Java Threads & Concurrency API — Part I

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Java Threads & Concurrency API — Part I

What is a Thread?

Thread is the smallest unit of execution that can be scheduled by the OS.

What is a Task?

A task is a single unit of work performed by a Thread.

But then how do we define this work (or create a task for a thread)?

  • A common way is to use a Runnable Interface (we will implement it using Lambda expression)
Runnable runnable = () -> System.out.println("I am another Thread");
Thread anotherThread = new Thread(runnable);
  • Or you create a Java class that implements a Runnable interface and then pass the instance of this class while creating an instance of Thread.
Thread someThread =
new Thread(new AUserDefinedClassThatImplementsRunnable());
  • With a class implementing Callable Interface. Instead of overriding run() method, for this one we override call() method, which unlike run() can throw an exception and can return a generic value (a String in the example below)
public class TestCallable implements Callable {

public static void main(String[] args) throws Exception {

TestCallable callable = new TestCallable();
String message =;
public String call() throws Exception {
return "Task creation for thread using Callable";
  • Or by creating a class that extends the Thread class. Then create an instance of this class and invoke “start()” method on it. Thread class itself implements a Runnable interface. (Avoid this way of Thread creation)

  • and also by creating Threads with the ExecutorService (discussed in detail in part 2 of this article)

What is the difference between the Runnable and Callable Interfaces?

  • Runnable interface has an abstract “run()” method whereas Callable has an abstract “call()” method.

  • Runnable interface’s “run()” method has void return type whereas Callable’s abstract “call()” method can have a generic return type.

  • run() method doesn’t throw an exception while call() can throw an exception

public interface Callable { 
V call() throws Exception;
public interface Runnable {
 public abstract void run();

What is a Process?

Now, envision a group of related threads executing within the same shared environment. This is what a process resembles.

Types of Threads?

  • System-defined threads — by the JVM and run in the background of the application (like garbage collection daemon thread)

  • User-defined threads — created by the application developer to accomplish a certain task

What is Concurrency?

The property of executing multiple threads at the same time is referred to as concurrency.
It can be misleading when we say "at the same time," because in a single-core processor, only one task is executed at any given moment. However, the context switch occurs rapidly enough to create the illusion of multiple tasks being accomplished simultaneously.

(like listening to music while preparing a Word document)

Part II in progress…

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