如何在2个不同的类上使用一个数组

Question

I have 2 classes right now, the first class has the arraylist in it. But on the second class when I try to access the arraylist it keeps giving me the red line underneath saying that the variable doesn't exist.

Here is class one...

public class BankMain {


    public static void main(String[] args) {
        // TODO Auto-generated method stub
        BankMain main = new BankMain();

        menu();
    }

    public static void cardNumbers(){
        ArrayList<Integer> cardNum = new ArrayList<Integer>();
        Scanner cards = new Scanner(System.in);
        Scanner input = new Scanner(System.in);
        Scanner keyboard = new Scanner(System.in);

        System.out.println("Please select a 5 digit card number");

        cardNum.add(input.nextInt());

        System.out.println("Thank you! You're card number is " +cardNum);

        System.out.println("Type 'c' to go back to main menu.");
        String value  = keyboard.next();
        if(value.equalsIgnoreCase("c")){
            menu();
        }
        else if (!keyboard.equals('c')){
            System.out.println("Invalid Entry!");
        }
    }

    public static void menu(){
        System.out.println("What Would you like to do today?");
        System.out.println();
        System.out.println("Create Account = 1");
        System.out.println("Login = 2");
        System.out.println("Exit = 3");
        query();
    }

    public static void query(){
        Scanner keyboard = new Scanner(System.in);
        double input = keyboard.nextInt();  

        if (input == 2){
            BankMainPart2 main2 = new BankMainPart2();
            System.out.println("Please enter your 5 digit card number.");
            main2.loginCard();
        }
        else if (input == 1){
            cardNumbers();
        }
        else if (input == 3){
            System.out.println("Thank you, have a nice day!");
            System.exit(0);
        }
    }
}

Here is the second class...

public class BankMainPart2 {
    public static void loginCard(){
        if (cardNum.contains(name)) {
        }
    }
}

I know I haven't entered anything in the if statement yet on the second class but I'm just trying to get my array list to work on both classes.

Answer 1

The code looks very naive. A very simple answer to your question is

You have not declared any cardNum in BankMainPart2 as global variable or in loginCard as local variable, how do you think it will be available in the loginCard method?

ArrayList<Integer> cardNum = new ArrayList<Integer>();

is local to cardNumbers method.

How can you access it from other class?

1. A local variable cannot be accessed from outside the method, so first thing, make cardNum class level variable
2. Make the variable public if you want other classes to be able to access it directly, else make the variable private and create getter method (setter if required).
3. You can also send the variable when calling the method as argument
4. If this is class level variable, make it static and use Classname.variable.

--Edit--

As you have asked for details let me give you a quick overview of the different approaches.

1. A variable declared inside a method is local. as name suggest "local", no one but the method knows there is such a variable. No other method in the class knows about existence of this variable, let alone some outside class.
2. I say you can make it static, but static should strictly be used for class level storage, not object level. Say a list which is modified by multiple objects of the same class (I hope you know concepts of objects, else go to the basics otherwise it will not be clear). Now as per your example, I guess this is not what you want.
3. A public variable is generally no - no, only in few cases it will be useful (for example in android programming where performance is utmost important). Normally we will create a variable and provide getter setters. A getter or setter is used normally when we want to give access to the variable, which again does not look like what you want.
4. Last, the variable is private to you class, but if you want some method to do something about it, you can pass it as argument, this looks the case for you.

Step by step

take the variable out of method and add to class level, note that I removed static from method names

public class BankMain {
private ArrayList<Integer> cardNum = new ArrayList<Integer>();
// rest of code as it is 
..
..
 BankMain main = new BankMain();
 //change
    main.menu();

 //no need foe static
  public void cardNumbers(){
//no need here now        
//ArrayList<Integer> cardNum = new ArrayList<Integer>();
        Scanner cards = new Scanner(System.in);
        Scanner input = new Scanner(System.in);
..
..

//public static void menu(){
  public void menu(){


//send the list
 //I see there are confusion at times regarding calling of static method.
 //please note objectname.staticMethod() or classname.staticMethod() is one 
 //and same thing. Just that classname.staticMethod() is more clear 
 BankMainPart2.loginCard(cardNum);

}

and

public class BankMainPart2 {
    public static void loginCard(ArrayList<Integer> cardNum){
        if (cardNum.contains(name)) {
        }
    }
}

Answer 2

Your method, BankMainPart2.loginCard has not context of "cardNum", it doesn't know what it is (type or value).

In order for the method to be able to act on the array list, you must pass a reference to it, something like...

public class BankMainPart2 {
    public static void loginCard(ArrayList<Integer> cardNum){
        if (cardNum.contains(name)) {
        }
    }
}

Answer 3

make the cardnum arraylist as an instance variable in BankMain class and extend BankMain in BankMainClass2 and using reference of BankMain you would be able to access cardNum like this

Class BankMain {
  public ArrayList<String> cardNum = new ArrayList<String>();

 }
 Class BankMain2 extends BankMain {
    public void method() {
  BankMain2 main = new BankMain2();
      sysout(main.cardNum.size());
      }
 }

but the above scenario would only work when cardNum ArrayList in BankMain class is either marked public,protected or default(Nomodifier). it wouldnt work if its marked as private and other non access modifier such as static and final

Answer 4

You can try any one of these

1.Declare the Arraylist as public then import the first class and use the cardNum in the second class

2.Make the cardNum a static var and use it directly in second class as BankMain.cardNum

3.Pass the Arraylist as argument to the second class.

Answer 5

The key problem is in the the way you are trying to create your classes. Your current problem can be solved by answer given by @MadProgrammer. But you should definitly have a look into the Object Oriented Programming Concepts. This section on How to identify and design a Class? should give you some clear pointers.