Java Error: NullPointerException [duplicate]

Question

This question already has answers here:

What is a NullPointerException, and how do I fix it? (12 answers)

Closed 9 years ago.

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.");
            }
        }
    }
}
}

Answer 1

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

Answer 2

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);
                }
}