MiniJava is a subset of Java.  The meaning of a MiniJava program
is given by its meaning as a Java program.

In a MiniJava class, each variable and method has a name which is different 
from all other names (of variables, methods, and formal parameters) which 
are declared in that class.  Notice that two methods of a class may have 
formal parameters with the same name.

The MiniJava statement 
  
   System.out.println( ... );

can only print integers.

You can generate a MiniJava tree builder as follows:

   jtb minijava.jj
   javacc jtb.out.jj
   javac Main.java

You can now parse a MiniJava program in, say, Fac.java:

   java Main < Fac.java  

The program in Main.java can invoke visitors.
For example, the current Main.java contains the call 

   root.accept(new Visitor());

MicroJava is also a subset of Java, with the same restrictions as
in MiniJava.