In Object-Oriented Programming, objects collaborate. The initial idea of collaboration, first found in Smalltalk, was for object A to send a message to object B. Languages designed later use method calling. In both cases, the same question stands: how does an object reference other objects to reach the desired results? Object-Oriented Programming In this post, I tackle the problem of passing dependencies to an object. I will go through several options and analyze their respective pros and cons. Constructor injection For constructor injection, you pass dependencies as parameters to the constructor. class Delivery(private val addressService: AddressService,
               private val geoService: GeoService,
               private val zoneId: ZoneId) {

    fun computeDeliveryTime(user: User, warehouseLocation: Location): ZonedDateTime {
        val address = addressService.getAddressOf(user)
        val coordinates = geoService.getCoordinates(location)
        // return date time
    }
} class Delivery(private val addressService: AddressService,
               private val geoService: GeoService,
               private val zoneId: ZoneId) {

    fun computeDeliveryTime(user: User, warehouseLocation: Location): ZonedDateTime {
        val address = addressService.getAddressOf(user)
        val coordinates = geoService.getCoordinates(location)
        // return date time
    }
} Constructor injection is by far the most widespread way to pass to an object its dependencies: for about ten years, every codebase I've seen has constructor injection. I've a slight issue with constructor injection: it stores dependencies as fields, just like state. Looking at the constructor's signature, it's impossible to distinguish between the state and dependencies without proper typing. It bugs me. Let's see other ways. Parameter passing Instead of storing the dependencies along with the state, we can pass the dependency when calling the method. class Delivery(private val zoneId: ZoneId) {

    fun computeDeliveryTime(addressService: AddressService,
                            geoService: GeoService,
                            user: User, warehouseLocation: Location): ZonedDateTime {
        val address = addressService.getAddressOf(user)
        val coordinates = geoService.getCoordinates(location)
        // return date time
    }
} class Delivery(private val zoneId: ZoneId) {

    fun computeDeliveryTime(addressService: AddressService,
                            geoService: GeoService,
                            user: User, warehouseLocation: Location): ZonedDateTime {
        val address = addressService.getAddressOf(user)
        val coordinates = geoService.getCoordinates(location)
        // return date time
    }
} The separation of state and dependencies is now clear: the former is stored in fields, while the latter is passed as function parameters. However, the responsibility of handling the dependency is moved one level up the call chain. The longer the call chain, the more unwieldy it gets. class Order() {

    fun deliver(delivery: Delivery, user: User, warehouseLocation: Location): OrderDetails {
        // Somehow get the address and the geo services
        val deliveryTime = delivery.computeDeliveryTime(addressService, geoService, user, warehouseLocation)
        // return order details
    }
} class Order() {

    fun deliver(delivery: Delivery, user: User, warehouseLocation: Location): OrderDetails {
        // Somehow get the address and the geo services
        val deliveryTime = delivery.computeDeliveryTime(addressService, geoService, user, warehouseLocation)
        // return order details
    }
} Note that the call chain length is also a problem with constructor injection. You need to design the code for the call site to be as close as possible to the dependency creation one. ThreadLocal Legacy design makes use of the ThreadLocal: ThreadLocal This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).
For example, the class below generates unique identifiers local to each thread. A thread's ID is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls.
import java.util.concurrent.atomic.AtomicInteger;

public class ThreadId {
   // Atomic integer containing the next thread ID to be assigned
   private static final AtomicInteger nextId = new AtomicInteger(0);

   // Thread local variable containing each thread's ID
   private static final ThreadLocal<Integer> threadId =
       new ThreadLocal<Integer>() {
           @Override protected Integer initialValue() {
               return nextId.getAndIncrement();
       }
   };

   // Returns the current thread's unique ID, assigning it if necessary
   public static int get() {
       return threadId.get();
   }
}

-- Class ThreadLocal<T> This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID). ThreadLocal For example, the class below generates unique identifiers local to each thread. A thread's ID is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls. ThreadId.get() import java.util.concurrent.atomic.AtomicInteger;

public class ThreadId {
   // Atomic integer containing the next thread ID to be assigned
   private static final AtomicInteger nextId = new AtomicInteger(0);

   // Thread local variable containing each thread's ID
   private static final ThreadLocal<Integer> threadId =
       new ThreadLocal<Integer>() {
           @Override protected Integer initialValue() {
               return nextId.getAndIncrement();
       }
   };

