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package org.example;

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class StringUniqueService {
	public String getFirstUniqueElement(String[] elements) {
		List<String> uniqueElements = Arrays.stream(elements)
				.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
				.entrySet()
				.stream()
				.filter(entry -> entry.getValue().equals(1L))
				.map(Map.Entry::getKey)
				.collect(Collectors.toList());

		return Arrays.stream(elements)
				.filter(uniqueElements::contains)
				.findFirst().orElse(null);
	}
}

package org.example;

import static org.junit.jupiter.api.Assertions.assertEquals;

import org.junit.jupiter.api.Test;

class StringUniqueServiceTest {

	StringUniqueService underTest = new StringUniqueService();

	@Test
	void testGetFirstUniqueElement_ShouldReturnCorrectValue_GivenNotEmptyStringArray() {
		String [] elements = new String[] {
				"a", "a", "b", "c", "d"
		};
		String firstUniqueElement = underTest.getFirstUniqueElement(elements);
		assertEquals(firstUniqueElement, "b");
	}


}