antlr/antlr4 · error · IllegalArgumentException

labels cannot be null

Error message

labels cannot be null

What it means

Error "labels cannot be null" thrown in antlr/antlr4.

Source

Thrown at runtime/Java/src/org/antlr/v4/runtime/tree/pattern/ParseTreeMatch.java:64

	 * {@link ParseTree} objects located by the tree pattern matching process.
	 * @param mismatchedNode The first node which failed to match the tree
	 * pattern during the matching process.
	 *
	 * @exception IllegalArgumentException if {@code tree} is {@code null}
	 * @exception IllegalArgumentException if {@code pattern} is {@code null}
	 * @exception IllegalArgumentException if {@code labels} is {@code null}
	 */
	public ParseTreeMatch(ParseTree tree, ParseTreePattern pattern, MultiMap<String, ParseTree> labels, ParseTree mismatchedNode) {
		if (tree == null) {
			throw new IllegalArgumentException("tree cannot be null");
		}

		if (pattern == null) {
			throw new IllegalArgumentException("pattern cannot be null");
		}

		if (labels == null) {
			throw new IllegalArgumentException("labels cannot be null");
		}

		this.tree = tree;
		this.pattern = pattern;
		this.labels = labels;
		this.mismatchedNode = mismatchedNode;
	}

	/**
	 * Get the last node associated with a specific {@code label}.
	 *
	 * <p>For example, for pattern {@code <id:ID>}, {@code get("id")} returns the
	 * node matched for that {@code ID}. If more than one node
	 * matched the specified label, only the last is returned. If there is
	 * no node associated with the label, this returns {@code null}.</p>
	 *
	 * <p>Pattern tags like {@code <ID>} and {@code <expr>} without labels are
	 * considered to be labeled with {@code ID} and {@code expr}, respectively.</p>

View on GitHub (pinned to 7d5770395b)

Solutions

1. Pass a non-null Map for labels to the ParseTreeMatch constructor.
2. Do not construct ParseTreeMatch directly; obtain it from ParseTreePattern.match(tree) which supplies the labels map.

Example fix

Map<String, List<ParseTree>> labels = new MultiMap<>();
ParseTreeMatch match = new ParseTreeMatch(tree, pattern, labels);

When it happens

Trigger: ParseTreeMatch constructed with a null labels map.

Common situations: Pass a non-null (possibly empty) MultiMap for labels when constructing ParseTreeMatch; internal matcher code always supplies one.

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/40b4cbfdcfc38a95. Report an issue: GitHub.