   // Returns the current thread's unique ID, assigning it if necessary
   public static int get() {
       return threadId.get();
   }
} import java.util.concurrent.atomic.AtomicInteger;

public class ThreadId {
   // Atomic integer containing the next thread ID to be assigned
   private static final AtomicInteger nextId = new AtomicInteger(0);

   // Thread local variable containing each thread's ID
   private static final ThreadLocal<Integer> threadId =
       new ThreadLocal<Integer>() {
           @Override protected Integer initialValue() {
               return nextId.getAndIncrement();
       }
   };

   // Returns the current thread's unique ID, assigning it if necessary
   public static int get() {
       return threadId.get();
   }
} -- Class ThreadLocal<T> Class ThreadLocal<T> We can rewrite the above code using ThreadLocal: ThreadLocal class Delivery(private val zoneId: ZoneId) {

    fun computeDeliveryTime(user: User, warehouseLocation: Location): ZonedDateTime {
        val addressService = AddressService.get()
        val geoService = GeoService.get()
        // return date time
    }
} class Delivery(private val zoneId: ZoneId) {

    fun computeDeliveryTime(user: User, warehouseLocation: Location): ZonedDateTime {
        val addressService = AddressService.get()
        val geoService = GeoService.get()
        // return date time
    }
} The ThreadLocal can be either set up in the call chain or lazily, on first access. Regardless, the biggest disadvantage of this approach is that it completely hides the dependency. There's no way to understand the coupling by only looking at the class constructor or the function signature; one needs to read the function's source code. ThreadLocal Additionally, the implementation could be a regular singleton pattern, with the same downsides. Kotlin context The last approach is Kotlin-specific and has just been promoted from experimental to beta in Kotlin 2.2. Context parameters allow functions and properties to declare dependencies that are implicitly available in the surrounding context.
With context parameters, you don't need to manually pass around values, such as services or dependencies, that are shared and rarely change across sets of function calls.
-- Context parameter Context parameters allow functions and properties to declare dependencies that are implicitly available in the surrounding context. With context parameters, you don't need to manually pass around values, such as services or dependencies, that are shared and rarely change across sets of function calls. -- Context parameter Context parameter Here's how we can migrate the above code to context parameters: class Delivery(private val zoneId: ZoneId) {

    context(addressService: AddressService, geoService: GeoService)
    fun computeDeliveryTime(user: User, warehouseLocation: Location): ZonedDateTime {
        // return date time
    }
} class Delivery(private val zoneId: ZoneId) {

    context(addressService: AddressService, geoService: GeoService)
    fun computeDeliveryTime(user: User, warehouseLocation: Location): ZonedDateTime {
        // return date time
    }
} And here's how to call it: context(addressService,geoService) {
	delivery.computeDeliveryTime(user, location)
} context(addressService,geoService) {
	delivery.computeDeliveryTime(user, location)
} Note that the call can be nested at any level inside the context. context Summary Approach

Pros

Cons



Constructor injection

Testable

Mix state and dependencies



Parameter passing

Testable

Noisy



ThreadLocal



Hides coupling



Context parameter

Get dependencies on deeply-nested

Limited to Kotlin Approach

Pros

Cons



Constructor injection

Testable

Mix state and dependencies



Parameter passing

Testable

Noisy



ThreadLocal



Hides coupling



Context parameter

Get dependencies on deeply-nested

Limited to Kotlin Approach

Pros

Cons Approach Approach Pros Pros Cons Cons Constructor injection

Testable

Mix state and dependencies Constructor injection Constructor injection Testable Testable Mix state and dependencies Mix state and dependencies Parameter passing

Testable

Noisy Parameter passing Parameter passing Testable Testable Noisy Noisy ThreadLocal



Hides coupling ThreadLocal ThreadLocal ThreadLocal Hides coupling Hides coupling Context parameter

Get dependencies on deeply-nested

Limited to Kotlin Context parameter Context parameter Get dependencies on deeply-nested Get dependencies on deeply-nested Limited to Kotlin Limited to Kotlin I guess I'll continue to use constructor injection, unless I'm coding in Kotlin. In this case, I'll be happy to use context parameters, even though they are in beta. Originally published at A Java Geek on October 12th, 2025 Originally published at A Java Geek on October 12th, 2025 A Java Geek

The is an opinion piece based on the author’s POV and does not necessarily reflect the views of HackerNoon.

Understanding Dependency Injection in Object-Oriented Programming

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