fix(searches): add null safety and make ExponentialSearch public (#7651)

Co-authored-by: Oleksandr Klymenko <19151554+alxkm@users.noreply.github.com>

Author: hardikkaurani

src/main/java/com/thealgorithms/searches/ExponentialSearch.java

@@ -19,7 +19,7 @@
  * Note: This algorithm requires that the input array be sorted.
  * </p>
  */
-class ExponentialSearch implements SearchAlgorithm {
+public final class ExponentialSearch implements SearchAlgorithm {
 
     /**
      * Finds the index of the specified key in a sorted array using exponential search.
@@ -31,13 +31,13 @@ class ExponentialSearch implements SearchAlgorithm {
      */
     @Override
     public <T extends Comparable<T>> int find(T[] array, T key) {
-        if (array.length == 0) {
+        if (array == null || array.length == 0 || key == null) {
             return -1;
         }
-        if (array[0].equals(key)) {
+        if (key.compareTo(array[0]) == 0) {
             return 0;
         }
-        if (array[array.length - 1].equals(key)) {
+        if (key.compareTo(array[array.length - 1]) == 0) {
             return array.length - 1;
         }
 

src/test/java/com/thealgorithms/searches/ExponentialSearchTest.java

@@ -1,6 +1,7 @@
 package com.thealgorithms.searches;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.stream.IntStream;
 import org.junit.jupiter.api.Test;
@@ -58,6 +59,17 @@ void testExponentialSearchSingleElementFound() {
         assertEquals(expectedIndex, exponentialSearch.find(array, key), "The index of the single element should be 0.");
     }
 
+    /**
+     * Test for exponential search with a single element not present.
+     */
+    @Test
+    void testExponentialSearchSingleElementNotFound() {
+        ExponentialSearch exponentialSearch = new ExponentialSearch();
+        Integer[] array = {1};
+        int key = 2; // Key not present
+        assertEquals(-1, exponentialSearch.find(array, key), "The element should not be found in the array.");
+    }
+
     /**
      * Test for exponential search with an empty array.
      */
@@ -70,6 +82,25 @@ void testExponentialSearchEmptyArray() {
         assertEquals(expectedIndex, exponentialSearch.find(array, key), "The element should not be found in an empty array.");
     }
 
+    /**
+     * Test for exponential search with a null array.
+     */
+    @Test
+    void testExponentialSearchNullArray() {
+        ExponentialSearch exponentialSearch = new ExponentialSearch();
+        assertEquals(-1, exponentialSearch.find(null, 5), "Should return -1 for null array.");
+    }
+
+    /**
+     * Test for exponential search with a null key.
+     */
+    @Test
+    void testExponentialSearchNullKey() {
+        ExponentialSearch exponentialSearch = new ExponentialSearch();
+        Integer[] array = {1, 2, 3, 4, 5};
+        assertEquals(-1, exponentialSearch.find(array, null), "Should return -1 for null key.");
+    }
+
     /**
      * Test for exponential search on large array.
      */
@@ -123,4 +154,26 @@ void testExponentialSearchNotFoundReturnsMinusOne() {
         assertEquals(-1, exponentialSearch.find(array, 0), "A key below the minimum should give -1.");
         assertEquals(-1, exponentialSearch.find(array, 12), "A key above the maximum should give -1.");
     }
+
+    /**
+     * Test for duplicate elements in the array.
+     */
+    @Test
+    void testExponentialSearchDuplicates() {
+        ExponentialSearch exponentialSearch = new ExponentialSearch();
+        Integer[] array = {1, 2, 2, 2, 3};
+        int index = exponentialSearch.find(array, 2);
+        assertTrue(index >= 1 && index <= 3, "Index should be one of the occurrences of 2.");
+    }
+
+    /**
+     * Test for generic Comparable types such as Strings.
+     */
+    @Test
+    void testExponentialSearchStrings() {
+        ExponentialSearch exponentialSearch = new ExponentialSearch();
+        String[] words = {"apple", "banana", "cherry", "date", "elderberry"};
+        assertEquals(2, exponentialSearch.find(words, "cherry"), "Should find 'cherry' at index 2.");
+        assertEquals(-1, exponentialSearch.find(words, "fig"), "Should return -1 for absent word.");
+    }
 }