Friday, January 11, 2013

Difference between Callable and Runnable in Java - Interview question

Difference between Callable and Runnable interface in Java is one of the interesting question from my list of Top 15 Java multi-threading questions, and it’s also very popular in various Java Interviews. Callable interface is newer than Runnable interface and added on Java 5 release along with other major changes e.g. Generics, Enum, Static imports and variable argument method. Though both Callable and Runnable interface are designed to represent task, which can be executed by any thread, there is some significant difference between them. In my opinion, major difference between Callable and Runnable interface is that Callable can return result of operation performed inside call() method, which was one of the limitation with Runnable interface. Another significant difference between Runnable and Callable interface is ability to throw checked exception. Callable interface can throw checked exception because it's call method throws Exception. By the way sometime this question is also asked as follow-up question of another classic difference between Runnable and Thread in Java. Commonly FutureTask is used along with Callable to get result of asynchronous computation task performed in call() method.

Callable vs Runnable interface

Difference between Runnable and Callable in JavaAs I explained major differences between Callable and Runnable interface in last section. Sometime this question is also asked as difference between call() and run()  method in Java. All the points discussed here is equally related to that question as well.  Let's see them in point format for better understanding :

1) Runnable interface is older than Callable, there from JDK 1.0, while Callable is added on Java 5.0.
2) Runnable interface has run() method to define task while Callable interface uses call() method for task definition.
3) run() method does not return any value, it's return type is void while call method returns value. Callable interface is a generic parameterized interface and Type of value is provided, when instance of Callable implementation is created.

4) Another difference on run and call method is that run method can not throw checked exception, while call method can throw checked exception in Java.

That's all on Difference between Callable and Runnable interface in Java or difference between call() and run() method. Both are very useful interface from core Java and good understanding of where to use Runnable and Callable is must for any good Java developer. In next article we will see example of Callable interface along with FutureTask to learn How to use Callable interface in Java.

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