parkingLot | Android mobile app , allows a search of empty parking lots | Mobile library

You can use the RabbitMQ REST api (Hop client) to get the status of the DLQ.

You can also use Spring's RabbitAdmin.getQueueProperties("my.dlq") to get the message count.

https://docs.spring.io/spring-amqp/docs/current/reference/html/#broker-configuration

Other options include adding another listener on the DLQ and run it periodically to either send it back to the original queue or send it to a parking lot queue if it fails too many times.

There's an example of that in the spring cloud stream documentation.

TL;DR: Use Constructor-based DI

1. move the @Autowired atop constructor
2. add field as parameter to constructor and assign therein first

You got this UnsatisfiedDependencyException because:

Error creating bean with name 'parking':

This bean could not be created because:

Unsatisfied dependency expressed through field 'parkingServices';

So the field could not be autowired by Spring with a dependency because of a NullPointerException (NPE):

nested exception is org.springframework.beans.factory.BeanCreationException: Constructor threw exception; nested exception is java.lang.NullPointerException

Continue to answer explaining M. Deinum's comment:

The problem is the accessing of a variable before it is available.

The NPE was raised when the constructor of ParkingServices tries to call init on the field parkingLot, because the field was null.

ParkingLot[] lot = new ParkingLot[10];

It feels like you imagine this creates a single parking lot with 10 spaces.

It doesn't.

It creates a plot of land upon which up to 10 parking lots can be placed, though none are there yet (a ParkingLot is an object, if you want one to exist, somebody, somewhere, must invoke new ParkingLot(), or no parking lot objects exist).

lot[1].getEmptySpaces()

This goes to the second lot (java is 0-indexed, so, lot[1] is the second lot), and asks it: Oi, how much free space is there? Given that you're yelling this at an empty space where a parking lot can be, but isn't, you get a NullPointerException.

lot.getEmptySpaces();

This makes no sense at all. How can you ask 10 parking lots at once? Even worse, how can you ask a bit of terrain reserved for 10 parking lots, but where no lots exist right now?

Relevant:

Some issues in your enter implementation:

• Don't create a new parkingLot. You need to use the current parkingLot instance (this) only.
• Don't call shift on the this.spaces array (Imagine all those cars that would so move to a neighboring spot!). Instead find the free spot (using indexOf) and write the license plate number there.
• You cannot update this.vacantSpaces as a property. It is a getter, not a number. There is no need to update anything, as this getter will dynamically count how many vacant spaces there are, at the moment it is called.

Here is the corrected version. I also added a leave method, and extended the test so the queue also gets used:

Remove:

Change your client query to this:

You need to add sample data in setUp method and use them in test methods. Add the sample data like following.

The modulo operator % gives back the remainder after floor division //. That is to say that 7 // 3 == 2, 7 % 3 == 1. Indeed these two are mutually defined such that (x // k) * k + (x % k) == x.

You should consider taking the modulo of your minutesPassed and 30 to see where the partial-half-hour lies, then comparing that with 15 to judge whether or not to round up or down.

The problem here is that mutable collections have invariant element types in Scala so you can't store Option[CarLot] nor Option[MotorcycleLot] elements in Array[Option[AbstractLot[Any]]]. The reason for this can be found here: relation between variance and mutabilty / immutability in Scala.

ParkingLot object is used mainly for space preallocation which is rarely needed in Scala. You have to rethink your design because it won't be possible to create any mutable collection with covariant type. The simplest way to represent a parking lot using a mutable collection would be something like this: