Java Error: NullPointerException

Question

I'm totally new in this world. I want to make a simple file move program. It works fine with only 1 file and until I add the new code for multiple files move. But I wanted more and I added multiple file selection to JFileChooser. To do the move of files I search around the web and found some users that asked for something similar to it. I tried to put it in my code but I've obtained an Error like this:

Exception in thread "main" java.lang.NullPointerException at jfile.main(jfile.java:27)

Line 27 is: for (int i = 0; i < files.length; i++) {

This is the code, thanks you and sorry for my bad English.

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import javax.swing.JFileChooser;
import org.apache.commons.io.FileUtils;
public class jfile {
public static void main (String[] args) throws IOException{

    System.out.println("Creado por: MarcosCT7");

    if (new File(System.getProperty("user.home"), "\\AppData\\Roaming\\.minecraft\\mods").exists());{
        System.out.println("Seleccione el mod a instalar:");
        JFileChooser chooser = new JFileChooser();
        chooser.setFileSelectionMode(JFileChooser.FILES_ONLY);
        chooser.setMultiSelectionEnabled(true);
        int returnVal = chooser.showOpenDialog(chooser);

        if(returnVal == JFileChooser.APPROVE_OPTION) {
                System.out.println("Se está instalando " + chooser.getSelectedFile().getName());
                    File fuente = new File(chooser.getSelectedFile().getAbsolutePath());
                    File destino = new File(System.getProperty("user.home"), "\\AppData\\Roaming\\.minecraft\\mods");

                    File[] files = fuente.listFiles(); //thats new added
                    for (int i = 0; i < files.length; i++) {
                        File destFile = new File(destino.getAbsolutePath()+File.separator+files[i].getName().replace(",", "")
                                .replace("[", "") 
                                .replace("]", "")
                                .replace(" ", "")); //until here its new added
                        FileUtils.moveFileToDirectory(files[i], destFile, true); //changed to multiple move, before it was: FileUtils.moveFileToDirectory(fuente, destino, true);
                    }

                }

         else {
            if(returnVal == JFileChooser.CANCEL_OPTION) {
            System.out.println("No se ha seleccionado ningun mod. Adios.");
            }
        }
    }
}
}

I've flaged it because I cannot delete the question, apologizes @l4mpi — MarcosCT7, May 01 '14 at 11:43

score 0 · Answer 1 · answered May 01 '14 at 11:10

0

Before you iterate through files using the loop, check using an if statement:

if(files==null){
    System.out.println("Files not found");
}
else{
    for (int i = 0; i < files.length; i++) {
                    File destFile = new File(destino.getAbsolutePath()+File.separator+files[i].getName().replace(",", "")
                            .replace("[", "") 
                            .replace("]", "")
                            .replace(" ", "")); //until here its new added
                    FileUtils.moveFileToDirectory(files[i], destFile, true); //changed to multiple move, before it was: FileUtils.moveFileToDirectory(fuente, destino, true);
                }
}

answered May 01 '14 at 11:10

Andrew Lobley

342
4
12

While that's correct, please don't answer duplicate questions - this one is going to be closed as a duplicate of http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it soon, and could then be automatically deleted after 9 days if it doesn't have any upvoted or accepted answers. If now someone upvotes your answer or OP accepts it, this would mean the question can't be deleted by the system anymore and would stay around indefinitely until it's deleted manually by either five high-rep users or a mod. – l4mpi May 01 '14 at 11:14
I'm sorry for duplicate a thread, but I can't understand well the link you show me, I know I'm totally a beginner and maybe I must do easier things to my level. Although this answer helped me a lot to understand what's happening in my error. Sorry I will delete this post, It wasn't my intention to be annoying others. – MarcosCT7 May 01 '14 at 11:21
Well, then hopefully @MarcosCT7 or someone else who finds this solution useful should upvote it. :) – Andrew Lobley May 01 '14 at 11:31

score 0 · Accepted Answer · answered May 01 '14 at 11:19

NullPointerException means that the variable doesn't hold any reference to a object. What I mean by reference is, For ex:

String path="";
File f=new File(path);
if(f.exists()) {
   // do something
}

f is a variable of type File which holds a reference to a object File defined by path and now you can use variable f just like any other variable call methods on that variable etc. Another example

File f;

if(f.exists()) {
   // do something
}

Now you will get NullPointerException in line if(f.exists()) because f doesn't hold any reference.

In JAVA new keyword is used to assign new reference. JVM will take care of all the low level details. It is similar to pointers in c and c++. In JAVA you don't have to explicitly delete the objects. JVM garbage collector will take care of these things. Java is object oriented language

Do read and understand OOP comcepts

Java Error: NullPointerException

2 Answers2