antlr/antlr4 · error · IllegalArgumentException

tree cannot be null

Error message

tree cannot be null

What it means

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

Source

Thrown at runtime/Java/src/org/antlr/v4/runtime/tree/pattern/ParseTreePatternMatcher.java:261

	// ---- SUPPORT CODE ----

	/**
	 * Recursively walk {@code tree} against {@code patternTree}, filling
	 * {@code match.}{@link ParseTreeMatch#labels labels}.
	 *
	 * @return the first node encountered in {@code tree} which does not match
	 * a corresponding node in {@code patternTree}, or {@code null} if the match
	 * was successful. The specific node returned depends on the matching
	 * algorithm used by the implementation, and may be overridden.
	 */

	protected ParseTree matchImpl(ParseTree tree,
								  ParseTree patternTree,
								  MultiMap<String, ParseTree> labels)
	{
		if (tree == null) {
			throw new IllegalArgumentException("tree cannot be null");
		}

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

		// x and <ID>, x and y, or x and x; or could be mismatched types
		if ( tree instanceof TerminalNode && patternTree instanceof TerminalNode ) {
			TerminalNode t1 = (TerminalNode)tree;
			TerminalNode t2 = (TerminalNode)patternTree;
			ParseTree mismatchedNode = null;
			// both are tokens and they have same type
			if ( t1.getSymbol().getType() == t2.getSymbol().getType() ) {
				if ( t2.getSymbol() instanceof TokenTagToken ) { // x and <ID>
					TokenTagToken tokenTagToken = (TokenTagToken)t2.getSymbol();
					// track label->list-of-nodes for both token name and label (if any)
					labels.map(tokenTagToken.getTokenName(), tree);
					if ( tokenTagToken.getLabel()!=null ) {

View on GitHub (pinned to 7d5770395b)

Solutions

1. Pass a non-null ParseTree to ParseTreePatternMatcher.match(...).
2. Verify the parse succeeded (no syntax errors) and the returned tree is not null before calling match.

Example fix

ParseTree tree = parser.expr();
if (parser.getNumberOfSyntaxErrors() == 0 && tree != null) {
    ParseTreeMatch m = pattern.match(tree);
}

When it happens

Trigger: ParseTreePatternMatcher.match() invoked with a null tree.

Common situations: Guard that the parse tree returned by the parser is non-null before calling match(tree, pattern).

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/ead8a4b00c1d3f54. Report an issue: GitHub.