Class-Based Object-Oriented Programming Example

This is specifically for typechecking and code generation. Key language features:

• Object-oriented classes
• Inheritance
• Subtyping

Assumptions:

• void is not a special value (Java/C/C++ semantics)
• Object is a built-in class with a no-arg constructor

public class MyClass {
  public MyClass(int x) {}
  public void myMethod() { println(true); }
}

public class Subclass extends MyClass {
  public Subclass(int y) { super(y); }
  //public void myMethod() { println(false); }
}

public class OtherClass { ... }
MyClass temp = new OtherClass(); // new SubClass(7);
temp.myMethod();

public class Foo extends Bar {}
public class Bar extends Foo {}

// forgetting to return from a function in C: undefined behavior
public static int doSomething(int x) {
  if (x < 3) {
    return 11;
  } else {

  }
}

x is a variable
i is an integer
classname is a class name
methodname is a method name
type ::= int | bool | void | classname
op ::= + | < | ==
exp ::= i | x | true | false | this | exp op exp | exp.methodname(exp*) | new classname(exp*)
vardec ::= type x
stmt ::= exp; |
         vardec = exp; |
         if (exp) stmt else stmt |
         while (exp) stmt |
         return exp; |
         return; |
         println(exp); |
         { stmt* }
methoddef ::= type methodname(vardec*) stmt
classdef ::= class classname extends classname {
               vardec*; // comma-separated instance variables
               constructor(vardec*) {
                 super(exp*);
                 stmt* // comma-separated
               }
               methoddef*
             }
program ::= classdef* stmt // stmt is the entry point