Error after labeling top alternatives in JSON grammar

Hi Terence,

when I label the top alternatives in the JSON grammar

json
    :   object                      # ObjectTree                
    |   array                       # ArrayTree
    ;

I get the following error:

 Exception in thread "main" java.lang.NullPointerException
    at org.antlr.v4.runtime.Parser.enterOuterAlt(Parser.java:441)
    at antlr.tests.listeners.JSONParser.json(JSONParser.java:97)
    at antlr.tests.listeners.JSON2XML.main(JSON2XML.java:138)

Looking at the source code, there is no null check for “parent.removeLastChild();” in method
enterOuterAlt of class org.antlr.v4.runtime.Parser:

    public void enterOuterAlt(ParserRuleContext<Token> localctx, int altNum) {
        // if we have new localctx, make sure we replace existing ctx
        // that is previous child of parse tree
        if ( _buildParseTrees && _ctx != localctx ) {
            ParserRuleContext<?> parent = (ParserRuleContext<?>)_ctx.parent;
            parent.removeLastChild();
            if ( parent!=null )    parent.addChild(localctx);
        }
        _ctx = localctx;
        _ctx.altNum = altNum;
    }

Best regards,
Kurt

Thanks. noted. what was your input?

I did add following lines

        
    @Override
    public void enterArrayTree(ArrayTreeContext ctx)
    {
      setXML(ctx, "<elements>");
    }

    @Override
    public void exitArrayTree(ArrayTreeContext ctx)
    {
      setXML(ctx, "</elements>");
    }

to JSON2XML.java from the book source code.

After removing the labels this code worked as expected:

    
    public void exitArrayOfValues(JSONParser.ArrayOfValuesContext ctx)
    {
      StringBuilder buf = new StringBuilder();
      buf.append("\n<elements>\n");
      for (JSONParser.ValueContext vctx : ctx.value())
      {
        buf.append("  <element>"); // conjure up element for valid XML
        buf.append(getXML(vctx));
        buf.append("  </element>\n");
      }
      buf.append("</elements>\n");
      setXML(ctx, buf.toString());
    }

I really enjoy reading your book :-)

ok, fixed for 4.0b3. thanks!