How to create .der file using RSA private and public key?

Question

I need to read private key and public key using RSA algorithm. So how to create .der file?

Answer 1

Using CertificateFactory

try{
    CertificateFactory cf = CertificateFactory.getInstance("X.509");
    Certificate cert = cf.generateCertificate(new FileInputStream("ca.cert"));
    System.out.println(cert);
}catch(Exception ex){
    ex.printStackTrace();
}