MainActivity

package bb.okhttpdemo;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import java.io.IOException;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {

            GetExample example = new GetExample();
            String response = null;
            try {
            response = example.run("https://raw.github.com/square/okhttp/master/README.md");
             } catch (IOException e) {
            e.printStackTrace();
            }
        System.out.println(response);
            }

        }).start();
    }


    public class GetExample {
        OkHttpClient client = new OkHttpClient();

        String run(String url) throws IOException {
            Request request = new Request.Builder()
                    .url(url)
                    .build();

            try (Response response = client.newCall(request).execute()) {
                return response.body().string();
            }
        }

    }
}

AndroidMainfest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="bb.okhttpdemo">
    <uses-permission android:name="android.permission.INTERNET"></uses-permission>
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>