Why do I get an error when trying to give a lexer as input of CommonTokenStream in ANTLRv4

Question

so I'm trying to create a launcher for my ANTLR4 parser in java and I get 'incompatible types' error when I try to run my code:

import org.antlr.runtime.*;
import java.io.IOException;
import static org.antlr.v4.runtime.CharStreams.fromFileName;

public class TLLauncher {
    public static void main(String[] args) {
        try{
            String inp = "input.txt";
            org.antlr.v4.runtime.CharStream in = fromFileName(inp);
            gramLexer lexer = new gramLexer(in);
            CommonTokenStream tokens = new CommonTokenStream(lexer); // ###HERE
            gramParser parser = new gramParser(tokens); // ###HERE
            parser.r();
        } catch (IOException e) {e.printStackTrace();}
    }
}

and i get the following errors.

incompatible types: gramLexer cannot be converted to org.antlr.runtime.TokenSource
incompatible types: org.antlr.runtime.CommonTokenStream cannot be converted to org.antlr.v4.runtime.TokenStream

can someone explain to me what I'm doing wrong?

ALSO, since we're here, can you give me a good source to learn ANTRL? I need to somehow make an AST of the input code and convert it to another language but I'm having trouble even understanding how ANTLR works and how i could get it done.

Answer 1

import org.antlr.runtime.*;

You're importing the wrong runtime. org.antlr.runtime is the ANTLR3 runtime. The ANTLR4 runtime lives in org.antlr.v4.runtime (from which you're already using CharStream and CharStreams, but not CommonTokenStream).