Library.java

import java.util.*;

public class Library
{ 
    public List<String> filterBooks(HashMap<String, Boolean> library, IBookFilter bookFilter)
    {
        List<String> filteredBooks = new ArrayList<String>();

        for (Map.Entry<String, Boolean> entry : library.entrySet())
        {
            String bookTitle = entry.getKey();
            if (bookFilter.keep(bookTitle, entry.getValue())
                filteredBooks.add(bookTitle);
        }
                
        return filteredBooks;
    }

    public void printBooks(String listTitle, List<String> books)
    {
        System.out.println(listTitle);
        System.out.println("-------------------------------");
        
        if (books.isEmpty())
            System.out.println("No books to display!");
        
        for (String book : books)
            System.out.println(book);
        
        System.out.println("");
    }
                
    public interface IBookFilter
    {
          boolean keep(String book, boolean finished);  
    }
    
    public static void main(String[] args)
    {
        // Filter to get only finished books
        IBookFilter finishedFilter = new IBookFilter() 
        {
            public boolean keep(String book, boolean finished) 
            {
                return finished;   
            }
        };
        
        // Filter to get only unfinished books
        IBookFilter unfinishedFilter = new IBookFilter()
        {
            public boolean keep(String book, boolean finished) 
            {
                return !finished;
            }
        };
        
        // Filter to get only books with the word "the" in the title
        IBookFilter booksWithTheFilter = new IBookFilter()
        {
            public boolean keep(String book, boolean finished)
            {
                return book.toLowerCase().contains("the");   
            }
        };

        HashMap<String, Boolean> myBooks = new HashMap<String, Boolean>;
        myBooks.put("Road Down the Funnel", true);
        myBooks.put("Rat: A Biology", false);
        myBooks.put("The Wizard of Oz", false);
        myBooks.put("TimeIn", true);
        myBooks.put("3D Food Printing", false);

        Library myLibrary = new Library();

        List<String> finishedBooks = myLibrary.filterBooks(myBooks, finishedFilter);
        List<String> unfinishedBooks = myLibrary.filterBooks(myBooks, unfinishedFilter);
        List<String> booksWithThe = myLibrary.filterBooks(myBooks, booksWithTheFilter);
        
        myLibrary.printBooks("Finished Books", finishedBooks);
        myLibrary.printBooks("Unfinished Books", unfinishedBooks);
        myLibrary.printBooks("Books with the word \"the\"", booksWithThe);
    }
}