Illustration of AST Visitor pattern. (This is bad java, just to give you an idea of how things fit together)

gistfile1.txt

public abstract class BaseAST {
  public abstract void accept(Visitor v);
}

public abstract class Visitor {
	public abstract void visitBinOpNode(Expression lhs,
					                   Op op,
					                   Expression rhs);
	public abstract void visitIdentNode(String ident);
}
public abstract class Expression extends BaseAST {}

public class BinaryOperator extends Expression {

	public BinaryOperator (Expression lhs,
		                   Op op,
		                   Expression rhs) {
		this.lhs = lhs.
		...
		...
	}

	@Override
	public void accept(Visitor v) {
		v.visitBinOpNode(this.lhs, this.op, this.rhs)
	}
}

public class Identity extends Expression {
	final String ident;
	public Identity(String ident) {
		this.ident = ident;
	}

	@Override
	public void accept(Visitor v) {
		v.visitIdentNode(this.ident);
	}
}