(Java, SSCE included) SwingWorker gives "Cannot find symbol"

/* Gives these compilation errors:
foo.java:21: error: cannot find symbol
                                        this.things.put(thing.id, thing);
                                            ^
  symbol: variable things
foo.java:22: error: cannot find symbol
                                        this.jtp.setText(String.valueOf(this.thi
ngs.size()));
                                                                            ^
  symbol: variable things
foo.java:22: error: cannot find symbol
                                        this.jtp.setText(String.valueOf(this.thi
ngs.size()));
                                            ^
  symbol: variable jtp
3 errors
*/

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.HashMap;
import java.util.List;

public class foo extends JFrame {
    HashMap<Integer,Thing> Things = new HashMap<Integer,Thing>();
    JTextPane jtp = new JTextPane();

    public void findThings() {
        SwingWorker<HashMap<Integer,Thing>,Thing> sw1 = new SwingWorker<HashMap<Integer,Thing>,Thing>() {
            protected HashMap<Integer,Thing> doInBackground() {
                HashMap<Integer,Thing> things = new HashMap<Integer,Thing>();
                Thread.sleep(1000);
                return things;
            }

            protected void process(List<Thing> chunks) {
                for(Thing thing : chunks) {
                    this.things.put(thing.id, thing);
                    this.jtp.setText(String.valueOf(this.things.size()));
                }
            }
        };

        sw1.execute();
    }

    public foo() {
        super();
        setLayout(new BoxLayout(getContentPane(), BoxLayout.PAGE_AXIS));
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JButton jbtn = new JButton("findThings()");
        jbtn.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                findThings();
            }
        });
        add(jbtn);
        this.jtp.setPreferredSize(new Dimension(300,300));
        add(this.jtp);

        setLocationRelativeTo(null);
        pack();
        setVisible(true);
    }

    public static void foo(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new foo();
            }
        });
    }

    private class Thing {
        public Thing() {
            id = 100;
            name = "Thing's name";
        }

        Integer id = null;
        String name = null;
    }
}