Start of lec 9.

2/www/lectures/8.scrbl

@@ -8,6 +8,8 @@
 
 @link["https://umd.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=41c2986c-d55e-4548-b43f-a8820148000b"]{Video}.
 
+@bold{Introducting Java Syntax}
+
 You've now seen several programming languages: BSL, ISL, ISL+, and
 @tt{class/0}.  Now time for one more: Java.  Let's start by looking at
 the @emph{syntax}, the way Java programs are written.
@@ -172,6 +174,8 @@ within a programming environment, like DrRacket but for Java, or use a
 Java @emph{compiler} and run the Java interpreter from the command
 line.  These will be covered in demos in class.
 
+@bold{Grammar: Simplest Java}
+
 Taking a step back, we can characterize the syntax of Java, or at
 least the small bit of Java we've covered so far, with the following
 @emph{grammar}:
@@ -215,4 +219,4 @@ least the small bit of Java we've covered so far, with the following
         (list @nonterm{meth-name} @nonterm{id})
         (list @nonterm{field-name} @nonterm{id})
         (list @nonterm{id}
-              @elem{any name except for reserved words like @litchar{class}, @litchar{this}, @litchar{return}, etc.})])
+              @elem{any name except for reserved words like @litchar{class}, @litchar{this}, @litchar{return}, etc.})])

2/www/lectures/9.scrbl

@@ -1,13 +1,203 @@
 #lang scribble/manual
-@(require scribble/eval
-          racket/sandbox
-          (for-label (only-in lang/htdp-intermediate-lambda define-struct ... check-expect))
+@(require (for-label (only-in lang/htdp-intermediate-lambda define-struct ... check-expect))
           (for-label (except-in class/0 define-struct ... check-expect))
           (for-label class/universe)
+          scribble/bnf
           "../utils.rkt")
 
 @lecture-title[9]{Union, Interfaces, and Lists in Java}
 
 @link["https://umd.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=15905f29-3a7a-4f9e-92a6-a88601577373"]{Video}.
 
