The problem

Suppose our code adds a value of some other type to this list:
```
0790:        tList.add("hello world");
```

When we come to retrieve this value:

0791:        Shape tShapeGet2 = (Shape)tList.get(2); 
0792:        System.out.println(tShapeGet2);

our program will collapse with a ClassCastException:

0797: Exception in thread "main" java.lang.ClassCastException
0798:         at ListShape.main(ListShape.java:17)

Unless we detect this when testing, our program embarassingly crashes when someone uses it.