How do you pass an ArrayList into the constructor?

Question

import java.util.ArrayList;

public class Finance {

ArrayList<String> title = new ArrayList<String>();
ArrayList<Double> moneySpent = new ArrayList<Double>();
Scanner input = new Scanner(System.in);


public Finance(ArrayList<String> title, ArrayList<Double> moneySpent){
this.title = title;
this.moneySpent = moneySpent;
System.out.println(title);
System.out.println(moneySpent);
}

public static void main(String[] args){
Finance entertainment = new Finance(title.add("Movies"), moneySpent.add(1));
}

When I run the code this, I found 2 errors:

File: D:\Java Programs\Finance.java [line: 19] Error: Cannot make a static reference to the non-static field title

File: D:\Java Programs\Finance.java [line: 19] Error: Cannot make a static reference to the non-static field moneySpent

score 0 · Answer 1 · answered Jun 06 '20 at 11:42

Although your focus is in creating an instance Finance there are few concepts I'd like to invite you to explore.

To start and to answer your question, the Finance's contructor signature defines the contract for a Finance oject to become an instance. It expects two arguments, each of which are of ArrayList type. To create an instance of Finance you have to create two arrays, one made of Strings and the other made of Doubles, on this thread you have many ideas on how to initialise arrays.

An example could be,

  new Finance(new ArrayList<>(List.of("Movies")), new ArrayList<>(List.of(1.0)));

Next, I'd like to drag your awareness on this expression in your code:

title.add("Movies")

title is what you want initialised inside your constructor and altough is not a bad idea to have it initialised as an empty ArrayList as you did, it's pointless to add an element to it and then pass it to your constructor for it to change the same instance object.

Next, I'd like to drag your awareness on the types you are using: In my personal opinion I would use java's List interface when declaring variables, use a plural to name any collection of objects, and in your case, use ArrayList as the List interface implementing class. For example,

List<String> titles; 
// and
List<Double> expenses;

Same on the constructor:

public Finance(List<String> titles, List<Double> expenses) {
  //...init variables here
}

Note that I renamed moneySpent to expenses. I did this because a money spent on multiple moments it's more likely to be expenses and moneySpent would be the amount spent on a single moment.

Applying Single Resposibility I would refactor the creation of a Finance instance onto a separete (static, why static?) method.

public class Finance {

    List<String> titles;
    List<Double> expenses;
    Scanner input = new Scanner(System.in);

    public Finance(List<String> titles, List<Double> expenses) {
        this.titles = titles;
        this.expenses = expenses;
        System.out.println(this.titles);
        System.out.println(this.expenses);
    }

    public static void main(String[] args) {
        Finance entertainment = createFinance();
    }

    private static Finance createFinance() {
        return new Finance(
                new ArrayList<>(List.of("Movies")),
                new ArrayList<>(List.of(1.0)));
    }
}

Lastly, Consider driving your implementation guided by tests. I can recommend: TDD by example, GOOS and Jason Gorman's TDD book

How do you pass an ArrayList into the constructor?

1 Answers1