1. Day 3: The Command and Visitor Patterns

2. Preliminaries The Java static type system uses simple rules to infer types for Java expressions. The inferred type for an expression is conservative; it is guaranteed to be correct, but it may be weaker than what is required for a particular computation. Example: recall the finger exercise from Day 1 involving the DeptDirectory class hierarchy. Given a variable d of type Cons bound to a directory containing two entries, the expression d.getRest().getRest() is ill-typed. Why?

3. Explanation The variable d has type Cons which supports the method getRest. What is the type of d.getRest() ? The return type of the method getRest, which is DeptDirectory. Does DeptDirectory support the method getRest ? No. Hence, the method call ( d.getRest()).getRest() is not well-typed.

4. Casting: The Standard Work-Around Java allows any expression of object (reference) type A to be cast to another object type B --provided that the types A and B overlap (have a non-empty intersection ignoring null. Given a Java expression e of type A, the notation (B)e means expression e of type B. The value of e is unchanged.

5. Finger Exercise In DrJava, open the file DeptDirectory.java from the code library at the TeachJava website. Compile it. In the Interactions pane, evaluate: e1 = new Entry("Corky","DH3104","x 6042"); e2 = new Entry("Matthias","DH3106","x 5732"); d = new Cons(e1, new Cons(e2, new Empty())); d d.getRest() e = (Cons) d.getRest() e.getRest()

6. More Practice with the Composite and Interpreter Patterns In DrJava, open the file IntList.java. We want to convert IntList.java to a program List.java that implements heterogeneous lists (lists that can hold arbitrary objects, not just elements of a particular type). The element type in the program List.java must be Object. Otherwise the code is unchanged. Rewrite the concat and rev methods for heterogeneous lists. Test your code on some lists of Integer.

7. List abstract class List { abstract Object getFirst(); abstract List getRest(); abstract String toStringHelp(); } class Empty extends List { static Empty ONLY = new Empty(); // singleton pattern private Empty() {} Object getFirst() { throw new IllegalArgumentException( "first requires a non Empty List"); } List getRest() { throw new IllegalArgumentException( "rest requires a non Empty List"); } public String toString() { return "()"; } String toStringHelp() { return ""; } } class Cons extends List { Object first; List rest; Cons(Object f, List r) { first = f; rest = r; } Object getFirst() { return first; } List getRest() { return rest; } public String toString() { return "(" + first + rest.toStringHelp() + ")"; } String toStringHelp() { return " " + first + rest.toStringHelp(); } }

8. Discussion Question We want to define a method to sort heterogeneous lists. The method should work on lists of Integer, Double, Float Long, Short, Byte, Char, String, etc. How can we do this?

9. Some Possible Answers Pass the comparison method as an argument to the sort method. How can we do this? We will discuss a pattern for doing this later. Exploit an interface in all of the built-in types with a sensible total ordering called Comparable.

10. Java Interfaces In Java, an interface is a language construct that resembles a "lightweight" abstract class (an abstract class with no concrete methods). An interface definition has the syntax interface { } which looks exactly like a class definition except for the use of the keyword interface instead of class. But the members of an interface are restricted to abstract methods and static final fields.

11. Examples The interface Comparable is built-in to Java (part of the core library java.lang ) has the following definition interface Comparable { int compareTo(Object other); } The value returned by compareTo is negative, zero, or positive depending on whether this is less than other, equal to other, or greater than other. The built-in class String also implements the interface CharSequence which includes methods such as int length(). The built-in class StringBuffer (mutable strings) also implements this interface.

12. Writing a Sort Program Our task is to add a method List sort() { … } to the List class. We will decompose this problem into smaller tasks. Our first subtask is to write a method List insert(Object o) { … } that inserts o in proper order in this assuming that this is in (ascending) sorted order. Your code will be littered with casts. Our remaining task is to write sort() using insert as a help function.

13. List Sort abstract class List { abstract Object getFirst(); abstract List getRest(); abstract List insert(Object e); abstract List sort(); abstract String toStringHelp(); abstract List sort(); } class Empty extends List { static Empty ONLY = new Empty(); // singleton pattern private Empty() {} Object getFirst() { throw new IllegalArgumentException( "first requires a non Empty List"); } List getRest() { throw new IllegalArgumentException( "rest requires a non Empty List"); } List sort() { return Empty.ONLY; } List insert(Object e) { return new Cons(e,Empty.ONLY); } public String toString() { return "()"; } String toStringHelp() { return ""; } }

14. List Sort class Cons extends List { Object first; List rest; Cons(Object f, List r) { first = f; rest = r; } Object getFirst() { return first; } List getRest() { return rest; } List insert(Object e) { Comparable ce = (Comparable) e; Comparable cf = (Comparable) first; if (ce.compareTo(cf) < 0) return new Cons(e,this); else return new Cons(first, rest.insert(e)); } List sort() { return rest.sort().insert(first); } public String toString() { // no leading space before first return "(" + first + rest.toStringHelp() + ")"; } String toStringHelp() { // leading space before each elt return " " + first + rest.toStringHelp(); }