antlr/antlr4 · error · NullPointerException

delegates

Error message

delegates

What it means

ProxyErrorListener's constructor throws NullPointerException("delegates") when the delegate collection is null — the proxy fans every error callback out to that collection during parsing, so a null collection cannot be iterated. An empty collection is fine (errors are simply dropped). This is a fail-fast guard for an internal composition helper most users meet indirectly.

Source

Thrown at runtime/Java/src/org/antlr/v4/runtime/ProxyErrorListener.java:26

import org.antlr.v4.runtime.atn.ATNConfigSet;
import org.antlr.v4.runtime.dfa.DFA;

import java.util.BitSet;
import java.util.Collection;

/**
 * This implementation of {@link ANTLRErrorListener} dispatches all calls to a
 * collection of delegate listeners. This reduces the effort required to support multiple
 * listeners.
 *
 * @author Sam Harwell
 */
public class ProxyErrorListener implements ANTLRErrorListener {
	private final Collection<? extends ANTLRErrorListener> delegates;

	public ProxyErrorListener(Collection<? extends ANTLRErrorListener> delegates) {
		if (delegates == null) {
			throw new NullPointerException("delegates");
		}

		this.delegates = delegates;
	}

	@Override
	public void syntaxError(Recognizer<?, ?> recognizer,
							Object offendingSymbol,
							int line,
							int charPositionInLine,
							String msg,
							RecognitionException e)
	{
		for (ANTLRErrorListener listener : delegates) {
			listener.syntaxError(recognizer, offendingSymbol, line, charPositionInLine, msg, e);
		}
	}

View on GitHub (pinned to 7d5770395b)

Solutions

1. Pass an empty list instead of null: new ProxyErrorListener(listeners != null ? listeners : Collections.emptyList())
2. Fix the producer of the delegate collection to never return null (return empty collections)
3. Skip constructing the proxy entirely when there are no listeners

Example fix

// before
Collection<ANTLRErrorListener> ls = maybeListeners(); // may be null
new ProxyErrorListener(ls); // NPE

// after
new ProxyErrorListener(ls == null ? List.of() : ls);

Collection<ANTLRErrorListener> safe = (delegates != null) ? delegates : Collections.emptyList();
ProxyErrorListener proxy = new ProxyErrorListener(safe);

When it happens

Trigger: new ProxyErrorListener(null), usually because a varargs/collect step produced null; a helper method that builds the delegate list conditionally and returns null; DI wiring that failed to provide the listener list.

Common situations: Aggregating multiple error listeners (console + in-memory collector) in tooling; refactoring that moved listener-list construction; Optional-averse code paths that use null as 'no listeners'.

Related errors

• tokenSource cannot be null
• tokens cannot be null
• listener cannot be null.
• tokenSource cannot be null
• tokens cannot be null

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/a6a938b848cc8185. Report an issue: GitHub.