public Object getValueAt(int row, int col) {
                synchronized(LOCK) {
                    if (row < stack.length) {
                        switch (col) {
                            case 0:
                                String name = stack[row].getFileName();
                                return name == null || name.isEmpty() ? "unknown" : name;
                            case 1:
                                int number = stack[row].getLineNumber();
                                return number > -1 ? number : "unknown";
                            case 2:
                                return stack[row].getClassName();
                            case 3:
                                return stack[row].getMethodName();

                        }
                    }
                    return null;
                }
            }