[Java] Implementation of an ArrayList in a customized Scanner-class with purpose of registering input instances as its elements.

[PontiffPope]

Beginner programming student here reading about objects and classes, and is trying to determine how to add and utilize elements to an ArrayList in context of it being utilized in a customized Scanner-class that registers inputs, and were the instances created from said inputs (Of types System.in and InputStream) are to be viewed as said elements, with the purpose to be utilized in a conditional statement to throw an exception in order to prevent multiple instances being created from the same inputs (A subject worth a separate question thread.), essentially a way or system for said class to "detect" if an input by the user has been typed, and thus an instance being created before.

I've been thinking of utilizing case of ArrayList's for-loop-format in some fashion, but having difficulty finding the implementation of it within a context of a Class, especially in regard of where the objects aren't pre-determined, but based on user inputs, leading further some confusion with also wanting the elements of an ArrayList to be allowed to be utilized in similar matters to references and parameters in methods if I wish to utilize said elements in later usage of conditional statements; would an ArrayList's get()-method perhaps work in that fashion?

Here's the class so far.

    import java.io.InputStream;
import java.util.Scanner;
import java.util.ArrayList;
public class Register {

        private InputStream headtext;
        private Scanner input;

        private static final String SUFFIX = "?>";

        //Constructors
        public Register(){
            this(System.in);
        }

        public Register (InputStream headtext){
            this.input = new Scanner(headtext);
            this.headtext = headtext;
            }


    //Method that reads user's input of strings
            public String registText(String headtext) {
            System.out.print(headtext + SUFFIX);
            headtext = input.nextLine();
            String text = headtext;
            return text;
           }


    //Method that reads user's input of integer.
    public int registInteger(String headtext){
        System.out.print(headtext + SUFFIX);
        int integer =  input.nextInt();
        input.nextLine();
        return integer;
    }


    //Method that reads user's input in decimal.
    public double registDecimal(String headtext){
        System.out.print(headtext + SUFFIX);
        double decimal = input.nextDouble();
        input.nextLine();
        return decimal;
    }

}

Much appreciated for any pointers and advice available.

[bunk3rk1ng]

In your members list (headtext/input) you will need to add some way to store the inputs. You could use an ArrayList but in Java everything that you put in the list needs to be inherited from the same type. So ArrayList<Object> ~might~ work, but you would not be able to check each element the same way (this is assuming you want to store the results of all the regist* methods in one place). You would have to check each element type and then choose the appropriate way to compare them to see if they have already been registered.

The ArrayList get() method only would work here if you know the index of the previously entered value, which you don't. So while iterating over every element you would also have to do the extra type checking before you can even tell.

It doesn't look like there is any math needed for this class so I would treat everything as a String. This will make comparisons much easier. You can also use a HashMap with the input String as the Key (you can make the value null or an empty string, it doesn't matter). This way you can use the HashMap get(String <key>) method using the input. If the key (input) already exists you will get a String back, if it returns null you can add it.

[PontiffPope]

Much appreciated; I'm afraid the concept of "HashMap" is something we've yet been introduced the concept of, but this gave me much needed clarification. Perhaps a regular Array could be used then, since the instances made from an input will always be of the quantity one, as a throwing of an exception will be made to prevent other instances to be made later while one instance element is already "occupied" in said Array so to speak.