title: How to union 2 string arrays, but keep the duplicate in Java?
body: <pre><code>s1={"phone","smartphone","dumpphone"} (all string elements in s1 are unique)
  s2={"phone","phone","smartphone"}
  </code></pre>
  
  <p>(string elements in s2 can be duplicated, but each element in s2 must belongs to s1, i.e. s2 can not contain string that does not exist in s1. For example, s2 can not contain "handheld" because s1 does not contain ""handheld"")</p>
  
  <pre><code>s2 union s1={"phone","phone", "smartphone","dumpphone"}
  </code></pre>
  
  <p>Set &amp; HashSet didn't allow duplicate</p>
  
  <p>I tried List but it didn't help.</p>
  
  <p>Do you know how to solve?</p>