### [Fast Fallback](https://square.github.io/okhttp/4.x/okhttp/okhttp3/-ok-http-client/-builder/fast-fallback/)

Since version 5.0, `OkHttpClient` supports fast fallback, which is our implementation of Happy Eyeballs [RFC 6555](https://datatracker.ietf.org/doc/html/rfc6555).

    val latch = CountDownLatch(1)
    val exceptionRef = AtomicReference<Throwable>()
    val call = client.newCall(request)
    call.timeout().timeout(250, TimeUnit.MILLISECONDS)
    call.enqueue(
      object : Callback {
        override fun onFailure(
          call: Call,
          e: IOException,
        ) {
          exceptionRef.set(e)
          latch.countDown()
        }

        @Throws(IOException::class)

import com.google.common.annotations.GwtCompatible;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A callback for accepting the results of a {@link java.util.concurrent.Future} computation
 * asynchronously.
 *
 * <p>To attach to a {@link ListenableFuture} use {@link Futures#addCallback}.
 *
 * @author Anthony Zana

 *
 * @author Kevin Damm
 * @author Kyle Maddison
 */
@ElementTypesAreNonnullByDefault
final class LittleEndianByteArray {

  /** The instance that actually does the work; delegates to Unsafe or a pure-Java fallback. */
  private static final LittleEndianBytes byteArray;

  /**
   * Load 8 bytes into long in a little endian manner, from the substring between position and

        /**
         * @param op The option of condition to be set up. (NotNull)
         */
        void callback(OP op);
    }

    @FunctionalInterface
    public interface OperatorCall<CA extends EsAbstractConditionAggregation> {

        void callback(CA query);
    }

        /**
         * @param op The option of condition to be set up. (NotNull)
         */
        void callback(OP op);
    }

    @FunctionalInterface
    public interface OperatorCall<CA extends EsAbstractConditionAggregation> {

        void callback(CA query);
    }

import org.apache.maven.eventspy.EventSpy;
import org.apache.maven.execution.ExecutionListener;
import org.eclipse.aether.RepositoryListener;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Dispatches callbacks to all registered EventSpies.
 * @since 3.0.2
 */
@Named
@Singleton
public class EventSpyDispatcher {
    private final Logger logger = LoggerFactory.getLogger(getClass());

        Function<String, String> callback = v -> {
            String r = request.getUserProperties().getProperty(v);
            if (r == null) {
                r = request.getSystemProperties().getProperty(v);
            }
            return r != null ? r : v;
        };
        return v -> interpolator.interpolate(v, callback);
    }

    static class SimpleSession extends DefaultSession {

	return nil
}

func TestCallback(t *testing.T) {
	user, err := schema.Parse(&UserWithCallback{}, &sync.Map{}, schema.NamingStrategy{})
	if err != nil {
		t.Fatalf("failed to parse user with callback, got error %v", err)
	}

	for _, str := range []string{"BeforeSave", "AfterCreate"} {
		if !reflect.Indirect(reflect.ValueOf(user)).FieldByName(str).Interface().(bool) {
			t.Errorf("%v should be true", str)
		}
	}

			_, ip, _ = strings.Cut(s.Addresses[0], "/")
		}
		if ip == "" {
			// fallback to None when a service does not have a VIP
			ip = "None"
		}
		serviceNames[ip] = s.Hostname
	}
	for _, s := range d.Workloads {
		var ip string
		if len(s.WorkloadIPs) != 0 {
			ip = s.WorkloadIPs[0]
		}
		if ip == "" {
			// fallback to None when a workload does not have an IP
			ip = "None"
		}