Thread vs Runnable

Thread is a block of code which can execute concurrently with other threads in the JVM. You can create and run a thread in either ways; Extending Thread class, Implementing Runnable interface.

Both approaches do the same job but there have been some differences. Almost everyone have this question in their minds: which one is best to use? We will see the answer at the end of this post.

The most common difference is

• When you extends Thread class, after that you can’t extend any other class which you required. (As you know, Java does not allow inheriting more than one class).
• When you implements Runnable, you can save a space for your class to extend any other class in future or now.

However, the significant difference is.

• When you extends Thread class, each of your thread creates unique object and associate with it.
• When you implements Runnable, it shares the same object to multiple threads.

The following example helps you to understand more clearly.

ThreadVsRunnable.java

class ImplementsRunnable implements Runnable {

 private int counter = 0;

 public void run() {
 counter++;
 System.out.println("ImplementsRunnable : Counter : " + counter);
 }
 }

 class ExtendsThread extends Thread {

 private int counter = 0;

 public void run() {
 counter++;
 System.out.println("ExtendsThread : Counter : " + counter);
 }
 }

 public class ThreadVsRunnable {

 public static void main(String args[]) throws Exception {
 //Multiple threads share the same object.
 ImplementsRunnable rc = new ImplementsRunnable();
 Thread t1 = new Thread(rc);
 t1.start();
 Thread.sleep(1000); // Waiting for 1 second before starting next thread
 Thread t2 = new Thread(rc);
 t2.start();
 Thread.sleep(1000); // Waiting for 1 second before starting next thread
 Thread t3 = new Thread(rc);
 t3.start();

 //Creating new instance for every thread access.
 ExtendsThread tc1 = new ExtendsThread();
 tc1.start();
 Thread.sleep(1000); // Waiting for 1 second before starting next thread
 ExtendsThread tc2 = new ExtendsThread();
 tc2.start();
 Thread.sleep(1000); // Waiting for 1 second before starting next thread
 ExtendsThread tc3 = new ExtendsThread();
 tc3.start();
 }
 }

Output of the above program.

ImplementsRunnable : Counter : 1
ImplementsRunnable : Counter : 2
ImplementsRunnable : Counter : 3
ExtendsThread : Counter : 1
ExtendsThread : Counter : 1
ExtendsThread : Counter : 1

In the Runnable interface approach, only one instance of a class is being created and it has been shared by different threads. So the value of counter is incremented for each and every thread access.

Whereas, Thread class approach, you must have to create separate instance for every thread access. Hence different memory is allocated for every class instances and each has separate counter, the value remains same, which means no increment will happen because none of the object reference is same.

When to use Runnable?
Use Runnable interface when you want to access the same resource from the group of threads. Avoid using Thread class here, because multiple objects creation consumes more memory and it becomes a big performance overhead.

Apart from this, object oriented designs have some guidelines for better coding.

• Coding to an interface rather than to implementation. This makes your software/application easier to extend. In other words, your code will work with all the interface’s subclasses, even ones that have not been created yet.
• Interface inheritance (implements) is preferable – This makes your code is loosely coupling between classes/objects.(Note : Thread class internally implements the Runnable interface)

Example: coding to an interface.

Map subject = new HashMap();

Assigning HashMap object to interface Map,  suppose in future if you want to change HashMap to Hashtable or LinkedHashMap you can simple change in the declaration part is enough rather than to all the usage places. This point has been elaborately explained here.

Which one is best to use?

Ans : Very simple, based on your application requirements you will use this appropriately. But I would suggest, try to use interface inheritance i.e., implements Runnable.

Example : 

Map subject = new HashMap();

Assigning HashMap object to interface Map,  suppose in future if you want to change HashMap to Hashtable or LinkedHashMap you can simple change in the declaration area is enough rather than to all the usage places. I will explain this elaborately in the upcoming post.