comparing two collections for comparing two text files for additions, deletions, modifications

Question

I have two collections as below which hold IDs for Students.

The ids are Strings in the format 111-1111. e.g. of ids 221-2534, 215-6365, etc.

 Collec         


        

Answer 1

Overall, I don't think this is the correct approach. Instead of storing all the information in a single String, I would create an object with fields for the various things you need to store.

public Student {
   String id; //or int, or char[8]
   String firstName, lastName;
   String address;
  //and so on

  //constructor - Given a line of input from the data file, create a Student object
  public Student(String line) {
     id = line.substring(0,8);
     //and so on

  }

As for comparing the two collections, let's declare them both as ArrayLists and then keep track of the indices of what they have in common.

ArrayList newKeys = new ArrayList<>();  //java 7 syntax
ArrayList oldKeys = new ArrayList<>();
//store keys from files.

TreeMap commonKeys = new TreeMap();
//stores the index values from newList as keys that get mapped to the old list index.

ArrayList removedKeys =ArrayList<>();  
// Store the indices from oldKeys that are not in newKeys.

int newListIndex = 0;
int oldListIndex = 0;
while(newListIndex < newKeys.size() && oldListIndex0 ) {
      removedKeys.add(oldListIndex);
      oldListIndex++
   }
   else {
      //maybe this is a newListIndex that is not in the old list, so it was added.
      newListIndex++;
   }
}

You will need to tweak the above code a bit to make it fail-safe. Another approach is to use the contains method like this:

for(int i=0; i