added push() stub, added testPushOneElement()

src/main/java/MyStack.java

@@ -6,4 +6,8 @@ public class MyStack {
     public int size() {
         return 0;
     }
+
+    public void push(Object element) {
+        ;
+    }
 }

src/test/java/MyStackTest.java

@@ -21,4 +21,10 @@ public class MyStackTest {
     public void testEmptyStack() {
         assertEquals(0, stack.size());
     }
+
+    @Test
+    public void testPushOneElement() {
+        stack.push(new Object());
+        assertEquals(1, stack.size());
+    }
 }
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