make sure a list has unique values

Question

How can I get a collection of unique pairs of values in Java?

I read this other posting and tried the following code from an answer to that posting as a Pair class:

public class Pair<L,R> {

  private final L left;
  private final R right;

  public Pair(L left, R right) {
    this.left = left;
    this.right = right;
  }

  public L getLeft() { return left; }
  public R getRight() { return right; }

  @Override
  public int hashCode() { return left.hashCode() ^ right.hashCode(); }

  @Override
  public boolean equals(Object o) {
    if (!(o instanceof Pair)) return false;
    Pair pairo = (Pair) o;
    return this.left.equals(pairo.getLeft()) &&
           this.right.equals(pairo.getRight());
  }

}

But how to I ensure that a collection of Pair objects only contains unique combinations of left+right using the Pair class?

Here is my first attempt:

            sel_person = this.serviceLayer.findSelPerson(patntId);
            ArrayList<Pair> ps = new ArrayList<Pair>();
            for(int ee=0;ee<sel_person.getRoles().size();ee++){
                Pair p1 = new Pair(sel_person.getRoles().get(ee).getId1Root(), sel_person.getRoles().get(ee).getId1Extension());
                ps.add(p1);
                Pair p2 = new Pair(sel_person.getRoles().get(ee).getId2Root(), sel_person.getRoles().get(ee).getId2Extension());
                ps.add(p2);
            }

My fear is that the ArrayList will not care if there are redundant combinations of left+right. I wonder if the hashcode method will somehow prevent instantiation of Pair objects with redundant combinations of left+right`, but how can I be sure? If the above code is correct, then a good answer would simply explain why. If something else needs to be done, then a good answer would explain what else needs to be done.

Assuming that `equals` will tell you whether two elements are unique, `HashSet` should do the job. — fdsa, May 11 '15 at 23:54
You may also want to check whether the parameter `Object o` to the `equals` method is null and return false if it is. — copeg, May 12 '15 at 00:05
You also have the possibility of getting a `NullPointerException` with `this.left.equals(pairo.getLeft())` since `this.left` could be `null` (same with right). This will also break the symmetric contract of `equals` since `a.equals(b)` could be `false` while `b.equals(a)` could throw a `NullPointerException`. — mkobit, May 12 '15 at 00:33

score 2 · Accepted Answer · answered May 11 '15 at 23:57

But how to I ensure that a collection of Pair objects only contains unique combinations of left+right using the Pair class?

A List cares not about duplicate elements. An implementation of the Set interface (for instance HashSet) is more appropriate, as a Set is defined as a Collection of unique elements (uniqueness as defined by hashCode and equals of the Objects it contains).

Set<Pair> ps = new HashSet<Pair>();
for(int ee=0;ee<sel_person.getRoles().size();ee++){
    Pair p1 = new Pair(sel_person.getRoles().get(ee).getId1Root(), sel_person.getRoles().get(ee).getId1Extension());
    ps.add(p1);
    Pair p2 = new Pair(sel_person.getRoles().get(ee).getId2Root(), sel_person.getRoles().get(ee).getId2Extension());
    ps.add(p2);
}
//now ps has only unique pairs as defined by the Pair class hashcode/equals

score 2 · Answer 2 · answered May 12 '15 at 00:01

2

HashSet does the trick for you

ArrayList<Pair> ps = new ArrayList<Pair>();
Set<Pair> set = new HashSet<Pair>(ps);

set will then contain the unique objects.

answered May 12 '15 at 00:01

Dino Tw

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make sure a list has unique values

2 Answers2