Fix comparer and add its unit tests

pom.xml

@@ -14,5 +14,19 @@
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <exec.mainClass>org.alda.Main</exec.mainClass>
     </properties>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <version>5.10.0</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <version>5.10.0</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
 
 </project>

src/main/java/org/alda/common/Comparer.java

@@ -50,13 +50,17 @@ public static <T> Integer compare(T a, T b) {
             // Compare as Numbers
             double numA = ((Number) a).doubleValue();
             double numB = ((Number) b).doubleValue();
-            if (numA >= numB) {
+            if (numA > numB) {
                 return 1;
             } else if (numA < numB) {
                 return -1;
             }
             return 0;
         } else {
+            if(a == null || b == null) {
+                throw new NullPointerException("Both arguments must be either Strings or Numbers.");
+            }
+
             throw new IllegalArgumentException("Both arguments must be either Strings or Numbers.");
         }
     }

src/test/java/org/alda/common/ComparerTest.java

@@ -0,0 +1,58 @@
+package org.alda.common;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Unit tests for {@link Comparer} class.
+ */
+class ComparerTest {
+
+    @Test
+    void testCompareStrings_LexicographicalOrder() {
+        assertTrue(Comparer.compare("apple", "banana") < 0);
+        assertTrue(Comparer.compare("banana", "apple") > 0);
+        assertEquals(0, Comparer.compare("cherry", "cherry"));
+    }
+
+    @Test
+    void testCompareNumbers_NumericalOrder() {
+        System.out.println(Comparer.compare(100, 100));
+        assertTrue(Comparer.compare(10, 5) > 0);
+        assertTrue(Comparer.compare(3.14, 4.2) < 0);
+        assertEquals(0, Comparer.compare(100, 100));
+    }
+
+    @Test
+    void testCompareMixedNumberTypes() {
+        assertTrue(Comparer.compare(42, 27.5) > 0);
+        assertTrue(Comparer.compare(3.14, 3) > 0);
+        assertTrue(Comparer.compare(2.5f, 3.5) < 0);
+    }
+
+    @Test
+    void testCompareWithInvalidTypes() {
+        Exception exception1 = assertThrows(IllegalArgumentException.class, () ->
+                Comparer.compare("apple", 42)
+        );
+        assertEquals("Both arguments must be either Strings or Numbers.", exception1.getMessage());
+
+        Exception exception2 = assertThrows(IllegalArgumentException.class, () ->
+                Comparer.compare(42, "apple")
+        );
+        assertEquals("Both arguments must be either Strings or Numbers.", exception2.getMessage());
+    }
+
+    @Test
+    void testCompareWithNullValues() {
+        Exception exception1 = assertThrows(NullPointerException.class, () ->
+                Comparer.compare(null, "banana")
+        );
+        Exception exception2 = assertThrows(NullPointerException.class, () ->
+                Comparer.compare(42, null)
+        );
+        Exception exception3 = assertThrows(NullPointerException.class, () ->
+                Comparer.compare(null, null)
+        );
+    }
+}