-TBD.
+@bold{An IDE for Java}
+
+We're going to be using the IntelliJ integrated development
+environment (IDE) to develop and run Java programs.  Like DrRacket, it
+will provide useful features to help us in the process of developing
+programs.
+
+We'll do a short demo in class and in lab.
+
+@bold{Unions in Java: Traffic Lights}
+
+Let's now turn back to modelling information in Java.  So far we've
+seen how to represent compound data such as coordinates.  Another
+common kind of data definition is a @emph{union} data definition.
+Let's look at a very simple union data definition (that we first
+looked at in @racket[class/0]: traffic lights.
+
+We define the union of objects in terms of their interface; the method
+signatures that each variant of the union has in common.  In
+@racket[class/0] this takes the form:
+
+@class-block{
+;; A Light implements
+;;
+;; Compute the next traffic light after this one
+;; next : -> Light
+}
+
+In Java, this interface definition looks like this:
+@java-block|{
+interface Light {
+  // Compute the next traffic light after this one
+  Light onTick();
+}
+}|
+
+In @racket[class/0], we would define a class and declare it implements
+the interface in a comment.  In Java, we can declare the intention to
+implement the interface as part of the class definition:
+
+@java-block|{
+class Red implements Light {
+  Red() {}
+}
+}|
+
+@bold{Nullary Constructor by Default.}  Note the @java{Red}
+constructor takes no arguments and does nothing, which is what we want
+since there are no fields in the @java{Red} class.  As a side note: we
+can omit this @emph{nullary} constructor, since it is the default
+constructor Java will use if no constructor is defined.
+
+@java-block|{
+class Red implements Light {}
+}|
+
+Now, this class @emph{says} that it implements the @java{Light}
+interface, but so far it has not actually defined the @java{next}
+method as required by the @java{Light} interface.
+
+The Java type system will catch this error for us.  If you tried to
+compile this program as-is, there would be a compile-time error:
+
+@centered{Error: Red is not abstract and does not override abstract
+method next() in Light.}
+
+In the IDE, this static error is often indicated with a red squiggly
+underlining of the class definition.
+
+Let's make a stub for @java{next} method:
+
+@java-block|{
+class Red implements Light {
+  // Compute the next traffic light after this red light
+  Light next() {
+    return this;
+  }
+}
+}|
+
+@bold{Visibility modifiers.}  As a small wrinkle, we need to place a
+@emph{visibility modifier} on this method indicating that the method
+can be invoked from outside of the methods in this class:
+@java-block|{
+class Red implements Light {
+  // Compute the next traffic light after this red light
+  public Light next() {
+    return this;
+  }
+}
+}|
+
+This @java{public} keyword is the visibility modifier.  Using it here
+indicates that @java{next} can be called from outside of the
+@java{Red} class, which is (implicitly) what's required by the
+interface.  The are also visibility modifiers @java{private} and
+@java{protected}.  We will mostly ignore visibility issues for now and
+assume that methods are public and fields are private.  We will add
+modifiers when needed to make programs type-check.  For more details,
+see the
+@link["https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html"]{Oracle
+tutorial}.
+
+We can define the other variants of @java{Light}, with method stubs:
+@java-block|{
+class Red implements Light {
+  // Compute the next traffic light after this red light
+  public Light next() {
+    return this;
+  }
+}
+
+class Yellow implements Light {
+  // Compute the next traffic light after this yellow light
+  public Light next() {
+    return this;
+  }
+}
+
+class Green implements Light {
+  // Compute the next traffic light after this green light
+  public Light next() {
+    return this;
+  }
+}
+}|
+
+Thinking through some examples, it's easy to come up with the correct
+code for the method definitions:
+
+@java-block|{
+class Red implements Light {
+  // Compute the next traffic light after this red light
+  public Light next() {
+    return new Green();
+  }
+}
+
+class Yellow implements Light {
+  // Compute the next traffic light after this yellow light
+  public Light next() {
+    return new Red();
+  }
+}
+
+class Green implements Light {
+  // Compute the next traffic light after this green light
+  public Light next() {
+    return new Yellow();
+  }
+}
+}|
+
+So now we've seen our first union data definition in Java.
+
+@bold{Grammar: Java with Interfaces}
+
+The grammar of Java programs has grown some now.  In particular a
+program now consists of some number of class @emph{or interface}
+definitions.  Since a set of objects may be defined as either a class
+or interface, we use @nonterm{type-name} to range over things defined
+as classes or interfaces.  The grammar is also updated to have
+optional visibility modifiers on method definitions.  Here are the
+revised parts of the grammar (only):
+
+@(let ([open @litchar|{{}|]
+       [close @litchar|{}}|])
+   @BNF[(list @nonterm{program}
+              @kleenestar[@nonterm{class-or-intf-defn}])
+        (list @nonterm{class-or-intf-defn}
+              @nonterm{class-defn}
+              @nonterm{intf-defn})
+        (list @nonterm{intf-defn}
+              @BNF-seq[@litchar{interface} @nonterm{int-name} open @nonterm{method-sigs} close])
+        (list @nonterm{field-decl}
+              @BNF-seq[@nonterm{type-name} @nonterm{field-name} @litchar{;}])
+        (list @nonterm{method-sigs}
+              @BNF-seq[]
+              @BNF-seq[@nonterm{method-sig} @litchar{;} @nonterm{method-sigs}])
+        (list @nonterm{method-sig}
+              @BNF-seq[@nonterm{type-name} @nonterm{meth-name} @litchar{(} @nonterm{param-list} @litchar{)}])
+        (list @nonterm{method-defn}
+              @BNF-seq[@optional{@nonterm{vis-modifier}} @nonterm{method-sig} open
+                         @litchar{return} @nonterm{expr} @litchar{;} 
+                       close])
+        (list @nonterm{vis-modifier} @litchar{public} @litchar{private})
+        (list @nonterm{type-name} @nonterm{class-name} @nonterm{intf-name})
+        (list @nonterm{intf-name} @nonterm{id})])
+
+@bold{Recursive Unions in Java: Lists}
+
+TBD