need to find the oldest book from the list

  import java.util.Objects; public class Roman implements Comparable<Roman> { private final String author; private final String name; private final Integer data; private final Integer size; private final String genre; public Roman(String author, String name, int data, int size, String genre) { this.author = author; this.name = name; this.data = data; this.size = size; this.genre = genre; } public String getAuthor() { return author; } public String getName() { return name; } public int getData() { return data; } public int getSize() { return size; } public String getGenre() { return genre; } @Override public int hashCode() { int hash = 3; hash = 53 * hash + Objects.hashCode(this.author); hash = 53 * hash + Objects.hashCode(this.name); hash = 53 * hash + Objects.hashCode(this.data); hash = 53 * hash + Objects.hashCode(this.size); hash = 53 * hash + Objects.hashCode(this.genre); return hash; } @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj == null) { return false; } if (getClass() != obj.getClass()) { return false; } final Roman other = (Roman) obj; if (!Objects.equals(this.author, other.author)) { return false; } if (!Objects.equals(this.name, other.name)) { return false; } if (!Objects.equals(this.genre, other.genre)) { return false; } if (!Objects.equals(this.data, other.data)) { return false; } if (!Objects.equals(this.size, other.size)) { return false; } return true; } @Override public String toString() { return "Roman{" + "author=" + author + ", name=" + name + ", data=" + data + ", size=" + size + ", genre=" + genre + '}'; } @Override public int compareTo(Roman roman) { return this.data.compareTo(roman.getData()); } } public static void main(String[] args) { TreeSet<Roman>roman = new TreeSet<>(); roman.add(new Roman("Иван Сергеевич Тургенев", "Отцы и дети", 1862, 300, "Любовный роман, Философский роман, Политическая фантастика")); roman.add(new Roman("Иван Сергеевич Тургенев", "Муму", 1854, 35, "Художественное произведение")); roman.add(new Roman("Иван Сергеевич Тургенев", "Записки охотника", 1852, 50, "Художественное произведение")); roman.add(new Roman("Лев Николаевич Толстой", "Война и мир", 1867, 1274, "Роман, Любовный роман, Военная проза, Исторический жанр, Философский роман")); roman.add(new Roman("Лев Николаевич Толстой", "Анна Каренина", 1877, 776, "Роман, Реализм, Художественное произведение")); //Collections.sort(roman); System.out.println(roman.first()); } 

Your Roman class cannot define a book with the smallest year (we are not talking about static methods now), because the class is a kind of “drawing” for creating objects. Creating an object through the operator new, each time you create an independent entity. Each of the objects of the Roman class contains information about only one book that is stored in the fields of the object, i.e. each object knows only its own year. Therefore, it is impossible to find out about the year in other books within the object. The Main method in the Main class creates a collection to which it adds many objects. Now our collection stores information about a variety of books, which means that within the collection you can make a comparison. In this case, I just sort the books by year. This is not the only option, just want to show how it works. If you have questions - ask. And more ... In the Roman class you have a huge amount of standard code (constructor getters, methods toString, equals, hashcode). To not write this look in the direction of lombok. This library dynamically generates these methods and is managed using annotations. Moreover, you can easily use a builder instead of a not very successful designer. There are logging and many other nice "buns"