Pick a random file from a directory in Java

I would like to pick a random file from any directory I want. such as C:\Users\Josh\Pictures\[randomFile]

I have no code to show I just would like to know how I would come by doing that.

What I am fully doing is using a class to change the background of my desktop, and now I want to add a random file to it so when it refreshes the background will be different and I wont have to stop the running code to manually change the name of the file in the path. If you are wondering heres what it looks like

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;


public class BackgroundTest {

    final static File dir = new File("C:\\Users\\Kevin\\Pictures\\Pyimgur\\");
    static int size = 10;
    static String [] fileArray = new String[size];





public static void main(String[] args) {
    File[] files = dir.listFiles();
    for(int i =0; i<size;i++){
        int idz = (int)(Math.random()*size);
        fileArray[i]=files[idz].getName();

    }

    for(String x: fileArray){
        System.out.print(x);
    }

    String path = "C:\\Users\\Kevin\\Pictures\\Pyimgur\\";
    String picture = "picture.jpg";
    //System.out.print(fileArray[0]);

    SPI.INSTANCE.SystemParametersInfo(
            new UINT_PTR(SPI.SPI_SETDESKWALLPAPER),
            new UINT_PTR(0),
            path + picture,
            new UINT_PTR(SPI.SPIF_UPDATEINIFILE | SPI.SPIF_SENDWININICHANGE));
}

public interface SPI extends StdCallLibrary {

    //from MSDN article
    long SPI_SETDESKWALLPAPER = 20;
    long SPIF_UPDATEINIFILE = 0x01;
    long SPIF_SENDWININICHANGE = 0x02;

    SPI INSTANCE = (SPI) Native.loadLibrary("user32", SPI.class, new HashMap<Object, Object>() {
        {
            put(OPTION_TYPE_MAPPER, W32APITypeMapper.UNICODE);
            put(OPTION_FUNCTION_MAPPER, W32APIFunctionMapper.UNICODE);
        }
    });

    boolean SystemParametersInfo(
            UINT_PTR uiAction,
            UINT_PTR uiParam,
            String pvParam,
            UINT_PTR fWinIni
    );
    }
}