Apply methods without knowing object type in java

Question

I have two classes in java: Movie and Book (it's a simplified example):

Book can have author:

    public class Book {

      public String author;

      public Book(String a) {
        this.author = a;
      }

      public String getAuthor(){
        return author;
      }

}

And Movie can have title:

    public class Movie {

      public String title;

      public Movie(String t) {
        this.title = t;
      }

      public String getAuthor(){
        return title;
      }

}

I'm trying to put all objects in a list like this:

    ArrayList myList = new ArrayList();

    Book book = new Book("William");
    Movie movie = new Movie("Titanic");


    myList.add(book);
    myList.add(movie);

And afterwards I want to count how many books written by John do I have (or any other specific titles). However I can't apply getAuthor() or getTitle() method since java doesn't know what type of object it is

    int counter = 0;
    for (int i =0;i<myList.size();i++){
        if (myList.get(i).getAuthor().equals("John") ){
            counter +=1;
        }

I would be able to use if clause, check every time for an object type, and apply different methods for different objects, but this is not viable, since in real-life case I have 20+ classes and it would make code very long and maintainable.

Can someone suggest a solution for this? Thanks in advance!

Answer 1

create an interface

public interface HasAuthor {
    String getAuthor();
}

implement this interface in both your classes and use this:

List<HasAuthor> list = new ArrayList<>();
list.add(new Book());
list.add(new Movie());

long count = list.stream().filter(smth -> "John".equals(smth.getAuthor())).count();

Answer 2

You cannot be using ArrayList myList = new ArrayList(); in 2017. The world has moved on from that archaic and error-prone style of programming. Generics were added to the Java programming language in 2004, and since then, any attempt to use a generic class without a generic type argument issues a warning. Which brings me to the next issue:

You cannot be ignoring warnings in 2017. Actually, there was never a good time to be ignoring warnings. Your IDE ought to be issuing warnings when you try to do ArrayList myList = new ArrayList(); heed them.

So, bottom line is, you should not be putting books and movies in the same collection. If you have a book class that has an author, and a movie class that has a director, (I will ignore your example of movies having a title and returning that as "author", because it is nonsensical,) then you can have either an interface or an abstract base class called, say, Item, with a String getAuthor() method, which is implemented (overridden) in both Book and Movie.

Then, your myList will be an ArrayList<Item>, and since Item has a getAuthor() method, you will be able to do myList.get( 0 ).getAuthor() and it will work without having to know whether it is a book or a movie.

Answer 3

First, myList.get(i).getAuthor() == "John" won't work since strings need to be compared via equals() (look up tutorials on why).

Second, you need to know the type of your objects and cast accordingly in order to call a method (you could do without the cast using reflection but please don't try that at home). Thus when iterating over your list you need to check:

for (Object o : myList ) {     
  if (o instanceof Book && ((Book)o).getAuthor().equals("John") ){
        counter +=1;
  }
}

However, if you want one list to contain all books and movies you'd better provide a common interface or superclass:

//Make it abstract to not allow instances of this class directly
abstract class PieceOfArt {
   private String creator;

   public String getCreator() {
     return creator;
   }
}

class Book extends PieceOfArt {
  //Access the creator as the author
  //note that I do this just for demonstration purposes, just using getCreator() would be perfectly fine
  public String getAuthor() {
    return getCreator();
  }
}

class Movie extends PieceOfArt {
  //Access the creator as the director
  //note that I do this just for demonstration purposes, just using getCreator() would be perfectly fine
  public String getDirector() {
    return getCreator();
  }
}

List<PieceOfArt> myList = ...;
for( PieceOfArt p : myList ) { 
  if( p.getCreator().equals("John") {
    ...
  }
}

Answer 4

Putting objects of different, unrelated types, such as Book and Movie, which don't have a common superclass (besides java.lang.Object) is bad practice.

You could define a common abstract superclass for these types, and then create a List of that type. For example:

public abstract class Product {
    private String title;
    private String author;

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }
}

class Book extends Product {
}

class Movie extends Product {
}

Create a List<Product> and work with that:

List<Product> products = new ArrayList<>();

Book book = new Book();
book.setTitle("Cooking");
book.setAuthor("Bob the Cook");
products.add(book);

Movie movie = new Movie();
movie.setTitle("Romance at sea");
movie.setAuthor("John");
products.add(movie);

int count = 0;
for (Product product : products) {
    if (product.getAuthor().equals("John")) {
        count++;
    }
}

NOTE: Do not make the getAuthor method actually return the title in case of a Movie, that would make your program really confusing.