Practice Homework 1
CMSC 433
Programming Language Technologies and Paradigms
Fall 2003
October 16, 2003
The follow are exercises to get you familiar with writing design patterns.
We’ll post answers to these exercises on Tuesday.
1 Iterator
BTree is a simple implementation of trees with arbitrary numbers of children,
where a value is stored at each node in the tree. Write a method that returns
an implementation of java.util.Iterator that returns the value at each node
using a pre-order traversal. Write another method that returns an iterator doing
post-order traversal instead.
2 Visitor
Expression is an abstract superclass for arithmetic expressions on integers,
where each expression can have up to two arguments. There are various sub-
classes of this class, including Constant, for representing integer constants;
AddExpression, for representing the sum of two expressions, etc.
Write two visitors for this class. The first will print out an expression, and
the second with compute its result. To do this, you will have to write accept
methods for each subclass of Expression, and write two implementations of
ExpressionVisitor, one for printing expressions (called PrintVisitor), and
one for computing expressions (called ComputeVisitor). Each visitor should
maintain its own state that it accumulates while visiting. For printing, this will
be a string, while for computation this will be a integer, holding the result of
the computation. Each visitor will define a method getResult that will return
its state.
Try writing these visitors in two ways (you’ll have to copy the code into
two separate directories). First, do internal traversal. That is, have the accept
method in each subclass of Expression invoke the visitor on each of its children
(if it has any). Second, do an external traversal, within the visitor. In this case,
the accept method will simply call the visitor on itself, while the calls to visit
