Add Goldbach's Conjecture algorithm (#6127)

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

@@ -0,0 +1,29 @@
+package com.thealgorithms.maths;
+
+import static com.thealgorithms.maths.GoldbachConjecture.getPrimeSum;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class GoldbachConjectureTest {
+    @Test
+    void testValidEvenNumbers() {
+        assertEquals(new GoldbachConjecture.Result(3, 7), getPrimeSum(10)); // 10 = 3 + 7
+        assertEquals(new GoldbachConjecture.Result(5, 7), getPrimeSum(12)); // 12 = 5 + 7
+        assertEquals(new GoldbachConjecture.Result(3, 11), getPrimeSum(14)); // 14 = 3 + 11
+        assertEquals(new GoldbachConjecture.Result(5, 13), getPrimeSum(18)); // 18 = 5 + 13
+    }
+    @Test
+    void testInvalidOddNumbers() {
+        assertThrows(IllegalArgumentException.class, () -> getPrimeSum(7));
+        assertThrows(IllegalArgumentException.class, () -> getPrimeSum(15));
+    }
+    @Test
+    void testLesserThanTwo() {
+        assertThrows(IllegalArgumentException.class, () -> getPrimeSum(1));
+        assertThrows(IllegalArgumentException.class, () -> getPrimeSum(2));
+        assertThrows(IllegalArgumentException.class, () -> getPrimeSum(-5));
+        assertThrows(IllegalArgumentException.class, () -> getPrimeSum(-26));
+    }
+}