This progam demonstrate the RSA keypair generation, and get the keys as String format.

KeyGen.java

import com.jcraft.jsch.*;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

class KeyGen {
    public static void main(String[] arg) {
        String email = "[email protected]";

        JSch jsch = new JSch();

        try (ByteArrayOutputStream privateKeyStream = new ByteArrayOutputStream();
             ByteArrayOutputStream publicKeyStream = new ByteArrayOutputStream()) {

            com.jcraft.jsch.KeyPair kpair = com.jcraft.jsch.KeyPair.genKeyPair(jsch, com.jcraft.jsch.KeyPair.RSA);

            kpair.writePrivateKey(privateKeyStream);
            kpair.writePublicKey(publicKeyStream, email);

            System.out.println(privateKeyStream.toString());
            System.out.println(publicKeyStream.toString());

            kpair.dispose();
        } catch (JSchException | IOException e) {
            e.printStackTrace();
        }
    }
}