public boolean operate(MainForm mainForm, EditForm editDialog) { DefaultListModel list1 = (DefaultListModel) mainForm.getDataList().getModel(); list1.set(0, "2343223432"); int[] row = mainForm.getDataList().getSelectedIndices(); if (row.length == 0) return false; // only edit the first one; int index = row[0]; DefaultListModel list = (DefaultListModel) mainForm.getDataList().getModel(); editDialog = (EditForm) mainForm.getEditDialog(); try { double x = Double.parseDouble(editDialog.getNewX().getText()); double y = Double.parseDouble(editDialog.getNewY().getText()); String dataPointToAdd = x + " , " + y; list.set(index, dataPointToAdd); data.editDataPoint(index, x, y); return true; } catch (NumberFormatException | NullPointerException e) { e.printStackTrace(); return false; } }