:56:21 java: non-static variable this cannot be referenced from a static context

I need two things in one project average grade without minimum and maximum and display of books from the list of which the date of publication is above a certain value

import java.awt.print.Book;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;

public class sredniaocen {

    class Book {
        int pubyear;
        String title;
        public Book(String title, int pubyear) {
            this.title = title;
            this.pubyear = pubyear;
        }
        public String getTitle() {
            return title;
        }
        public void setTitle(String title) {
            this.title = title;
        }
        public int getPubyear() {
            return pubyear;
        }
        public void setPubyear(int pubyear) {
            this.pubyear = pubyear;
        }
    }

    public static void main(String args[]) {

        ArrayList<Double> list = new ArrayList<Double>();
        list.add(3.0);
        list.add(5.0);
        list.add(3.0);
        list.add(4.0);
        list.add(4.0);
        list.add(5.0);

        double sum = 0;

        for (double x : list) {
            System.out.println("Wartość elementu: " + x);
            sum += x;
        }

        double min = Collections.min(list);
        double max = Collections.max(list);

    System.out.println("Suma elementów: " + sum);
    System.out.println("Min: " + min);
    System.out.println("Max: " + max);
    double average = (sum - min - max) / (list.size() - 2);
    System.out.println(average);

        Book book = new Book();
        book.pubyear = 2000;
        book.title = "abcd";


        LinkedList<Book> booklist = new LinkedList<Book>();
        booklist.add(book.pubyear, book.title);


        // Retrieve and display an element at index 7
        System.out.println("Książka = " + list.get(book));
    }
}

:62:40 java: incompatible types: java.lang.String cannot be converted to sredniaocen.BookList :66:52 java: incompatible types: sredniaocen.Book cannot be converted to int ============================= working code complete

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;

public class sredniaocen {
    class Book {
        int pubyear;
        String title;

        public Book(String title, int pubyear) {
            this.title = title;
            this.pubyear = pubyear;
        }

        public String getTitle() {
            return title;
        }

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

        public int getPubyear() {
            return pubyear;
        }

        public void setPubyear(int pubyear) {
            this.pubyear = pubyear;
        }

        class BookList {
        }
    }
        public static void main(String args[]) {

            ArrayList<Double> list = new ArrayList<Double>();
            list.add(3.0);
            list.add(5.0);
            list.add(3.0);
            list.add(4.0);
            list.add(4.0);
            list.add(5.0);

            double sum = 0;

            for (double x : list) {
                System.out.println("Wartość elementu: " + x);
                sum += x;
            }

            double min = Collections.min(list);
            double max = Collections.max(list);

            System.out.println("Suma elementów: " + sum);
            System.out.println("Min: " + min);
            System.out.println("Max: " + max);
            double average = (sum - min - max) / (list.size() - 2);
            System.out.println("Średnia bez najwyższej i najniższej oceny: " + average);


            LinkedList<Book> booklist = new LinkedList<Book>();
            booklist.add(new Book("abc", 1998));
            booklist.add(new Book("def", 1999));
            booklist.add(new Book("ghi", 2000));
            booklist.add(new Book("jkl", 2001));
            booklist.add(new Book("mno", 1978));
            booklist.add(new Book("prs", 2012));
            booklist.add(new Book("tuw", 2006));


            for (Book book : booklist) {
                if (book.getPubyear() > 2000) {
                    System.out.println(book.getTitle());
                }

        }

}

LinkedList<Book> booklist = new LinkedList<Book>(); booklist.add(new Book("abc", 1998)); booklist.add(new Book("def", 1999)); booklist.add(new Book("ghi", 2000)); booklist.add(new Book("jkl", 2001)); booklist.add(new Book("mno", 1978)); booklist.add(new Book("prs", 2012)); booklist.add(new Book("tuw", 2006)); for (Book book : booklist) { if (book.getPubyear() > 2000) { System.out.println(book.getTitle()); }

Comments (11)

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Karol Grzeszczak

Level 31 , Poland, Poland

28 May 2022, 12:14

i put working code above but i guess if i used your code it will be more complicated to display books above pubyear 2000 for-each

Lisa L

Level 47 , Nuremberg, Germany

28 May 2022, 11:09

no big problems as far as I can see. In line 61 you need to add a book, not an int and a title in line 65... you can not get the element of a list using the object you look for. You need to access it by index. But you could search for the index first using the indexOf() method. However for these use cases you better could opt for a set. Also you probably meant to access booklist and not list? The main problem is the Book class and the creation of the Book object. You've chosen to use an inner class. For that you'd have to create an object of the outer class first.

// eg. if you do not further need the outer object
rednicaon.Book book = new srednicaon().new Book("some book", 1999);

That's advanced topics and I won't do that if not really needed. Better make the class static or make it a top level class (in a separate file or put it outside the outer class and make it not public but default, with no access modifier). Then you just need to pass a few parameters when creating the object (as I did above). That's what your one and only constructor requires, a string and an int

28 May 2022, 11:46

now i have only :68:56 java: incompatible types: sredniaocen.Book cannot be converted to int

28 May 2022, 11:55

ok i change it and now i need to get books above 2000

28 May 2022, 13:24

maybe a loop over the list and then compare the year of each book against 2000?

for (Book currentBook : booklist) {
    if (book.getPubyear() > 2000)
        System.out.println(book); // for a meaningful output you'll have to add a toString method
}

28 May 2022, 13:30

theres no error but also no result

you need to add books, you need to add a toString method and you need to fix my above code and use currentBook instead of book everywhere

28 May 2022, 13:43

add books to list ?

28 May 2022, 14:45

if i use this

for (Book thisBook : booklist) {
    if (book.getPubyear() < 2000) {
        System.out.println(book.title);
    }
}

i get abcd 6 times so he stop at the first book and only return value if i change to > 2000 theres nothing to display in first position

28 May 2022, 15:23

see above, you need to change the code a lil... instead of book you need to use thisBook. And booklist needs to be filled with some content like (first make Book static) booklist.add(new Book("some title", 2022));

28 May 2022, 15:58

now i understand :) i change