1. Introduction

Multithreading and concurrency are essential concepts in modern software development, enabling programs to efficiently handle multiple tasks simultaneously. In Kotlin, developers have several tools at their disposal to control the timing and execution of threads or coroutines.

In this tutorial, we’ll explore and compare three commonly used methods for introducing delays: wait(), sleep(), and delay().

2. Understanding the Basics of Threads and Coroutines

Before diving into the specific delay methods, it’s crucial to understand the underlying concepts of threads and coroutines.

Threads are the smallest unit of execution within a process. In Java and Kotlin, the Thread class provides a way to create and manage threads. However, directly working with threads can be error-prone and complex.

Coroutines, on the other hand, are a lightweight alternative introduced in Kotlin to simplify asynchronous programming. They provide a way to write asynchronous code in a sequential and more readable manner.

Now, let’s delve into the details of each method.

3. The wait() Method

The wait() method is a part of the Object class in Java and Kotlin and is used for thread synchronization. It should be noted that wait() is not a direct member of the Kotlin language but is accessible through Java interoperability.

In Kotlin, it’s important to note that the wait() and notify() methods are not directly accessible on instances of the Any type. Therefore, to use wait() and notify() we need to make our lock of type Object:

val lock = Object()
fun main() {
    runBlocking(Dispatchers.Default) {
        launch(Dispatchers.IO) {
        launch(Dispatchers.IO) {
fun testWaitThread1() = synchronized(lock) {
    println("Print first")
fun testWaitThread2() = synchronized(lock) {
    println("Print second")

In this example, our first coroutine testWaitThread1() waits for a signal before printing “Print first”. Our second coroutine testWaitThread2() prints “Print second” and signals the waiting thread to proceed by invoking notify() on our shared object lock.

The use of synchronized(), wait(), and notify() ensures proper coordination and synchronization between the two threads. When the code runs, we’ll notice that it prints “Print second” followed by “Print first” due to this locking.

4. The sleep() Method

The sleep() method is a static method of the Thread class and pauses the execution of a thread for a specified amount of time. Unlike wait(), it doesn’t involve synchronization or inter-thread communication:

fun sleepMethod() {
    println("Thread 1 is sleeping...")
    Thread.sleep(2000) // Sleep for 2 seconds
    println("Thread 1 woke up!")

In this example, the main thread sleeps for two seconds before continuing its execution. While sleep() is straightforward, it’s important to note that it pauses the entire thread.

5. The delay() Method

The delay() method is part of Kotlin’s coroutine framework and introduces delays in coroutine execution. Coroutines are a more modern and flexible approach to concurrency compared to traditional threads:

fun delayMethod() = runBlocking {
    println("Coroutine 1 is delaying...")
    delay(2000) // Delay for 2 seconds
    println("Coroutine 1 resumed!")

In this example, the runBlocking() coroutine builder creates a coroutine, and delay() introduces a non-blocking delay of two seconds. Unlike threads, coroutines don’t block the underlying thread, making them more suitable for asynchronous and responsive applications.

6. Comparative Analysis: When to Use Each Method

Now that we’ve explored wait(), sleep(), and delay(), let’s discuss when to use each method.

6.1. wait()

It is advisable to use the wait() method when we’re dealing with thread synchronization and inter-thread communication. This method lets one thread wait for a condition or notification from another thread before resuming.

6.2. sleep()

The sleep() method introduces a delay in thread execution without the need for any synchronization or inter-thread communication. It’s suitable for simple timing-related tasks within a thread.

6.3. delay()

Frequently, we use the delay() method when working with coroutines for non-blocking delays. This method is the preferred choice in modern Kotlin applications for asynchronous programming, ensuring responsiveness without blocking threads.

7. Conclusion

Understanding the differences between wait(), sleep(), and delay() is crucial for effective multithreading and coroutine-based programming in Kotlin. The choice between these methods depends on the specific requirements of our application, with coroutines and delay() being the more modern and flexible options for asynchronous programming.

As Kotlin continues to evolve, coroutines are likely to become even more integral to the language’s concurrency model.

The full implementation of these examples is available over on GitHub.

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