Add tests for null & empty strings

Author: Hardvan

src/main/java/com/thealgorithms/stacks/InfixToPrefix.java

@@ -13,11 +13,21 @@ private InfixToPrefix() {
      *
      * @param infixExpression the infix expression to convert
      * @return the prefix expression
-     * @throws Exception if the infix expression has unbalanced brackets
+     * @throws IllegalArgumentException if the infix expression has unbalanced brackets
+     * @throws NullPointerException     if the infix expression is null
      */
-    public static String infix2Prefix(String infixExpression) throws Exception {
+    public static String infix2Prefix(String infixExpression) throws IllegalArgumentException {
+        // Check for unbalanced brackets
         if (!BalancedBrackets.isBalanced(filterBrackets(infixExpression))) {
-            throw new Exception("invalid expression");
+            throw new IllegalArgumentException("invalid expression");
+        }
+        // Null input
+        if (infixExpression == null) {
+            throw new NullPointerException("null input");
+        }
+        // Empty string
+        if (infixExpression.isEmpty()) {
+            return "";
         }
         StringBuilder output = new StringBuilder();
         Stack<Character> stack = new Stack<>();

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

@@ -1,8 +1,10 @@
 package com.thealgorithms.stacks;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
@@ -15,6 +17,18 @@ void testValidExpressions(String infix, String expectedPrefix) throws Exception
         assertEquals(expectedPrefix, InfixToPrefix.infix2Prefix(infix));
     }
 
+    @Test
+    void testEmptyString() {
+        // Assuming that an empty string returns an empty prefix or throws an exception
+        assertEquals("", InfixToPrefix.infix2Prefix(""));
+    }
+
+    @Test
+    void testNullValue() {
+        // Assuming that a null input throws a NullPointerException
+        assertThrows(NullPointerException.class, () -> InfixToPrefix.infix2Prefix(null));
+    }
+
     private static Stream<Arguments> provideValidExpressions() {
         return Stream.of(Arguments.of("3+2", "+32"), // Simple addition
             Arguments.of("1+(2+3)", "+1+23"), // Parentheses