Question

How to create a list of unique/distinct objects (no duplicates) in Java?

Right now I am using HashMap<String, Integer> to do this as the key is overwritten and hence at the end we can get HashMap.getKeySet() which would be unique. But I am sure there should be a better way to do this as the value part is wasted here.

Answer 1

You can use a Set implementation:

Some info from the JAVADoc:

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.`

These are the implementations:

• HashSet

  This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance’s size (the number of elements) plus the “capacity” of the backing HashMap instance (the number of buckets). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

  When iterating a HashSet the order of the yielded elements is undefined.

• LinkedHashSet

  Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order). Note that insertion order is not affected if an element is re-inserted into the set. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.)

  So, the output of the code above…

   Set<Integer> linkedHashSet = new LinkedHashSet<>();
   linkedHashSet.add(3);
   linkedHashSet.add(1);
   linkedHashSet.add(2);
  
   for (int i : linkedHashSet) {
       System.out.println(i);
   }
  

  …will necessarily be

  3
  1
  2
  

• TreeSet

  This implementation provides guaranteed log(n) time cost for the basic operations (add, remove and contains). By default he elements returned on iteration are sorted by their “natural ordering“, so the code above…

   Set<Integer> treeSet = new TreeSet<>();
   treeSet.add(3);
   treeSet.add(1);
   treeSet.add(2);
  
   for (int i : treeSet) {
       System.out.println(i);
   }
  

  …will output this:

  1
  2
  3
  

  (You can also pass a Comparator instance to a TreeSet constructor, making it sort the elements in a different order.)

  Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all element comparisons using its compareTo (or compare) method, so two elements that are deemed equal by this method are, from the standpoint of the set, equal. The behavior of a set is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Set interface.