ANTLR4 Flattening a ParserRuleContext Tree into an Array - antlr4

Question

How to flatten a ParserRuleContext with subtrees into an array of tokens? The ParserRuleContext.getTokens(int ttype) looks good. but what is ttype? Is it token type? What value to use if I want to include all token types?

Answer 1

ParserRuleContext.getTokens(int ttype) only retrieves certain child nodes of a parent: it does not recursively go into the parent-tree.
However, it is easy enough to write yourself:
/**
* Retrieves all Tokens from the {#code tree} in an in-order sequence.
*
* #param tree
* the parse tee to get all tokens from.
*
* #return all Tokens from the {#code tree} in an in-order sequence.
*/
public static List<Token> getFlatTokenList(ParseTree tree) {
List<Token> tokens = new ArrayList<Token>();
inOrderTraversal(tokens, tree);
return tokens;
}
/**
* Makes an in-order traversal over {#code parent} (recursively) collecting
* all Tokens of the terminal nodes it encounters.
*
* #param tokens
* the list of tokens.
* #param parent
* the current parent node to inspect for terminal nodes.
*/
private static void inOrderTraversal(List<Token> tokens, ParseTree parent) {
// Iterate over all child nodes of `parent`.
for (int i = 0; i < parent.getChildCount(); i++) {
// Get the i-th child node of `parent`.
ParseTree child = parent.getChild(i);
if (child instanceof TerminalNode) {
// We found a leaf/terminal, add its Token to our list.
TerminalNode node = (TerminalNode) child;
tokens.add(node.getSymbol());
}
else {
// No leaf/terminal node, recursively call this method.
inOrderTraversal(tokens, child);
}
}
}