Java – Comparing two Collections in Java

collectionsjava

I have two Collections in a Java class.The first collection contains previous data, the second contains updated data from the previous collection.

I would like to compare the two collections but I'm not sure of the best way to implement this efficiently.Both collections will contain the same amount of items.

Based then on the carType being the same in each collection I want to execute the carType method.

Any help is appreciated

Best Solution

Difficult to help, because you didn't tell us how you like to compare the (equal-size) collections. Some ideas, hoping one will fit:

Compare both collections if they contain the same objects in the same order

Iterator targetIt = target.iterator();
for (Object obj:source)
  if (!obj.equals(targetIt.next()))
    // compare result -> false

Compare both collections if they contain the same objects in the any order

for (Object obj:source)
  if (target.contains(obj))
    // compare result -> false

Find elements in other collection that has changed

Iterator targetIt = target.iterator();
for (Object obj:source)
  if (!obj.equals(targetIt.next())
    // Element has changed

Based on your comment, this algorithm would do it. It collects all Cars that have been updated. If the method result is an empty list, both collections contain equal entries in the same order. The algorithm relies on a correct implementation of equals() on the Car type!

public List<Car> findUpdatedCars(Collection<Car> oldCars, Collection<Car> newCars)
  List<Car> updatedCars = new ArrayList<Car>();
  Iterator oldIt = oldCars.iterator();
  for (Car newCar:newCars) {
    if (!newCar.equals(oldIt.next()) {
      updatedCars.add(newCar);
    }
  }
  return updatedCars;
}