Android提示并验证用户输入对话框

Question

用户输入应仅是“ \ u0030”或“ 0030”形式的unicode字符串。用户输入（toTrain）将传递给unicdoetochar方法，如果不是unicode，则返回错误。我想验证用户输入，以便使警报对话框重新出现在用户面前。

builder
                .setCancelable(false)
                .setPositiveButton("SUBMIT",
                        new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int id) {
                                // get user input and set it to result
                                try {
                                    toTrain = SymbolLib.unicodeToChar(userInput.getText().toString());
                                    Toast.makeText(MainActivity.this, "Symbol: " + toTrain, Toast.LENGTH_SHORT).show();
                                }catch (Exception e){
                                    Toast.makeText(MainActivity.this, "Please enter only unicode!", Toast.LENGTH_SHORT).show();
                                    //alertdialog.show();  symbol cant be resolved

                                }
                                Toast.makeText(MainActivity.this, "Symbol: " + toTrain, Toast.LENGTH_SHORT).show();
                            }
                        })
                .setNegativeButton("Cancel",
                        new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int id) {
                                dialog.cancel();
                                simpleswitch.setChecked(false);
                            }
                        });
        // create alert dialog
        final AlertDialog alertDialog = builder.create();