import java.util.HashMap;
import java.util.Map;

public class App {

    public static void main(String[] args) {
        Object object = new Object();
        Map<Book, Object> map = new HashMap<>();

        for(int i = 0;i<100 ;i++){
            Book book =new Book(i);
            map.put(book, object);
        }

        System.out.println(map.size());
        map.remove(new Book(50));
        System.out.println(map.size());

    }
}
class Book {
    int i;

    public Book(int i) {
        this.i = i;
    }

    @Override
    public int hashCode() {
        return i; //or just 1
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Book book = (Book) o;

        return i == book.i;
    }

}