Enhance docs, remove main, add tests in NodeStack (#6017)

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

@@ -0,0 +1,85 @@
+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.Test;
+
+class NodeStackTest {
+
+    @Test
+    void testPush() {
+        NodeStack<Integer> stack = new NodeStack<>();
+        stack.push(10);
+        stack.push(20);
+        assertEquals(20, stack.peek(), "Top element should be 20 after pushing 10 and 20.");
+    }
+
+    @Test
+    void testPop() {
+        NodeStack<String> stack = new NodeStack<>();
+        stack.push("First");
+        stack.push("Second");
+        assertEquals("Second", stack.pop(), "Pop should return 'Second', the last pushed element.");
+        assertEquals("First", stack.pop(), "Pop should return 'First' after 'Second' is removed.");
+    }
+
+    @Test
+    void testPopOnEmptyStack() {
+        NodeStack<Double> stack = new NodeStack<>();
+        assertThrows(IllegalStateException.class, stack::pop, "Popping an empty stack should throw IllegalStateException.");
+    }
+
+    @Test
+    void testPeek() {
+        NodeStack<Integer> stack = new NodeStack<>();
+        stack.push(5);
+        stack.push(15);
+        assertEquals(15, stack.peek(), "Peek should return 15, the top element.");
+        stack.pop();
+        assertEquals(5, stack.peek(), "Peek should return 5 after 15 is popped.");
+    }
+
+    @Test
+    void testPeekOnEmptyStack() {
+        NodeStack<String> stack = new NodeStack<>();
+        assertThrows(IllegalStateException.class, stack::peek, "Peeking an empty stack should throw IllegalStateException.");
+    }
+
+    @Test
+    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.");
+        stack.pop();
+        assertTrue(stack.isEmpty(), "Stack should be empty after popping the only element.");
+    }
+
+    @Test
+    void testSize() {
+        NodeStack<Integer> stack = new NodeStack<>();
+        assertEquals(0, stack.size(), "Size of empty stack should be 0.");
+        stack.push(3);
+        stack.push(6);
+        assertEquals(2, stack.size(), "Size should be 2 after pushing two elements.");
+        stack.pop();
+        assertEquals(1, stack.size(), "Size should be 1 after popping one element.");
+        stack.pop();
+        assertEquals(0, stack.size(), "Size should be 0 after popping all elements.");
+    }
+
+    @Test
+    void testPrint() {
+        NodeStack<Integer> stack = new NodeStack<>();
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+
+        // Output verification would ideally be handled through a different means
+        // but you can print as a basic check to confirm method runs without errors.
+        stack.print();
+    }
+}