BAEL-5663 code for the filter java stream to only one element article

core-java-modules/core-java-streams-4/src/test/java/com/baeldung/streams/filteronlyoneelement/FilterUtilsUnitTest.java

@@ -0,0 +1,88 @@
+package com.baeldung.streams.filteronlyoneelement;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.function.Predicate;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.Test;
+
+public class FilterUtilsUnitTest {
+
+    private static final Predicate<Integer> IS_STRICTLY_GREATER_THAN5 = i -> i > 5;
+    private static final Predicate<Integer> IS_STRICTLY_GREATER_THAN4 = i -> i > 4;
+    private static final Predicate<Integer> IS_STRICTLY_GREATER_THAN3 = i -> i > 3;
+
+    private Stream getIntegers() {
+        return Stream.of(1, 2, 3, 4, 5);
+    }
+
+    @Test
+    void givenNoElementMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithReduction_ThenNoneFound() {
+        assertTrue(FilterUtils.findUniqueElementMatchingPredicate_WithReduction(getIntegers(), IS_STRICTLY_GREATER_THAN5)
+            .isEmpty());
+    }
+
+    @Test
+    void givenTwoElementsMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithReduction_ThenEmpty() {
+        assertTrue(FilterUtils.findUniqueElementMatchingPredicate_WithReduction(getIntegers(), IS_STRICTLY_GREATER_THAN3)
+            .isEmpty());
+    }
+
+    @Test
+    void givenOnlyOneElementMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithReduction_ThenFindsIt() {
+        assertEquals(5, FilterUtils.findUniqueElementMatchingPredicate_WithReduction(getIntegers(), IS_STRICTLY_GREATER_THAN4)
+            .get());
+    }
+
+    @Test
+    void givenNoElementMatchingPredicate_WhenGetUniqueElementMatchingPredicateWithReduction_ThenThrows() {
+        assertThrows(IllegalStateException.class, () -> FilterUtils.getUniqueElementMatchingPredicate_WithReduction(getIntegers(), IS_STRICTLY_GREATER_THAN5));
+    }
+
+    @Test
+    void givenTwoElementsMatchingPredicate_WhenGetUniqueElementMatchingPredicateWithReduction_ThenThrows() {
+        assertThrows(IllegalStateException.class, () -> FilterUtils.getUniqueElementMatchingPredicate_WithReduction(getIntegers(), IS_STRICTLY_GREATER_THAN3));
+    }
+
+    @Test
+    void givenOnlyOneElementMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithReduction_ThenGetIt() {
+        assertEquals(5, FilterUtils.getUniqueElementMatchingPredicate_WithReduction(getIntegers(), IS_STRICTLY_GREATER_THAN4));
+    }
+
+    @Test
+    void givenNoElementMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithCollectingAndThen_ThenEmpty() {
+        assertTrue(FilterUtils.findUniqueElementMatchingPredicate_WithCollectingAndThen(getIntegers(), IS_STRICTLY_GREATER_THAN5)
+            .isEmpty());
+    }
+
+    @Test
+    void givenTwoElementsMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithCollectingAndThen_ThenEmpty() {
+        assertTrue(FilterUtils.findUniqueElementMatchingPredicate_WithCollectingAndThen(getIntegers(), IS_STRICTLY_GREATER_THAN3)
+            .isEmpty());
+    }
+
+    @Test
+    void givenOnlyOneElementMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithCollectingAndThen_ThenFindsIt() {
+        assertEquals(5, FilterUtils.findUniqueElementMatchingPredicate_WithCollectingAndThen(getIntegers(), IS_STRICTLY_GREATER_THAN4)
+            .get());
+    }
+
+    @Test
+    void givenNoElementMatchingPredicate_WhenGetUniqueElementMatchingPredicateWithCollectingAndThen_ThenThrows() {
+        assertThrows(IllegalStateException.class, () -> FilterUtils.getUniqueElementMatchingPredicate_WithCollectingAndThen(getIntegers(), IS_STRICTLY_GREATER_THAN5));
+    }
+
+    @Test
+    void givenTwoElementsMatchingPredicate_WhenGetUniqueElementMatchingPredicateWithCollectingAndThen_ThenThrows() {
+        assertThrows(IllegalStateException.class, () -> FilterUtils.getUniqueElementMatchingPredicate_WithCollectingAndThen(getIntegers(), IS_STRICTLY_GREATER_THAN3));
+    }
+
+    @Test
+    void givenOnlyOneElementMatchingPredicate_WhenFindUniqueElementMatchingPredicateWithCollectingAndThen_ThenGetIt() {
+        assertEquals(5, FilterUtils.getUniqueElementMatchingPredicate_WithCollectingAndThen(getIntegers(), IS_STRICTLY_GREATER_THAN4));
+    }
+
+}