CryptarithmSolverGUI.java

// ...
                final ActionListener listener = event -> {
                if (working) {
                    return;
                }
                working = true;
                solveButton.setEnabled(false);
                solutionArea.setText("Computing solutions\u2026");
                progress.setIndeterminate(true);
                new SwingWorker<List<Map<Character, Integer>>, Void>() {
                    @Override
                    protected List<Map<Character, Integer>> doInBackground()
                            throws Exception {
                        return solver.solve(input.getText());
                    }

                    @Override
                    protected void done() {
                        try {
                            final List<Map<Character, Integer>> solutions = this
                                    .get();
                            solutionArea.setText(stringify(solutions));
                        } catch (InterruptedException e) {
                            // Cancelled (somehow). Nothing to do.
                            e.printStackTrace();
                        } catch (ExecutionException e) {
                            solutionArea
                                    .setText("Bad input: " + e.getMessage());
                        } finally {
                            working = false;
                            solveButton.setEnabled(true);
                            progress.setIndeterminate(false);
                        }
                    }
                }.execute();
            };
            solveButton.addActionListener(listener);
            input.addActionListener(listener);