Passing all tests i guess

src/ijvm/stack.rs

@@ -0,0 +1,40 @@
+use ijvm::Result;
+
+#[derive(Debug)]
+pub struct Stack {
+    pub data: Vec<i32>
+}
+
+impl Stack {
+    pub fn new() -> Stack {
+        Stack {
+            data: Vec::new()
+        }
+    }
+
+//    pub fn len(&self) -> usize {
+//        return self.data.len();
+//    }
+
+    pub fn top(&self) -> Result<i32> {
+        Ok(match  self.data.last() {
+            Some(a) => *a,
+            None => return Err("Stack empty"),
+        })
+    }
+
+    pub fn pop(&mut self) -> Result<i32> {
+        Ok(match self.data.pop() {
+            Some(a) => a,
+            None => return Err("Stack empty"),
+        })
+    }
+
+    pub fn push(&mut self, val: i32) {
+        self.data.push(val);
+    }
+
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+}