ANTLR4 - Tree pattern matching code error

Question

I basically follow the step of the answer in the url: ANTLR 4 - Tree pattern matching . However, I get the error:

Error:(29, 5) java: cannot find symbol symbol: class JavaLexer location: class Main.

My code is exactly same. It seems like Java does not know that JavaLexer is. Can anyone help me?

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.ParseTree;
import org.antlr.v4.runtime.tree.pattern.ParseTreeMatch;
import org.antlr.v4.runtime.tree.pattern.ParseTreePattern;

import java.util.List;

public class Main {

  public static void main(String[] args) {

    String source = "package sampleCodes;\n" +
            "\n" +
            "public class fruits {\n" +
            "\n" +
            "  static { int q = 42; }\n" +
            "\n" +
            "  public static void main(String[] args){\n" +
            "    int a = 10;\n" +
            "    System.out.println(a);\n" +
            "  }\n" +
            "}\n";

    JavaLexer lexer = new JavaLexer(CharStreams.fromString(source));
    JavaParser parser = new JavaParser(new CommonTokenStream(lexer));
    ParseTree tree = parser.compilationUnit();

    ParseTreePattern p = parser.compileParseTreePattern("<IDENTIFIER> = <expression>", JavaParser.RULE_variableDeclarator);
    List<ParseTreeMatch> matches = p.findAll(tree, "//variableDeclarator");

    for (ParseTreeMatch match : matches) {
      System.out.println("\nMATCH:");
      System.out.printf(" - IDENTIFIER: %s\n", match.get("IDENTIFIER").getText());
      System.out.printf(" - expression: %s\n", match.get("expression").getText());
    }
  }
}

Answer 1

You have to generate the lexer and parser classes first. Do the following:

1. download the ANTLR JAR: https://www.antlr.org/download/antlr-4.8-complete.jar (place it in the same folder as your Main.java file)
2. also in this folder, download the Java grammar files: https://raw.githubusercontent.com/antlr/grammars-v4/master/java/java/JavaLexer.g4 and https://raw.githubusercontent.com/antlr/grammars-v4/master/java/java/JavaParser.g4

3. open a terminal, navigate to the folder you've downloaded the files and generate the lexer and parser classes from the Java grammar:

    java -jar antlr-4.8-complete.jar *.g4

4. compile all .java source files:

    javac -cp antlr-4.8-complete.jar *.java

5. run the Main class:

    # For Mac & *nix java -cp antlr-4.8-complete.jar:. Main # or on Windows java -cp antlr-4.8-complete.jar;. Main

The following will be printed on your console:

MATCH:
 - IDENTIFIER: q
 - expression: 42

MATCH:
 - IDENTIFIER: a
 - expression: 10