How to inher variables values from one page to another in java?

I’m new to java and I’m stuck with this. I have one BasePage class that contains a “global” List and an int value like this;

public class BasePage {
    public int number;
    public List<String> stringList = new ArrayList<>;
}

Moreover, I’ve two classes that extend that page;

public class ClassA extends BasePage {
   
      public void addStringToList {
            stringList.add("String1");
            number++;
       }

}

And other:

public class ClassB extends BasePage {
   
      public void addStringToList {
            stringList.add("String2");
            number++;
       } 
}

Then, when I access “stringList” and “number” after calling that who classes, the values are not “stacked” in BasPage. Is there a way to achieve this? Thanks!

Answer

Your fields aren’t static, meaning, each and every instance of a BasePage object has its own ‘version’ of it. (and an instance of a ClassA is a BasePage instance as well; ClassA extends BasePage which means it’s a more specific kind of BasePage. It’s a BasePage with extra things, in other words).

Thus, when you run:

BasePage z = new BasePage();
ClassA a = new ClassA();
ClassB b = new ClassB();

You have 3 different numbers and 3 different stringList.

You can reduce this down to having just one for the entire VM by making them static but beware, this is almost universally derided as bad code style: You’ve created global state, which is hard to test, and makes code hard to read.

A better idea would be to have a separate class such that instances of it represent ‘an application’, ‘a library’, ‘a ledger’, or something similar: A thing that encompasses the idea of ‘a whole bunch of BasePage instances’.

This stringList variable would then be in the library class, not the book class. Example:

Bad design

public class Book {
    private static int booksInLibrary = 0;
    private String title;

    public Book(String title) {
        this.title = title;
        booksInLibrary++;
    }
}

Good design

public class Library {
    private final List<Book> books = new ArrayList<>();

    public int countBooks() {
        return books.size();
    }

    public void addBook(Book book) {
        books.add(book);
    }
}

public class Book {
    private final String title;

    public Book(String title) {
        this.title = title;
    }
}