Java- How to get private data from a class in the same package

Question

I have two classes under the same package.
Class A has a private hashmap _books = new ConcurrentHashMap<String,Book>(); String title is the key.
In class B, I would like to retrieve a certain Book object by its title from the private map in class A.

I tried to add a public method in class A

public Book getBookFromMap(String title ){
    return _books.get(title);
} 

And in class B I tried to call the method, but that didn't work. Eclipse says "the method getBookFromMap is undefined". In class I tried to do this:

Book bookFromMap= getBookFromMap(booktitle);

That's when the red line appeared under the getBookFromMap in class B. I reckon that maybe I'm not calling it correctly? How can I solve the problem? Many thanks in advance!

Answer 1

Book bookFromMap= aObjectOfClassA.getBookFromMap(booktitle);

Answer 2

you can't directly access a method that is defined in another class.

what you need to do is to initialize an instance of class A using its constructor for Example:

A myInstance = new A();

then you will be able to access the method inside class A by using

Book myBook = myInstance.getBookFromMap(booktitle);

An Alternative solution

By using static : its normally preferable not to use static, but in this case, its probably easier approach. (try to avoid static as much as you can, but use it as necessary)

so you need to define shared things as static:

in A class you'll have :

private static hashmap _books = new ConcurrentHashMap<String,Book>();


public static Book getBookFromMap(String title ){
    return _books.get(title);
} 

and in B class :

Book bookFromMap= A.getBookFromMap(booktitle); //accessing getBookFromMap method in class A

Answer 3

You can use Java reflections API, but it's not recommended in most cases. Here's example

Answer 4

You need to create an instance of class A and reference its getBookFromMap via that instance.

This is what class A should look like:

public class A
{
  private Map<String, Book> booksByTitle;

  public A(Book... books)
  {
    this.booksByTitle = new ConcurrentHashMap<>();

    for (Book book : books)
    {
      this.booksByTitle.put(book.getTitle(), book);
    }
  }

  public Book getBookForTitle(String title) throws BookNotFoundException
  {
    Book book = booksByTitle.get(title);

    if (book == null)
    {
      throw new BookNotFoundException("Book not found for title: " + title);
    }

    return book;
  }
}

While the tests might look like this:

@Test
public void testGetBookForTitle() throws Exception
{
  Book warAndPeace = new Book("War & Peace")

  A a = new A(warAndPeace);

  assertEquals(warAndPeace, a.getBookForTitle(warAndPeace.getTitle));
}

@Test(expected = BookNotFoundException.class)    
public void testGetBookForInvalidTitle() throws Exception
{
  A a = new A();
  a.getBookForTitle("invalid");
}

Answer 5

Change _books to

ConcurrentHashMap<String,Book> _books;

(just remove the private at the start of the line).

Or use reflection to access the field, see How do I read a private field in Java?