feat: Add stack implementation

src/main/java/collections/stack/Stack.java

@@ -0,0 +1,35 @@
+package collections.stack;
+
+import collections.exception.OverflowException;
+import collections.exception.UnderflowException;
+
+public class Stack {
+    private static final int CAPACITY = 10;
+
+    private final int[] elements = new int[CAPACITY];
+    private int size = 0;
+
+    public boolean isEmpty() {
+        return size == 0;
+    }
+
+    private boolean isFull() {
+        return size == elements.length;
+    }
+
+    public void push(int element) {
+        if (isFull()) {
+            throw new OverflowException();
+        }
+
+        elements[size++] = element;
+    }
+
+    public int pop() {
+        if (isEmpty()) {
+            throw new UnderflowException();
+        }
+
+        return elements[--size];
+    }
+}

src/test/java/collections/stack/StackTest.java

@@ -0,0 +1,62 @@
+package collections.stack;
+
+import collections.exception.UnderflowException;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class StackTest {
+
+    private Stack stack;
+
+    @BeforeEach
+    void setUp() {
+        stack = new Stack();
+    }
+
+    @Test
+    void newStack_IsEmpty() {
+        Assertions.assertTrue(stack.isEmpty());
+    }
+
+    @Test
+    void afterOnePush_IsNotEmpty() {
+        stack.push(0);
+        Assertions.assertFalse(stack.isEmpty());
+    }
+
+    @Test
+    void willThrowUnderflowException_WhenEmptyStackIsPopped() {
+        Assertions.assertThrows(UnderflowException.class, stack::pop);
+    }
+
+    @Test
+    void afterOnePushAndOnePop_WillBeEmpty() {
+        stack.push(0);
+        stack.pop();
+        Assertions.assertTrue(stack.isEmpty());
+    }
+
+    @Test
+    void afterTwoPushesAndOnePop_WillNotBeEmpty() {
+        stack.push(0);
+        stack.push(0);
+        stack.pop();
+        Assertions.assertFalse(stack.isEmpty());
+    }
+
+    @Test
+    void afterPushingX_WillPopX() {
+        stack.push(42);
+        Assertions.assertEquals(42, stack.pop());
+    }
+
+    @Test
+    void afterPushingXAndY_WillPopYAndX() {
+        stack.push(42);
+        stack.push(100);
+        Assertions.assertEquals(100, stack.pop());
+        Assertions.assertEquals(42, stack.pop());
+    }
+
+}