testing: improve test coverage NodeStackTest (#6432)

* testing: improve test coverage NodeStackTest

* style: fix style issues

* testing: fix type

* testing: fix import

* testing: fix import

Author: alxkm

src/test/java/com/thealgorithms/datastructures/stacks/NodeStackTest.java

@@ -1,15 +1,26 @@
 package com.thealgorithms.datastructures.stacks;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
 class NodeStackTest {
 
+    private NodeStack<Integer> intStack;
+    private NodeStack<String> stringStack;
+
+    @BeforeEach
+    void setUp() {
+        intStack = new NodeStack<>();
+        stringStack = new NodeStack<>();
+    }
+
     @Test
+    @DisplayName("Test push operation")
     void testPush() {
         NodeStack<Integer> stack = new NodeStack<>();
         stack.push(10);
@@ -18,6 +29,7 @@ void testPush() {
     }
 
     @Test
+    @DisplayName("Test pop operation")
     void testPop() {
         NodeStack<String> stack = new NodeStack<>();
         stack.push("First");
@@ -27,12 +39,14 @@ void testPop() {
     }
 
     @Test
+    @DisplayName("Test pop on empty stack throws exception")
     void testPopOnEmptyStack() {
         NodeStack<Double> stack = new NodeStack<>();
         assertThrows(IllegalStateException.class, stack::pop, "Popping an empty stack should throw IllegalStateException.");
     }
 
     @Test
+    @DisplayName("Test peek operation")
     void testPeek() {
         NodeStack<Integer> stack = new NodeStack<>();
         stack.push(5);
@@ -43,22 +57,25 @@ void testPeek() {
     }
 
     @Test
+    @DisplayName("Test peek on empty stack throws exception")
     void testPeekOnEmptyStack() {
         NodeStack<String> stack = new NodeStack<>();
         assertThrows(IllegalStateException.class, stack::peek, "Peeking an empty stack should throw IllegalStateException.");
     }
 
     @Test
+    @DisplayName("Test isEmpty method")
     void testIsEmpty() {
         NodeStack<Character> stack = new NodeStack<>();
         assertTrue(stack.isEmpty(), "Newly initialized stack should be empty.");
         stack.push('A');
-        assertFalse(stack.isEmpty(), "Stack should not be empty after a push operation.");
+        org.junit.jupiter.api.Assertions.assertFalse(stack.isEmpty(), "Stack should not be empty after a push operation.");
         stack.pop();
         assertTrue(stack.isEmpty(), "Stack should be empty after popping the only element.");
     }
 
