Runnable Vs Callable in Java

Java programming language provides two ways to implement a class whose instances are to be executed by a thread.

By implementing Runnable interface
By implementing Callable interface

Since there are two options so they must have some differences in the features they offer, that’s what we’ll discuss in this post; differences between Runnable and Callable in Java.

Runnable Vs Callable in Java

1- Part of Java programming language

Runnable interface is there since Java 1.0.
Callable interface is added in Java 1.5

2- Interface method

Runnable interface has a single method run() with the method signature as following.

public interface Runnable {
  public abstract void run();
}

Callable interface has a single method call(), a generic method with the method signature as following.

public interface Callable<V> { 
  V call() throws Exception;
}

3- Return value

This is one of major difference between Runnable and Callable in Java. Runnable doesn’t return a result. You can see from the method signature of the run method that the return type is void.

Callable can return a result. Return type of the result is provided at the time of creating Callable instance.

For Example a Callable that returns String.

Callable<String> callableObj = new Callable<String>() {
  @Override
  public String call() throws Exception {
    return "Hello";
  }
};

4- Exception handling

Since Runnable’s run method doesn’t include a throws clause so it is not possible for a Runnable to throw a checked exception.

Following implementation of Runnable results in compile time error “Exception IOException is not compatible with throws clause in Runnable.run()” as IOException is a checked exception.

Runnable runnable = new Runnable() {
  @Override
  public void run() throws IOException{
	  
  }
};

But it is ok if RunTimeException is thrown.

public void run() throws RuntimeException{

}

Callable’s call method does include a throws clause- V call() throws Exception; so it is possible for a Callable to throw a checked exception.

Callable<String> callable = new Callable<String>() {
  @Override
  public String call() throws InterruptedException {
    return "Hello";
  }
};

5- How to execute.

To execute a Runnable task you can use one of the following options-

Thread class has a constructor where you can pass a Runnable as a parameter.
You can use execute method of the Executor interface to run a Runnable task.
You can use submit method of the ExecutorService interface to run a Runnable task.

To execute a Callable task there is no support provided by the Thread class but the concurrent API provides the following options-

You can use submit method of the ExecutorService interface to run a Callable task.
ExecutorService also has invokeAll() and invokeAny() methods where Callable is passed as a parameter.
You can also convert a Runnable object to Callable object by using Executors.callable(Runnable task) provided by utility class Executors.

That's all for the topic Runnable Vs Callable in Java. If something is missing or you have something to share about the topic please write a comment.

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KnpCode

October 20, 2022

Runnable Vs Callable in Java

Runnable Vs Callable in Java

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