Récupération d'un élément aléatoire dans une liste de tableaux (ArrayList)

Question

Je suis en train d'apprendre Java et j'ai un problème avec ArrayList y Random .

J'ai un objet appelé catalogue qui contient une liste d'objets créés à partir d'une autre classe appelée item .

J'ai besoin d'une méthode dans catalogue qui renvoie toutes les informations sur l'un des item dans la liste.
Les item doit être choisi au hasard.

import java.util.ArrayList;
import java.util.Random;

public class Catalogue
{
    private Random randomGenerator = new Random();
    private ArrayList<Item> catalogue;

    public Catalogue ()
    {
        catalogue = new ArrayList<Item>();  
    }

    public Item anyItem()
    {
        int index = randomGenerator.nextInt(catalogue.size());
        System.out.println("Managers choice this week" + catalogue.get(index) + "our recommendation to you");
        return catalogue.get(index);
    }

Lorsque j'essaie de compiler, j'obtiens une erreur qui pointe vers le fichier System.out.println ligne disant

Impossible de trouver le symbole de la variable anyItem

Answer 1

La solution n'est pas bonne, même si vous avez corrigé le nom et l'énoncé inaccessible de cette sortie imprimée.

Ce à quoi vous devez également prêter attention 1. graine aléatoire, et données volumineuses, le nombre d'éléments est si grand que le nombre d'éléments aléatoires retournés est < itemlist.size().

1. vous n'avez pas géré le multithread, vous risquez d'obtenir une exception de type "index out of bound".

Answer 2

Voici une meilleure façon de procéder :

import java.util.ArrayList;
import java.util.Random;

public class facultyquotes
{
    private ArrayList<String> quotes;
    private String quote1;
    private String quote2;
    private String quote3;
    private String quote4;
    private String quote5;
    private String quote6;
    private String quote7;
    private String quote8;
    private String quote9;
    private String quote10;
    private String quote11;
    private String quote12;
    private String quote13;
    private String quote14;
    private String quote15;
    private String quote16;
    private String quote17;
    private String quote18;
    private String quote19;
    private String quote20;
    private String quote21;
    private String quote22;
    private String quote23;
    private String quote24;
    private String quote25;
    private String quote26;
    private String quote27;
    private String quote28;
    private String quote29;
    private String quote30;
    private int n;
    Random random;

    String teacher;

    facultyquotes()
    {
        quotes=new ArrayList<>();
        random=new Random();
        n=random.nextInt(3) + 0;
        quote1="life is hard";
        quote2="trouble shall come to an end";
        quote3="never give lose and never get lose";
        quote4="gamble with the devil and win";
        quote5="If you don’t build your dream, someone else will hire you to help them build theirs.";
        quote6="The first step toward success is taken when you refuse to be a captive of the environment in which you first find yourself.";
        quote7="When I dare to be powerful – to use my strength in the service of my vision, then it becomes less and less important whether I am afraid.";
        quote8="Whenever you find yourself on the side of the majority, it is time to pause and reflect";
        quote9="Great minds discuss ideas; average minds discuss events; small minds discuss people.";
        quote10="I have not failed. I’ve just found 10,000 ways that won’t work.";
        quote11="If you don’t value your time, neither will others. Stop giving away your time and talents. Value what you know & start charging for it.";
        quote12="A successful man is one who can lay a firm foundation with the bricks others have thrown at him.";
        quote13="No one can make you feel inferior without your consent.";
        quote14="Let him who would enjoy a good future waste none of his present.";
        quote15="Live as if you were to die tomorrow. Learn as if you were to live forever.";
        quote16="Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do.";
        quote17="The difference between a successful person and others is not a lack of strength, not a lack of knowledge, but rather a lack of will.";
        quote18="Success is about creating benefit for all and enjoying the process. If you focus on this & adopt this definition, success is yours.";
        quote19="I used to want the words ‘She tried’ on my tombstone. Now I want ‘She did it.";
        quote20="It is our choices, that show what we truly are, far more than our abilities.";
        quote21="You have to learn the rules of the game. And then you have to play better than anyone else.";
        quote22="The successful warrior is the average man, with laser-like focus.";
        quote23="Develop success from failures. Discouragement and failure are two of the surest stepping stones to success.";
        quote24="If you don’t design your own life plan, chances are you’ll fall into someone else’s plan. And guess what they have planned for you? Not much.";
        quote25="The question isn’t who is going to let me; it’s who is going to stop me.";
        quote26="If you genuinely want something, don’t wait for it – teach yourself to be impatient.";
        quote27="Don’t let the fear of losing be greater than the excitement of winning.";
        quote28="But man is not made for defeat. A man can be destroyed but not defeated.";
        quote29="There is nothing permanent except change.";
        quote30="You cannot shake hands with a clenched fist.";

        quotes.add(quote1);
        quotes.add(quote2);
        quotes.add(quote3);
        quotes.add(quote4);
        quotes.add(quote5);
        quotes.add(quote6);
        quotes.add(quote7);
        quotes.add(quote8);
        quotes.add(quote9);
        quotes.add(quote10);
        quotes.add(quote11);
        quotes.add(quote12);
        quotes.add(quote13);
        quotes.add(quote14);
        quotes.add(quote15);
        quotes.add(quote16);
        quotes.add(quote17);
        quotes.add(quote18);
        quotes.add(quote19);
        quotes.add(quote20);
        quotes.add(quote21);
        quotes.add(quote22);
        quotes.add(quote23);
        quotes.add(quote24);
        quotes.add(quote25);
        quotes.add(quote26);
        quotes.add(quote27);
        quotes.add(quote28);
        quotes.add(quote29);
        quotes.add(quote30);
    }

    public void setTeacherandQuote(String teacher)
    {
        this.teacher=teacher;
    }

    public void printRandomQuotes()
    {
        System.out.println(quotes.get(n++)+"  ~ "+ teacher);  
    }

    public void printAllQuotes()
    {
        for (String i : quotes)
        {
            System.out.println(i.toString());
        }
    }
}