Why do I get a nul pointer exception? From what I see from the debugger, `File d` is not null.

WalkDir.java

package je3.io;

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

/**
 * Created by IDEA on 31/01/15.
 */
public class WalkDir {
    private List<File> recursiveList;

    public void walkDir(String pathname) {
        File d = new File(pathname);
        recursiveList.add(d);
        if(d.isDirectory()) {
            for(String f : d.list()) {
                walkDir(f);
            }
        }
    }

    public List<File> getRecursiveList() {
        return recursiveList;
    }

    public static void main(String[] args) {
        WalkDir dirWalker = new WalkDir();
        dirWalker.walkDir("/tmp/test");
        for(File f : dirWalker.getRecursiveList()) {
            System.out.println(f);
        }
    }
}