     @Test
+    @DisplayName("Test size method")
     void testSize() {
         NodeStack<Integer> stack = new NodeStack<>();
         assertEquals(0, stack.size(), "Size of empty stack should be 0.");
@@ -70,4 +87,164 @@ void testSize() {
         stack.pop();
         assertEquals(0, stack.size(), "Size should be 0 after popping all elements.");
     }
+
+    @Test
+    @DisplayName("Test push and pop with null values")
+    void testPushPopWithNull() {
+        stringStack.push(null);
+        stringStack.push("not null");
+        stringStack.push(null);
+
+        assertEquals(3, stringStack.size(), "Stack should contain 3 elements including nulls");
+        org.junit.jupiter.api.Assertions.assertNull(stringStack.pop(), "Should pop null value");
+        assertEquals("not null", stringStack.pop(), "Should pop 'not null' value");
+        org.junit.jupiter.api.Assertions.assertNull(stringStack.pop(), "Should pop null value");
+        assertTrue(stringStack.isEmpty(), "Stack should be empty after popping all elements");
+    }
+
+    @Test
+    @DisplayName("Test LIFO (Last In First Out) behavior")
+    void testLifoBehavior() {
+        int[] values = {1, 2, 3, 4, 5};
+
+        // Push values in order
+        for (int value : values) {
+            intStack.push(value);
+        }
+
+        // Pop values should be in reverse order
+        for (int i = values.length - 1; i >= 0; i--) {
+            assertEquals(values[i], intStack.pop(), "Elements should be popped in LIFO order");
+        }
+    }
+
+    @Test
+    @DisplayName("Test peek doesn't modify stack")
+    void testPeekDoesNotModifyStack() {
+        intStack.push(1);
+        intStack.push(2);
+        intStack.push(3);
+
+        int originalSize = intStack.size();
+        int peekedValue = intStack.peek();
+
+        assertEquals(3, peekedValue, "Peek should return top element");
+        assertEquals(originalSize, intStack.size(), "Peek should not change stack size");
+        assertEquals(3, intStack.peek(), "Multiple peeks should return same value");
+        org.junit.jupiter.api.Assertions.assertFalse(intStack.isEmpty(), "Peek should not make stack empty");
+    }
+
+    @Test
+    @DisplayName("Test mixed push and pop operations")
+    void testMixedOperations() {
+        // Test interleaved push/pop operations
+        intStack.push(1);
+        assertEquals(1, intStack.pop());
+        assertTrue(intStack.isEmpty());
+
+        intStack.push(2);
+        intStack.push(3);
+        assertEquals(3, intStack.pop());
+        intStack.push(4);
+        assertEquals(4, intStack.peek());
+        assertEquals(2, intStack.size());
+
+        assertEquals(4, intStack.pop());
+        assertEquals(2, intStack.pop());
+        assertTrue(intStack.isEmpty());
+    }
+
+    @Test
+    @DisplayName("Test stack with duplicate values")
+    void testStackWithDuplicates() {
+        intStack.push(1);
+        intStack.push(1);
+        intStack.push(1);
+
+        assertEquals(3, intStack.size(), "Stack should handle duplicate values");
+        assertEquals(1, intStack.peek(), "Peek should return duplicate value");
+
+        assertEquals(1, intStack.pop(), "Should pop first duplicate");
+        assertEquals(1, intStack.pop(), "Should pop second duplicate");
+        assertEquals(1, intStack.pop(), "Should pop third duplicate");
+        assertTrue(intStack.isEmpty(), "Stack should be empty after popping all duplicates");
+    }
+
+    @Test
+    @DisplayName("Test stack with different data types")
+    void testDifferentDataTypes() {
+        NodeStack<Character> charStack = new NodeStack<>();
+        NodeStack<Boolean> booleanStack = new NodeStack<>();
+
+        // Test with Character
+        charStack.push('A');
+        charStack.push('Z');
+        assertEquals('Z', charStack.peek(), "Should handle Character values");
+
+        // Test with Boolean
+        booleanStack.push(Boolean.TRUE);
+        booleanStack.push(Boolean.FALSE);
+        assertEquals(Boolean.FALSE, booleanStack.peek(), "Should handle Boolean values");
+    }
+
+    @Test
+    @DisplayName("Test stack state consistency after exceptions")
+    void testStateConsistencyAfterExceptions() {
+        // Stack should remain consistent after exception-throwing operations
+        intStack.push(1);
+        intStack.push(2);
+
+        // Try to peek and pop normally first
+        assertEquals(2, intStack.peek());
+        assertEquals(2, intStack.pop());
+        assertEquals(1, intStack.size());
+
+        // Pop remaining element
+        assertEquals(1, intStack.pop());
+        assertTrue(intStack.isEmpty());
+
+        // Now stack is empty, operations should throw exceptions
+        assertThrows(IllegalStateException.class, intStack::peek);
+        assertThrows(IllegalStateException.class, intStack::pop);
+
+        // Stack should still be in valid empty state
+        assertTrue(intStack.isEmpty());
+        assertEquals(0, intStack.size());
+
+        // Should be able to push after exceptions
+        intStack.push(3);
+        org.junit.jupiter.api.Assertions.assertFalse(intStack.isEmpty());
+        assertEquals(1, intStack.size());
+        assertEquals(3, intStack.peek());
+    }
+
+    @Test
+    @DisplayName("Test single element stack operations")
+    void testSingleElementStack() {
+        intStack.push(2);
+
+        org.junit.jupiter.api.Assertions.assertFalse(intStack.isEmpty(), "Stack with one element should not be empty");
+        assertEquals(1, intStack.size(), "Size should be 1");
+        assertEquals(2, intStack.peek(), "Peek should return the single element");
+        assertEquals(1, intStack.size(), "Peek should not change size");
+
+        assertEquals(2, intStack.pop(), "Pop should return the single element");
+        assertTrue(intStack.isEmpty(), "Stack should be empty after popping single element");
+        assertEquals(0, intStack.size(), "Size should be 0 after popping single element");
+    }
+
+    @Test
+    @DisplayName("Test toString method if implemented")
+    void testToString() {
+        // This test assumes NodeStack has a toString method
+        // If not implemented, this test can be removed or NodeStack can be enhanced
+        intStack.push(1);
+        intStack.push(2);
+        intStack.push(3);
+
+        String stackString = intStack.toString();
+        // Basic check that toString doesn't throw exception and returns something
+        assertTrue(stackString != null, "toString should not return null");
+        assertTrue(stackString.length() > 0, "toString should return non-empty string");
+    }
 }