Joiner クラスは、Guava Java ライブラリのクラスです。

import com.google.common.base.Joiner;
import java.util.*;
public class Demo{
   public static void main(String[] args){
      String[] my_arr = { "hel", null, "lo", "wo", "r", null, "ld" };
      System.out.println("The original array is : "+ Arrays.toString(my_arr));
      String my_result = Joiner.on('+').skipNulls().join(my_arr);
      System.out.println("The joined string is : " + my_result);
   }
}

The original array is [hel, null, lo, wo, r, null, ld]
The joined string is hel+lo+wo+r+ld