add stack ADT

stack/Stack.java

@@ -0,0 +1,19 @@
+/** Stack ADT */
+public interface Stack<E> {
+    /** Reinitialize the stack. The user is responsible for reclaiming the storage used by the stack elements. */
+    public void clear();
+
+    /** Push an element ontp the top of the stack.
+	@param it The element being pushed onto the stack. */
+    public void push(E it);
+
+    /** Remove and return the element at the top of the stack.
+	@return The element at the top of the stack. */
+    public E pop();
+
+    /** @return A copy of the top element. */
+    public E topValue();
+
+    /** @return The number of elements in the stack. */
+    public int length();
+};