vmp123

package com.company;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Semaphore;

public class Main {

    static Semaphore semaphore = new Semaphore(4);
    static List<Book> books = new ArrayList<Book>();

    public static void main(String[] args) {
        System.out.println("Max 4 books");
        StudentThread t1 = new StudentThread("1 Student");
        StudentThread t2 = new StudentThread("2 Student");
        StudentThread t3 = new StudentThread("3 Student");
        StudentThread t4 = new StudentThread("4 Student");
        StudentThread t5 = new StudentThread("5 Student");

        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
    }

    static class StudentThread extends Thread {
        private String name = "";

        public StudentThread(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            super.run();
            try {
                System.out.println(name + " : lock");
                System.out.println("Available books: " + semaphore.availablePermits());

                semaphore.acquire();
                System.out.println(name + " got permit for book");

                Book book = books.get(0);
                books.remove(0);
                System.out.println(name + " got " + book);

                try {
                    Thread.sleep(2000);
                } finally {
                    books.add(book);
                    System.out.println(name + " returned " + book.name);

                    semaphore.release();
                }

            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    static class Book {
        public String name;

        public Book(String name) {
            this.name = name;
        }
    }

}