A DESCRIPTION OF THE PROBLEM :
public boolean renameTo(File dest) {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkWrite(path);
            security.checkWrite(dest.path);
        }
        if (dest == null) {
            throw new NullPointerException();
        }
        if (this.isInvalid() || dest.isInvalid()) {
            return false;
        }
        return fs.rename(this, dest);
    }

if a SecurityManager is exists, and dest is null, then execute security.checkWrite(dest.path) will throw nullpoint exception, and if (dest == null) is no meaning.

the first step should be test if dest is null.

public boolean renameTo(File dest) {
        if (dest == null) {
            throw new NullPointerException();
        }
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkWrite(path);
            security.checkWrite(dest.path);
        }
        if (this.isInvalid() || dest.isInvalid()) {
            return false;
        }
        return fs.rename(this, dest);
    }


FREQUENCY : always