Successive pair algo (eugenp#17136)

* Successive Pair

* Create SuccessivePairsUnitTest.java

* Update SuccessivePairs.java

* Update SuccessivePairsUnitTest.java

---------

Co-authored-by: Neetika Khandelwal <kwal.neetika2398@gmail.com>

Author: Neetika23

algorithms-modules/algorithms-miscellaneous-9/src/main/java/com/baledung/algorithms/successivepairs/SuccessivePairs.java

@@ -0,0 +1,28 @@
+package com.baledung.algorithms.successivepairs;
+
+import java.util.AbstractMap.SimpleEntry;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+public class SuccessivePairs {
+
+    public static <T> List<SimpleEntry<T, T>> collectSuccessivePairs(Stream<T> stream) {
+        List<T> list = stream.collect(Collectors.toList());
+        return IntStream.range(0, list.size() - 1)
+            .mapToObj(i -> new SimpleEntry<>(list.get(i), list.get(i + 1)))
+            .collect(Collectors.toList());
+    }
+
+    public static <T> Stream<List<T>> pairwise(Stream<T> stream) {
+        List<T> list = stream.collect(Collectors.toList());
+        List<List<T>> pairs = new ArrayList<>();
+        for (int i = 0; i < list.size() - 1; i++) {
+            pairs.add(Arrays.asList(list.get(i), list.get(i + 1)));
+        }
+        return pairs.stream();
+    }
+}

algorithms-modules/algorithms-miscellaneous-9/src/test/java/com/baeldung/algorithms/successivepairs/SuccessivePairsUnitTest.java

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+package com.baeldung.algorithms.successivepairs;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.AbstractMap.SimpleEntry;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.Test;
+
+import com.baledung.algorithms.successivepairs.SuccessivePairs;
+
+public class SuccessivePairsUnitTest {
+
+    @Test
+    public void givenEmptyStream_whenUsingSimpleEntry_thenCollectPairs() {
+        Stream<Integer> stream = Stream.empty();
+        List<SimpleEntry<Integer, Integer>> pairs = SuccessivePairs.collectSuccessivePairs(stream);
+        assertTrue(pairs.isEmpty());
+    }
+
+    @Test
+    public void givenSingleElement_whenUsingSimpleEntry_thenCollectPairs() {
+        Stream<Integer> stream = Stream.of(1);
+        List<SimpleEntry<Integer, Integer>> pairs = SuccessivePairs.collectSuccessivePairs(stream);
+        assertTrue(pairs.isEmpty());
+    }
+
+    @Test
+    public void givenMultipleElementStream_whenUsingSimpleEntry_thenCollectPairs() {
+        Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5);
+        List<SimpleEntry<Integer, Integer>> pairs = SuccessivePairs.collectSuccessivePairs(stream);
+        assertEquals(4, pairs.size());
+
+        assertEquals(new SimpleEntry<>(1, 2), pairs.get(0));
+        assertEquals(new SimpleEntry<>(2, 3), pairs.get(1));
+        assertEquals(new SimpleEntry<>(3, 4), pairs.get(2));
+        assertEquals(new SimpleEntry<>(4, 5), pairs.get(3));
+    }
+
+    @Test
+    public void givenStreamWithDuplicate_whenUsingSimpleEntry_thenCollectPairs() {
+        Stream<Integer> stream = Stream.of(1, 1, 2, 2, 3, 3);
+        List<SimpleEntry<Integer, Integer>> pairs = SuccessivePairs.collectSuccessivePairs(stream);
+        assertEquals(5, pairs.size());
+
+        assertEquals(new SimpleEntry<>(1, 1), pairs.get(0));
+        assertEquals(new SimpleEntry<>(1, 2), pairs.get(1));
+        assertEquals(new SimpleEntry<>(2, 2), pairs.get(2));
+        assertEquals(new SimpleEntry<>(2, 3), pairs.get(3));
+        assertEquals(new SimpleEntry<>(3, 3), pairs.get(4));
+    }
+
+    @Test
+    public void givenStreamWithLetters_whenUsingStatefulTransform_thenCollectPairs() {
+        Stream<String> stream = Stream.of("a", "b", "c", "d", "e");
+        List<List<String>> result = SuccessivePairs.pairwise(stream)
+            .collect(Collectors.toList());
+
+        List<List<String>> expected = Arrays.asList(Arrays.asList("a", "b"), Arrays.asList("b", "c"), Arrays.asList("c", "d"), Arrays.asList("d", "e"));
+
+        assertEquals(expected, result);
+    }
+}