Directory and Filesystem Operations - Java - page 449

Changing File Attributes

Problem

You want to change attributes of a file other than its name.

Solution

Use setReadOnly() or setLastModified() .

Discussion

As we saw in Getting File Information , many methods report on a file. By contrast, only a
few change the file.

setReadOnly() turns on read-only for a given file or directory. This method returns true if
it succeeds, otherwise false .

setLastModified() allows you to play games with the modification time of a file. This is
normally not a good game to play, but it is useful in some types of backup/restore programs.
This method takes an argument that is the number of milliseconds (not seconds) since the be-
ginning of Unix time (January 1, 1970). You can get the original value for the file by calling
getLastModified() (see Getting File Information ), or you can get the value for a given date
by calling the ZonedDateTime 's toInstant().getEpochSecond() method (see Converting
Among Dates/Times, YMDHMS, and Epoch Seconds ) and multiplying by 1,000 to convert
seconds to mSec. setLastModified() returns true if it succeeded and false otherwise.

The interesting thing is that the documentation reads, in part, that “Instances of the File
class are immutable,” which normally means that an object's fields don't change after the ob-
ject is constructed. But does calling setReadOnly() affect the return value of canRead() ?
Let's find out:

public public class class ReadOnly ReadOnly {

public public static static void void main ( String [] a ) throws throws IOException {

File f = new new File ( "f" );

iif (! f . createNewFile ()) {
System . out . println ( "Can't create new file." );