ANTLR is a powerful parser generator for multiple programming languages including Java.

ANTLR contains 2 major modules:

• Runtime - For building and executing parsers/lexers generated in Java.
• Tool (The Parser Generator) - For generating parsers/lexers Java class.

ANTLR Runtime Source Code files are provided in the distribution packge (antlr4-4.10.1.zip). You can download them at ANTLR Website.

You can also browse the source code below:

⏎ org/antlr/v4/runtime/atn/AtomTransition.java

/*
 * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.runtime.atn;

import org.antlr.v4.runtime.misc.IntervalSet;

/** TODO: make all transitions sets? no, should remove set edges */
public final class AtomTransition extends Transition {
	/** The token type or character value; or, signifies special label. */
	public final int label;

	public AtomTransition(ATNState target, int label) {
		super(target);
		this.label = label;
	}

	@Override
	public int getSerializationType() {
		return ATOM;
	}

	@Override

	public IntervalSet label() { return IntervalSet.of(label); }

	@Override
	public boolean matches(int symbol, int minVocabSymbol, int maxVocabSymbol) {
		return label == symbol;
	}

	@Override
	public String toString() {
		return String.valueOf(label);
	}
}

⏎ org/antlr/v4/runtime/atn/AtomTransition.java

Or download all of them as a single archive file:

File name: antlr-runtime-4.10.1-sources.jar
File size: 308953 bytes
Release date: 2022-04-15
Download 

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