Java：数组返回null

Question

我试图理解为什么我的数组不断返回为null或超出范围。我必须使用来自文本字段的用户输入来填充数组。有两种方法：一种用于检查密码，另一种用于存储密码。

//instance variables
Account [] myAccount = new Account[10];
    int lastAccountInteger = -1;
    final int MAX_ACCOUNTS_INTEGER = 9;
    `enter code here`public void processNewAccount()
    {
        //Create local variables
        int pinCheckInteger;
        String firstNameString = "", lastNameString = "", pinString;

        firstNameString = firstNameTextField.getText();
        lastNameString = lastNameTextField.getText();
        pinString = newPinTextField.getText();

        lastAccountInteger++;
        if(lastAccountInteger == 0)
        {
            if(firstNameString.equals("") && lastNameString.equals(""))
            {
                AlertDialog("Account Name", "Name on account", "Enter your name");
                firstNameTextField.requestFocus();
            }

                if(pinString.equals(""))
                {
                    AlertDialog("Pin", "Pin number is blank", "Enter a Pin");
                    newPinTextField.requestFocus();
                }
                else
                {
                    if(checkPin(pinString) == 0)
                    {
                        AlertDialog("Pin", "Pin number does not exist", "Enter a Pin");
                        newPinTextField.selectAll();
                        newPinTextField.requestFocus();
                    }
                    else
                    {
                        pinCheckInteger = checkPin(pinString);
                        displayAllAccountInfo(pinCheckInteger);
                    }

                }

        }
        else
        {
            AlertDialog("Pin", "Pin", "pin");
        }
    }//end of processNewAccount method
public int checkPin(String checkPinString)
    {
        boolean checkPinBoolean = false;
        int indexInteger = 0;
        String pinString;

        while(!checkPinBoolean && indexInteger <= lastAccountInteger)
lastAccountInteger++ )
        {

            //Stores the item of the myAccount array that is being checked
            pinString = String.valueOf(myAccount[lastAccountInteger].getPin());
            //Checking if the user input and the item are the same
            if(pinString.length() == 4)

            {
                checkPinString.equals(pinString);
                //Assigns to true
                checkPinBoolean = true;
            }
            else
            {
                //Add to one
                indexInteger++;
            }
        }
        //If the boolean is true then return the indexInteger
        if(checkPinBoolean == false)
        {
            indexInteger = -1;
        }
        //Return the position in the array where the pin matches or not
        return indexInteger;
    }

我正试图存储图钉并取回它，然后递增到下一个元素，并对10个帐户执行相同的操作。