need this or why I am doing it this way. I would really just like help on the algorithm to perform this transformation (there isn't that much data so I would prefer readability over complex optimizations which reduce complexity). (Oh and I am using Java, but pseudocode would be fine).

Answer 1

You want a Map<Worker,Set<Employer>> for this, or possibily a Multimap<Worker,Employer> from Guava.

In pseudocode:

initialize employerMap(worker => set of employers)
for every employer n do
    for every worker x in n.workers do
         employerMap[x].add(n)

Essentially a Worker can be mapped to multiple Employer, so you either:

• Have a Map<Worker,Set<Employer>>
  • each key in a Map can only have one value, so the value in this case is a Set of values
• Have a Multimap<Worker,Employer>
  • Multimap can map a key to multiple values

---

Example

Here's an example of doing the mapping. Note that storing the data in Object[][] arr like this is definitely not recommended, and is only used as part of the example. Concentrate on the actual mapping part.

import java.util.*;
public class MultiMapExample {
    public static void main(String[] args) {
        Object[][] arr = {
            { "IBM",    new String[] { "Joe", "Jack", "Carol"   }},
            { "MS",     new String[] { "Jack", "Andy", "Carol" }},
            { "Google", new String[] { "Bob", "Alice", "Carol"  }},
        };
        Map<String,Set<String>> employerMap =
            new HashMap<String,Set<String>>();

        for (Object[] data : arr) {
            String employer = (String) data[0];
            String[] workers = (String[]) data[1];
            for (String worker : workers) {
                Set<String> employers = employerMap.get(worker);
                if (employers == null) {
                    employerMap.put(worker, employers = new HashSet<String>());
                }
                employers.add(employer);
            }
        }

        for (String worker : employerMap.keySet()) {
            System.out.println(worker + " works for " + employerMap.get(worker));
        }
    }
}

This prints (order may vary):

Alice works for [Google]
Jack works for [IBM, MS]
Bob works for [Google]
Andy works for [MS]
Carol works for [IBM, Google, MS]
Joe works for [IBM]

I recommend keeping the data in a Map like this, but if you must transform the list of employees to an array for some reason, you can use Collection.toArray(T[]).

You should generally prefer List and other Java Collections Framework classes to arrays, though.

---

On implementing equals, hashCode, etc

The above example uses String for simplicity. You probably should have an actual Worker and Employer types instead, which is a good thing. You have to make sure that they implement equals and hashCode properly, though.

See also

• Effective Java 2nd Edition
  • Item 8: Obey the general contract when overriding equals
  • Item 9: Always override hashcode when you override equals

API links

• Object.equals(Object)
• Object.hashCode()
• java.lang.Comparable - not needed here, but another important Java API contract

Related questions

On equals/hashCode combo:

• Overriding equals and hashcode in Java
• Why both hashCode() and equals() exist
• How to ensure hashCode() is consistent with equals()?

On equals vs ==:

• Difference between equals and ==
• why equals() method when we have == operator?
• Java String.equals versus ==

Answer 2

Not java-specific, but here's the approach:

• make a dictionary of <Worker, Set<Employer>>,
• iterate through all the workers of each employer, and
• for every worker, add the current employer to the Dict[Worker] set

You'll need to add a check to create the set the first time each worker is found.