Add Collatz Conjecture (#3290)

src/test/java/com/thealgorithms/maths/CollatzConjectureTest.java

@@ -0,0 +1,40 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CollatzConjectureTest {
+
+    static CollatzConjecture cConjecture;
+
+    @BeforeAll
+    static void setUp() {
+        cConjecture = new CollatzConjecture();
+    }
+
+    @Test
+    void nextNumberFromEvenNumber() {
+        assertEquals(25, cConjecture.nextNumber(50));
+    }
+
+    @Test
+    void nextNumberFromOddNumber() {
+        assertEquals(154, cConjecture.nextNumber(51));
+    }
+
+    @Test
+    void collatzConjecture() {
+        final List<Integer> expected = List.of(35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1);
+        assertIterableEquals(expected, cConjecture.collatzConjecture(35));
+    }
+
+    @Test
+    void sequenceOfNotNaturalFirstNumber() {
+        assertThrows(IllegalArgumentException.class, () -> cConjecture.collatzConjecture(0));
+        assertThrows(IllegalArgumentException.class, () -> cConjecture.collatzConjecture(-1));
+    }
+}