Why does Files.isWritable() method return true for a directory even when it is set to read only?

I have explicitly set read only mode both through setReadOnly method as well as setWritable(false).

File file  = new File(myDir);
file.setReadOnly();
file.setWritable(false);
file.setExecutable(false);
return file.canWrite();

I also checked through windows explorer that the specified directory is marked read-only:

In the above example , the file.canWrite returns true. Is there a stricter way of enforcing read only mode?

The api specification for java.io.File states the following as kind of disclaimer -

> A file system may implement restrictions to certain operations on the
> actual file-system object, such as reading, writing, and executing.
> These restrictions are collectively known as access permissions. The
> file system may have multiple sets of access permissions on a single
> object. For example, one set may apply to the object's owner, and
> another may apply to all other users. The access permissions on an
> object may cause some methods in this class to fail.

And definitely this is the case with you. On Linux(Ubuntu-18.04) things look to be very normal and as per specification.
Outputs for the following code using Java sdk-1.8.0_222 in the 2 environments is printed below

import java.io.File;
public class FileDemo {
public static void main(String[] args) {
File f = null;
boolean bool = false;
try {
// create new file
f = new File("test");
f.setWritable(false);
bool = f.canWrite();
System.out.println("Can write to test: "+bool);
f.setWritable(true);
bool = f.canWrite();
System.out.println("Can write to test: "+bool+"\n\n");
f.mkdir();
//f.createNewFile();
f.setWritable(false);
bool = f.canWrite();
System.out.println("Can write to test: "+bool);
f.setWritable(false);
bool = f.canWrite();
System.out.println("Can write to test: "+bool);
} catch(Exception e) {
// if any I/O error occurs
e.printStackTrace();
}
}
}

Windows (file) -

C:\Users\rranjan\Desktop>java FileDemo
Can write to test: false
Can write to test: false
Can write to test: false
Can write to test: true
C:\Users\rranjan\Desktop>java FileDemo
Picked up _JAVA_OPTIONS: -Djava.net.preferIPv4Stack=true
Can write to test: false
Can write to test: true
Can write to test: false
Can write to test: true

Linux (file) -

rranjan@my-workstation:~/scratch/java$ java FileDemo
Can write to test: false
Can write to test: false
Can write to test: false
Can write to test: true
rranjan@my-workstation:~/scratch/java$ java FileDemo
Can write to test: false
Can write to test: true
Can write to test: false
Can write to test: true

Windows (directory) -

C:\Users\rranjan\Desktop>java FileDemo
Can write to test: false
Can write to test: false
Can write to test: true
Can write to test: true
C:\Users\rranjan\Desktop>java FileDemo
Picked up _JAVA_OPTIONS: -Djava.net.preferIPv4Stack=true
Can write to test: true
Can write to test: true
Can write to test: true
Can write to test: true

Linux (directory) -

rranjan@my-workstation:~/scratch/java$ java FileDemo
Can write to test: false
Can write to test: false
Can write to test: false
Can write to test: true
rranjan@my-workstation:~/scratch/java$ java FileDemo
Can write to test: false
Can write to test: true
Can write to test: false
Can write to test: true

Now, you can observe that for a file or directory, if it doesn't exist, canWrite() is always False.
After the file/directory is created on a Linux platform, for both a file as well as a directory, setting writeable works seamlessly. Whereas, on a Windows box, directory seems to return true whatsoever. This is the kind of behavior which I quoted in the above lines.