How to accept/find the certificate from an opc ua server?

I am new to opc ua and not a pro in java. While setting up a client in java I'm having trouble with the certificate dealing. I want to connect to the server via Basic 256, SignAndEncrypt. As I understand, in this stage of security a certificate, created or loaded by the client, is send to the server, where it must be accepted. The server then sends a certificate back to the client, which then needs to be accepted by the client. Please, correct me, if I'm wrong.

Creating/Loading a certificate on the client side and sending it to the server does already work fine (see code below) and I can then accept it on the server side manually. But after that I'm stuck: How can I see this certificate validation in my code and how can I find the server certificate, let alone accept it? I used the SampleConsoleClient of opc ua for some orientation during implementation. But in contrast to there, I do not use any user input.

Here's some of my code so far.

Initialization:

try {
        client = new UaClient(serverUri);
    } catch (final URISyntaxException e) {
        throw new InitializationException("The server uri has an invalid syntax.", e);
    }
    try {
        client.setApplicationIdentity(createApplicationIdentity());
    } catch (final SecureIdentityException e) {
        throw new InitializationException(
                "Application Identity could not be created due to a Security Identity Exception.", e);
    } catch (final IOException e) {
        throw new InitializationException("Application Identity could not be created due to an IO Exception.",
                e);
    }

createApplicationIdentity():

final ApplicationDescription appDescription = new ApplicationDescription();
    appDescription.setApplicationName(new LocalizedText(APPLICATION_NAME, Locale.ENGLISH));
    appDescription.setApplicationUri(APPLICATION_URI);
    appDescription.setProductUri(PRODUCT_URI);
    appDescription.setApplicationType(ApplicationType.Client);

    // Setting security features
    client.setSecurityMode(SecurityMode.BASIC256_SIGN_ENCRYPT);
    client.setCertificateValidator(validator);
    validator.setValidationListener(myValidationListener); //myValidationListener is similar to most lines in MyCertificateValidationListener in the opc ua samples
    final File privatePath = new File(validator.getBaseDir(), "private");
    final KeyPair issuerCertificate = null;
    final int[] keySizes = null;
    final ApplicationIdentity identity = ApplicationIdentity.loadOrCreateCertificate(appDescription,
            "Sample Organisation", "opcua", privatePath, issuerCertificate, keySizes, true);
    identity.setApplicationDescription(appDescription);
    return identity; 

After initializing, I try to connect like this (with annotation, how I imagine the connection could work properly):

final String securityPolicy = client.getEndpoint() == null
            ? client.getSecurityMode().getSecurityPolicy().getPolicyUri()
                    : client.getEndpoint().getSecurityPolicyUri();

            client.setSessionName(String.format("%s@%s/Session%d", APPLICATION_NAME,
                    ApplicationIdentity.getActualHostNameWithoutDomain(), ++sessionCount));
            try {
                //Idea: catch the server certificate and accept it. Only if that was possible: connect
                client.connect();
            } catch (final ServiceException e) {
                e.printStackTrace();
            }
            client.setKeepSubscriptions(false);
            // After that resolving namespace index (works fine)
            }

And the error, that is thrown:

WARN  (?:?): /<IPofServer> Error org.opcfoundation.ua.common.ServiceResultException: Bad_SecurityChecksFailed
(0x80130000) "An error occurred verifying security." at
org.opcfoundation.ua.transport.tcp.io.TcpConnection$ReadThread.run(Unknown Source)
com.prosysopc.ua.client.ConnectException: Failed to create secure channel to server: : opc.tcp://<IPofServer>
[http://opcfoundation.org/UA/SecurityPolicy#Basic256,SignAndEncrypt]
ServiceResult=Bad_SecurityChecksFailed (0x80130000) "An error occurred verifying security."
at com.prosysopc.ua.client.UaClient.n(Unknown Source)
at com.prosysopc.ua.client.UaClient.connect(Unknown Source)
at *lineOfCode*
Caused by: org.opcfoundation.ua.common.ServiceResultException:
Bad_SecurityChecksFailed (0x80130000) "An error occurred verifying security."
at org.opcfoundation.ua.transport.tcp.io.TcpConnection$ReadThread.run(Unknown Source)

With the lineOfCode being client.connect().

Thanks in advance for the help!!