Demonstration of caching issue where OkHttpClient doesn't check either the modified date or the Etag on subsequent requests, it just serves from the cache. Comment out the last-modified header and the test passes as the Etag is checked.

OkHttpCachingTest

import junit.framework.Assert;

import org.junit.Rule;
import org.junit.Test;

import java.io.File;
import java.util.UUID;
import java.util.concurrent.TimeUnit;

import okhttp3.Cache;
import okhttp3.Call;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.mockwebserver.MockResponse;
import okhttp3.mockwebserver.MockWebServer;
import okhttp3.mockwebserver.RecordedRequest;

import static junit.framework.Assert.assertEquals;
import static junit.framework.Assert.assertNotNull;
import static junit.framework.Assert.assertNull;

public class OkHttpTest {

    private static final String LOCATION = "/android.json";

    @Rule public final MockWebServer server = new MockWebServer();

    @Test
    public void test() throws Exception {
        server.enqueue(new MockResponse()
                               .setResponseCode(200)
                               .addHeader("content-type: application/json")
                               .addHeader("date: Mon, 18 Jun 2018 09:53:22 GMT")
                               // Comment the following line to make the test pass
                               .addHeader("last-modified: Fri, 11 May 2018 01:07:42 GMT")
                               .addHeader("etag: \"ff1f3f00586dea6efc5b60c2b689c5b0\"")
                               .addHeader("accept-ranges: bytes")
                               .addHeader("server: AmazonS3")
                               .addHeader("age: 10878")
                               .addHeader("x-cache: Hit from cloudfront")
                               .addHeader("via: 1.1 x.cloudfront.net (CloudFront)")
                               .addHeader("x-amz-cf-id: x")
                               .setBody("{}"));

        Request request = new Request.Builder()
                .url(server.url(LOCATION))
                .header("User-Agent", "Test Case")
                .build();
        File cacheDir = new File(System.getProperty("java.io.tmpdir"),
                                 UUID.randomUUID()
                                         .toString()
        );
        Cache cache = new Cache(cacheDir, 1024);
        OkHttpClient client = new OkHttpClient().newBuilder()
                .cache(cache)
                .connectTimeout(60000,
                                TimeUnit.MILLISECONDS
                               )
                .readTimeout(60000,
                             TimeUnit.MILLISECONDS
                            )
                .build();

        // Request once
        Call call = client.newCall(request);
        Response response = call.execute();

        assertEquals("{}", response.body()
                             .string()
                    );
        assertEquals(200, response.code());
        assertNull(response.cacheResponse());
        assertEquals(1, cache.networkCount());

        RecordedRequest recordedRequest = server.takeRequest();
        assertEquals(LOCATION, recordedRequest.getPath());
        Assert.assertNull(recordedRequest.getHeader("If-None-Match"));
        Assert.assertNull(recordedRequest.getHeader("If-Modified-Since"));
        assertEquals(0, cache.hitCount());

        server.enqueue(new MockResponse()
                               .setResponseCode(304)
                               .addHeader("date: Mon, 18 Jun 2018 09:53:22 GMT")
                               .addHeader("etag: \"ff1f3f00586dea6efc5b60c2b689c5b0\"")
                               .addHeader("server: AmazonS3")
                               .addHeader("age: 10878")
                               .addHeader("x-cache: Hit from cloudfront")
                               .addHeader("via: 1.1 x.cloudfront.net (CloudFront)")
                               .addHeader("x-amz-cf-id: x")
                      );
        // Request twice
        call = client.newCall(request);
        response = call.execute();

        assertEquals("{}", response.body()
                             .string()
                    );
        assertEquals(200, response.code());
        assertNotNull(response.cacheResponse());

        assertEquals(2, server.getRequestCount());
        recordedRequest = server.takeRequest();
        assertEquals(LOCATION, recordedRequest.getPath());
        Assert.assertEquals("\"ff1f3f00586dea6efc5b60c2b689c5b0\"", recordedRequest.getHeader("If-None-Match"));
        assertEquals(1, cache.hitCount());
    }
}