Find similar integers inside of an ArrayList with Class Method

Question

I need to iterate through and ArrayList of objects and look for a variable that has the same value for multiple objects. As the example below, I am looking through an ArrayList, that has a sub-class . All I want to do is find out if a house or condo share the same listing number for this example code. I tried using a double loop, and an enhanced loop with a method (sameListingNum) and am having trouble working it out.

Thank you

// main client class
public static void main(String[] args) 
    {
      ArrayList<House> listings = new ArrayList();

      listings.add(new House(0001, 200000.00));
      listings.add(new House(0201, 200000.00));
      listings.add(new House(0001, 200000.00));
      listings.add(new House(0401, 200000.00));
      listings.add(new House(0031, 200000.00));
      listings.add(new Condo(0401, 200000.00, 4));
      listings.add(new Condo(0001, 120000.00, 3));
      listings.add(new Condo(0301, 220000.00, 2));
      listings.add(new Condo(0001, 130000.00, 3));
      listings.add(new Condo(0201, 130000.00, 3));

      for(House currentHouse: listings)
        System.out.println(currentHouse);
      for(int i=0; i<listings.size()-1; i++)
      {
        for(int j=i+1; j<listings.size(); j++)
        {

        }
      } 

// House Class
public class House 
{
  public int listingNum;
  public double price; 

  public House()
  {
    listingNum = 0; 
    price = 0.00; 
  }  
  public House(int newListingNum, double newPrice)
  {
    listingNum = newListingNum;
    price = newPrice; 
  }  
  public int getListingNum()
  {
    return listingNum;
  }  
  public double getPrice()
  {
    return listingNum; 
  }
  public String toString()
  {
    return ("Listing number: "+listingNum+", Price: "+price);
  }
  public boolean sameListingNum(Object other)
  {
    if(!(other instanceof House))
      return false; 
    else {
      House objHouse = (House)other;
      if(listingNum - objHouse.getListingNum() == 0)
      {
        System.out.println("Same listing numbers: "
        +listingNum+", "+objHouse.getListingNum());
        return true;
      }
      else 
        return false;
    }
  }
}

// Condo Class
public class Condo extends House
{
  public int connectedUnits;

  public Condo()
  {
    super();
    connectedUnits = 0; 
  }
  public Condo(int newListingNum, double newPrice, int newConUnits)
  {
    super(newListingNum, newPrice);
    connectedUnits = newConUnits;
  }

  public double getPrice()
  {
    return price;
  }
  public int getListingNum()
  {
    return listingNum;
  }
  public int getConnectedUnits()
  {
    return connectedUnits; 
  }
  public String toString()
  {
    return super.toString()+", Number of connected unints: "+connectedUnits;
  }

public boolean sameListingNum(Object other)
          {
            if(!(other instanceof House))
              return false; 
            else {
              House objHouse = (House)other;
              if(listingNum - objHouse.getListingNum() == 0)
              {
                System.out.println("Same listing numbers: "
                +listingNum+", "+objHouse.getListingNum());
                return true;
              }
              else 
                return false;
            }
        }

Answer 1

You can group it using a Map of List, like Map> groups.

Then you loop over you list and for each house/condo you put it in the group of the same listing number. At the end you will have a map where for each entry there are all houses/condos with same listing number.

Here a sample:

Map<Integer, List<House>> groups = new HashMap<Integer, List<House>>();
for (House house:listings) {
   List<House> group = groups.get(house.getListingNum());
   if (group == null) {
     group = new ArrayList<House>();
     groups.put(house.getListingNum(), group);
   }
   group.add(house);
}

for (Entry<Integer, List<House>> entry:groups.entrySet()) System.out.println("Listing Number "+entry.getKey()+" Houses/condos: "+entry.getValue());

Answer 2

Both of the other 2 answers will work alternativly you can implement comparable on the House... eg.

public class House implements Comparable<House> 

@Override
public int compareTo(final House o) {
    return listingNum - o.listingNum;
}

then inside your main method. Sort the Collection and check to see if the previous row has the same Id all the time.

    Collections.sort(listings);

    int previousListing = Integer.MIN_VALUE;

    for (House currentHouse : listings) {
        if (currentHouse.getListingNum() == previousListing){
            System.out.println("Duplicate for " + currentHouse.getListingNum());
        }
        previousListing = currentHouse.getListingNum();
    }

take your pick.

Answer 3

Try this:

foreach(House h in listings)
{
if(listings.Exists(p => p.sameListingNum(h)))
{
//do something
}
}