Error with making Linked List have Images in it

Question

I'm working on a memory game and I'm trying to use a get a value from a LinkedList to put in a selected Linked List. Here is my code:

import java.awt.GridLayout;
import java.awt.Image;
import java.io.IOException;
import java.io.InputStream;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.ArrayList;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.swing.JFrame;


public class Runner{

    public static void main(String[] args) {
        JFrame frame =  new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        LinkedList<Image> icon = new LinkedList<Image>();


        for (int i = 0; i < 20; i++) {
            ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
            InputStream input = classLoader.getResourceAsStream((i + 1) + ".jpg");
            try {
                Image logo = ImageIO.read(input);
                icon.push(logo);
            } 

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


        Random rand = new Random();

        LinkedList<Image> selected = new LinkedList<Image>();

        for (int i = 0; i < 10; i++) {
            int randomNum = rand.nextInt(20);


            // Randomly pick one from the array of faces
            Image face = icon.get(randomNum);
            // Push 2 copies onto array
            selected.push(face);
            selected.push(face);
            // Remove from faces array so we don't re-pick
            icon.remove(randomNum);
        }

        Collections.shuffle(selected);  





        List<Card> tiles = new ArrayList<Card>();




        int NUM_COLS = 5;
        int NUM_ROWS = 4;

        frame.setLayout(new GridLayout(NUM_ROWS, NUM_COLS));


        for (int i = 0; i < NUM_COLS; i++) {
            for (int j = 0; j < NUM_ROWS; j++) {
                tiles.add(new Card(i * 39 + 10, j * 39 + 40, selected.pop()));
            }
        }

        for (int i = 0; i < tiles.size(); i++) {
            frame.add(tiles.get(i));
            tiles.get(i).drawFaceDown();
        }

        for (int i = 0; i < tiles.size(); i++) {
            frame.add(tiles.get(i));
            tiles.get(i).drawFaceUp();
        }

        frame.pack();
        frame.setVisible(true);



        }
}

My problem is that I've tried putting images in my icon LinkedList however I'm getting an error:

Exception in thread "main" java.lang.IllegalArgumentException: input == null!
    at javax.imageio.ImageIO.read(Unknown Source)
    at Runner.main(Runner.java:30)

Don't mind the Unknown Source, you should fix the actual error at line 44 of your code — OneCricketeer, May 25 '16 at 14:23
I recommend that you don't have empty catch blocks in your code. — khelwood, May 25 '16 at 14:24
Your random int generation looks wrong. Your icon list has 20 elements (0-19). — Mohamad Elghawi, May 25 '16 at 14:25
`(20 - 1) + 1` is just 20. Hint: Magic numbers are typically not good, and your error says the list is empty — OneCricketeer, May 25 '16 at 14:26
@SruthiKurada: This is a great time to familiarize yourself with the use of a debugger. When you step through the code while debugging, what is the value of `randomNum`? And how many elements are in `icon`? That's the problem. — David, May 25 '16 at 14:27
Have you stepped through your code? Have you read about an `IndexOutOfBoundsException` to understand what it is, why it happens, and how it can be fixed? This is an extremely common and basic exception. — tnw, May 25 '16 at 14:27
`int randomNum = rand.nextInt((20 - 1) + 1) + 1;` This means that randomNum can be 20 which is out of bounds. — Mohamad Elghawi, May 25 '16 at 14:27
You could oversubscript your array. Change `rand.nextInt((20 - 1) + 1) + 1` to `rand.nextInt(20)`. — Frecklefoot, May 25 '16 at 14:28
@SruthiKurada: Because nothing is being added to it. Where do you *think* something is being added to it? When you step through the code in your debugger, is that actually happening? (Hint: Empty `catch` blocks are your way of telling the code that you don't want to know when an error happens. So why are you surprised that you don't know when an error happens?) — David, May 25 '16 at 14:34

score 1 · Answer 1 · answered May 25 '16 at 14:40

1

This is where the exception occur.

Image face = icon.get(randomNum);

According to the exception you get, the icon contains no elements. Which means following code doesn't add elements to your linked list.

try {
            for (int i = 0; i < 20; i++) {
                icon.push(ImageIO.read(new File(i+1 + ".jpg")));

            }
        } 

        catch (IOException e) {

        }

Your catch statement does nothing here. Print something and see whether adding fails. Since you haven't posted your card class, I can't test it. If adding elements to icon fails, it must be due to failiure in File reading.

answered May 25 '16 at 14:40

pahan

567
1
8
22

The problem is the images aren't in the right folder – Sruthi Kurada May 25 '16 at 14:42
Then move them to the right folder, or give the path to the current location in `icon.push(ImageIO.read(new File("path/path"+i+1 + ".jpg")));`. – pahan May 25 '16 at 14:45
I tried to put all my images in src/images and then made path/path to src/images, but it still didn't work. – Sruthi Kurada May 25 '16 at 15:12
Are you on windows or linux? – pahan May 25 '16 at 15:14
Did I put my images in wrong? You can see the picture of the folder with images in it above – Sruthi Kurada May 25 '16 at 15:20
If windows path should be given as `"C:\\Users\\user\\Desktop"`, (extra \ for every \ in string as escape character). – pahan May 25 '16 at 15:22

score 0 · Answer 2 · answered May 25 '16 at 14:31

0

Are you using JRE instead of JDK?
You aren't adding any images to a List, your list is empty. Your random function returns random int between 0 and 20, but your image list size can be 0 or any number.

answered May 25 '16 at 14:31

arseniyandru

760
1
7
16

score 0 · Answer 3 · answered May 25 '16 at 14:37

It looks like the code in the try block that pushes Images into the 'icon' Linked List is failing, throwing an error, but since your catch block is empty, you don't see it. As a result your Linked List is empty and therefore any .get() call will throw an error. I suggest printing the error being thrown in the catch block and that may tell you the root of the problem.

Error with making Linked List have Images in it

3 Answers3