Lecture 7: Introducing Java: Syntax and Semantics

Video.

Introducting Java Syntax

You’ve now seen several programming languages: BSL, ISL, ISL+, and class/0. Now time for one more: Java. Let’s start by looking at the syntax, the way Java programs are written.

Notice here that the convention for class names in Java is to capitalize them. Also notice that, because Java is a typed language, it requires us to specify the type of the fields. In this case, the fields x and y are both objects belonging to the Integer class. So in Java, some of the information that we’re used to writing down as part of data definitions becomes part of the actual program text; not just a comment.

The above class definition defines a new class of values, called Coords. A Coord value is an object with an x and y field, both of which hold Integer objects.

One thing that Java requires us to write explicitly is a constructor, whereas class/0 made the constructors for us. The way constructors work in class/0 is to say new, a class name, and then give the appropriate number of expressions (one for each field); the value of these expressions are used to populate the fields of the object. We can write such a constructor in Java by revising the class definition:

This constructor definition says if you call the constructor (we will see how in a moment) with two integers, it will populate the fields of a Coord object with the given integers.

In Java, the notation for send is a “dot” written between the object expression and the field name:

There are a few things to note here. First, all of the peices from the class/0 definition are present, but the signature that existed in comments is now part of the code. The part of the signature to the right of the -> is now to the left of the method name in Java. The other thing to note is the use of the return keyword and ; around the expression in the body of the method; this is signalling that the method produces whatever the expression evaluates to.

Invoking the method uses the send notation similar to accessing a field:

The above expression will produce new Coord(4, 6) when run.

To actually run programs, we will have to either run the programs from within a programming environment, like DrRacket but for Java, or use a Java compiler and run the Java interpreter from the command line. These will be covered in demos in class.

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 grammar: