2 problems with java?

Question

the first is :
I have a variable "root" that the type is "File" and I want to convert "File" to "String".
so it must be like that:
String path = (String) root + fileNames[i].getName();
so how can I change the type of "root"?
the second is :
I have imported all this libraries :
import java.util.*;
import java.io.*;
import java.nio.*;
import java.lang.*;
import java.*;
but I have like error :
symbol : method isDirectory()
location: class java.lang.String
if ( fileNames[i].isDirectory()) {
symbol : method getName()
location: class java.lang.String
String path = root + fileNames[i].getName()
it don't know the methods getName() and isDirectory()

the first problem has resolve by Jim Mary
the response was the methode toString.
but the second is not resolve.

the second problem has resolve.
the solution was the type of "fileNames" because the type was String and isDirectory() and getName() are the methodes of type File.

I have a third question:
I have like error message this one:
Exception in thread "main" java.lang.NullPointerException
at frag.Main.main(Main.java:41)
and the code is :
................
int inode=0;
try {
fs=frag.FileSystem.getInstance(3);
}
catch(IOException exception) {
System.out.println("IOexception");
fs=null;
}

map=new CompleteMemoryMap(4096);
newMap=new CompleteMemoryMap(4096);

Vector files=fs.getRemixFiles();

Iterator list=files.iterator();
while (list.hasNext()) {
RemixFile rf;
rf=list.next();
map.add(rf.getFileMap());
newMap.add(rf.getFileMap());
}
.......
what can I do?

Answer 1

The class File has a "toString()" method that will return the name of the file. What you are trying to do with the line "String path = (String)root ..." is cast a File object to a String object and you should be seeing a ClassCastException.

** Edit **
Have you declared the "frag" variable anywhere? Has it been initialized? I can't tell if the initialization for the "frag" variable was done earlier as you only have a fragment of your code posted (which is fine, just hard to know what has been set earlier). Generally a NullPointerException is when you have a variable that you haven't initialized.

Example:

// Initialize variables outside of a try block
String abc = null;

try {
// some code
// ...
//

// should have a line initializing abc to a string but for
// the example, I'll just leave it commented.

// abc = foo.getString();

}

// This throws the NullPointerException because it wasn't actually initialized to anything.
System.out.println("abc = " + abc);

These types of errors can be a real pain as most IDEs don't check for that error and you only catch it at runtime.

Answer 2

http://answers.yahoo.com/question/index?qid=20070509063938AAaWa4x&pa=FYd1D2bwHTHwI7JhFe0yScaO00h8F6j7fgcdtEeec5wcTw--&paid=asked&msgr_status=
pls need help