After upgrading Android studio, I had this problem where I only need to fetch the API from my server, I've tried to create a new .pem but I was stuck. Also have I mention that I need to use AndroidX as well so every code need to adjust, Double KO.

echo | openssl s_client -connect {{IP}}:443 2>&1 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > mycert.pem

But hey I'm just starting this out and it's overwhelming. I need something to keep on going with my learning so I found this and it worked. It is said that It's not the best approach but hey need to move on and I'll be back to this when I'm comfortable with android.

import java.security.cert.CertificateException;

import javax.net.ssl.HostnameVerifier;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSession;
import javax.net.ssl.SSLSocketFactory;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;

import okhttp3.OkHttpClient;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
import view.utils.AppConstants;

/**
 * Created by Hitesh.Sahu on 11/23/2016.
 */

public class NetworkHandler {

    public static Retrofit getRetrofit() {

        return new Retrofit.Builder()
                .baseUrl(AppConstants.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .client(getUnsafeOkHttpClient())
                .build();
    }


    private static OkHttpClient getUnsafeOkHttpClient() {
        try {
            // Create a trust manager that does not validate certificate chains
            final TrustManager[] trustAllCerts = new TrustManager[] {
                    new X509TrustManager() {
                        @Override
                        public void checkClientTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {
                        }

                        @Override
                        public void checkServerTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {
                        }

                        @Override
                        public java.security.cert.X509Certificate[] getAcceptedIssuers() {
                            return new java.security.cert.X509Certificate[]{};
                        }
                    }
            };

            // Install the all-trusting trust manager
            final SSLContext sslContext = SSLContext.getInstance("SSL");
            sslContext.init(null, trustAllCerts, new java.security.SecureRandom());
            // Create an ssl socket factory with our all-trusting manager
            final SSLSocketFactory sslSocketFactory = sslContext.getSocketFactory();

            OkHttpClient.Builder builder = new OkHttpClient.Builder();
            builder.sslSocketFactory(sslSocketFactory);
            builder.hostnameVerifier(new HostnameVerifier() {
                @Override
                public boolean verify(String hostname, SSLSession session) {
                    return true;
                }
            });

            OkHttpClient okHttpClient = builder.build();
            return okHttpClient;
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

For Future me - try to Drop the https to http for your test server, then the logic doesn't have to change.

Reference - https://stackoverflow.com/questions/37686625/disable-ssl-certificate-check-in-retrofit-library