Filter out object with a value 'a' who appears more than once among all the objects

FilterObject.java

package me.ifthiskills.map;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class FilterObject {

  public int a = 0;

  public FilterObject(int a) {
    this.a = a;
  }

  public void setA(int a) {
    this.a = a;
  }

  public int getA() {
    return a;
  }
  
  public static void main(String[] args) {
    // initialize
    List<FilterObject> originalList = new LinkedList<FilterObject>();
    
    originalList.add(new FilterObject(2));
    originalList.add(new FilterObject(1));
    originalList.add(new FilterObject(2));
    originalList.add(new FilterObject(3));
    originalList.add(new FilterObject(0));
    originalList.add(new FilterObject(3));
    
    Map<Integer, List<FilterObject>> filterMap =
        new HashMap<Integer, List<FilterObject>>();
    Set<Integer> moreThanOneSet = new HashSet<Integer>();
    
    // count the occurrence of 'a'
    for (FilterObject object : originalList) {
      int key = object.getA();
      if (filterMap.containsKey(key)) {
        filterMap.get(key).add(object);
        moreThanOneSet.add(key);
      } else {
        List<FilterObject> list = new LinkedList<FilterObject>();
        list.add(object);
        filterMap.put(key, list);
      }
    }
    
    // keep FilterObject with 'a' who appears more than once
    List<FilterObject> filteredList = new LinkedList<FilterObject>();
    for (Iterator<Integer> iterator = moreThanOneSet.iterator(); iterator
        .hasNext();) {
      filteredList.addAll(filterMap.get(iterator.next()));
    }
    // print result
    for (Iterator<FilterObject> iterator = filteredList.iterator(); iterator
        .hasNext();) {
      System.out.println(iterator.next().getA());
    }
  }
}