Merging lists into a single array with unique elements

You can add the list contents into a Set.

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.

You can then get an array back by calling Set.toArray().

A standard set won't preserve order, but duplicates will be eliminated.

Create HashSet and call addAll with your Lists

Same question was asked to me in interview, but combine two vectors without repeating common object and without using collection framework methods.

package com.pra.sss;
/*
 * Vector v1 and V2 contain some common objects
 * Create vector V3 which will contain all V1 and V2 
 * and common objects only once 
 * without using colletion framework functions 
 */import java.util.Vector;

public class VectorTest {
public static void main(String[] args) {
   Vector<String> v1 = new Vector<String>();
   Vector<String> v2 = new Vector<String>();
   Vector<String> v3 = new Vector<String>();
   /*Vector v1*/
   v1.add("A");
   v1.add("B");
   v1.add("C");
   v1.add("D");
   /*Vector v2*/
   v2.add("W");
   v2.add("B");
   v2.add("C");
   v2.add("Z");

   System.out.println("V1"+v1);
   System.out.println("V2"+v2);

   for(String ss : v1) {
       v3.add(ss);
   }

   for (int i = 0; i < v1.size(); i++) {
       for (int j = i; j < v2.size(); j++) {
           if(v1.get(i).equals(v2.get(j))) {
               break;
           }
           else{
               v3.add(v2.get(j));
               break;
           }
    }
  }
   System.out.println("v3:"+v3);
}
}

/*
output
V1[A, B, C, D]
V2[W, B, C, Z]
v3:[A, B, C, D, W, Z]
*/

The best way to merge lists depends if you want have the same order of elements or not. i.d. elements 1st list before elements 2nd.

In @Johan Sjöberg solution it is used HashSet. The elements are returned in no particular order. If you want save the same order better use collection with predictable iteration order e.g. LinkedHashSet.