Java DOM/AST - why is MethodInvocation an Expression?

Question

In the Java DOM/AST (http://help.eclipse.org/indigo/topic/org.eclipse.jdt.doc.isv/reference/api/org/eclipse/jdt/core/dom/package-tree.html) why does an Initializer contain a Block and why is a MethodInvocation an Expression and not a Statement?

I mean, given the code

int a = Integer.parseInt("1");

the ASTView plugin shows me only INITIALIZER > MethodInvocation
Can there ever be an Initializer that really has a Block element?

Moreover, if I have a method like this

public void thisMethod(){
    System.out.println();
}

MethodInvocation is wrapped into an ExpressionStatement. But why isn't MethodInvocation a Statement? Just a simple System.out.println() is a valid "Statement". An Expression in the sense of the Java DOM/AST lacks the ability to stand for itself.

Maybe I just didn't the get the whole idea of the separation between Expression and Statement.

Answer 1

The AST tree is designed to reflect the syntax rules of the programming language.

Syntactically, an Initializer consists of a block with an optional static modifier before it. So it’s the most natural implementation having an Initializer class consisting of a Block and modifiers (inherited from BodyDeclaration). So I don’t understand why you question it.

Example of initializers:

class Foo {
  static {
    System.out.println("static initializer");
    System.out.println("class Foo now initialized");
  }

  {
    System.out.println("instance initializer");
    System.out.println("an instance of Foo has been created");
  }
}

---

Expressions and statements are two different syntactic constructs. There are places where only either an expression or a statement is allowed. But there are constructs like method invocations which are Expression Statements which means the can fulfill both roles, Expression and Statement. They can be invoked stand-alone for their side-effects but also at places where a value is required.

But since Java does not allow multiple inheritance you cannot create an AST class ExpressionStatement inheriting from both Expression and Statement. So you need a solution like in the Eclipse AST where ExpressionStatement inherits from one and wraps the other. The decision which one to inherit and which one to wrap is easy: you can create a Statement implementation (subclass) which drops the result of the Expression it has wrapped but you cannot create an Expression implementation which provides a result for a Statement that provides no result through its interface. An alternative to such an implementation would be the use of interfaces.