testing: improving PostfixEvaluatorTest (#6405)

* testing: improving PostfixEvaluatorTest

* testing: redundant cases

Author: alxkm

src/test/java/com/thealgorithms/stacks/PostfixEvaluatorTest.java

@@ -4,24 +4,41 @@
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.EmptyStackException;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 public class PostfixEvaluatorTest {
 
-    @Test
-    public void testValidExpressions() {
-        assertEquals(22, PostfixEvaluator.evaluatePostfix("5 6 + 2 *"));
-        assertEquals(27, PostfixEvaluator.evaluatePostfix("7 2 + 3 *"));
-        assertEquals(3, PostfixEvaluator.evaluatePostfix("10 5 / 1 +"));
+    @ParameterizedTest(name = "Expression: \"{0}\" → Result: {1}")
+    @CsvSource({"'5 6 + 2 *', 22", "'7 2 + 3 *', 27", "'10 5 / 1 +', 3", "'8', 8", "'3 4 +', 7"})
+    @DisplayName("Valid postfix expressions")
+    void testValidExpressions(String expression, int expected) {
+        assertEquals(expected, PostfixEvaluator.evaluatePostfix(expression));
     }
 
     @Test
-    public void testInvalidExpression() {
+    @DisplayName("Should throw EmptyStackException for incomplete expression")
+    void testInvalidExpression() {
         assertThrows(EmptyStackException.class, () -> PostfixEvaluator.evaluatePostfix("5 +"));
     }
 
     @Test
-    public void testMoreThanOneStackSizeAfterEvaluation() {
+    @DisplayName("Should throw IllegalArgumentException for extra operands")
+    void testExtraOperands() {
         assertThrows(IllegalArgumentException.class, () -> PostfixEvaluator.evaluatePostfix("5 6 + 2 * 3"));
     }
+
+    @Test
+    @DisplayName("Should throw ArithmeticException for division by zero")
+    void testDivisionByZero() {
+        assertThrows(ArithmeticException.class, () -> PostfixEvaluator.evaluatePostfix("1 0 /"));
+    }
+
+    @Test
+    @DisplayName("Should throw IllegalArgumentException for invalid characters")
+    void testInvalidToken() {
+        assertThrows(IllegalArgumentException.class, () -> PostfixEvaluator.evaluatePostfix("1 a +"));
+    }
 }