The Callable is an interface which is available in java.util.concurrent package. It is similar to Runnable Interface but there is some difference.
- Runnable interface has the run() method and its return type is void
- Callable interface has the call() method and its return type is Future
- Callable interface can’t be executed using thread class similarly runnable, Through Thread ExecutorService only we can run it.
Sample Code
In below example I try to run the callable object using thread but getting the compile time exception.
package com.samplecoder.threading;
import java.util.concurrent.Callable;
public class ThreadWithCallable {
public static void main(String[] args) {
// TODO Auto-generated method stub
Thread t1 = new Thread(new Callable<String>() {
@Override
public String call() throws Exception {
// TODO Auto-generated method stub
return null;
}
})
}
}
Exception
Exception in thread "main" java.lang.Error: Unresolved compilation problems:
The constructor Thread(new Callable<String>(){}) is undefined
Syntax error, insert ";" to complete BlockStatements
at com.samplecoder.threading.ThreadWithCallable.main(ThreadWithCallable.java:10)
Callable with ExecutorService
package com.samplecoder.threading;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class ThreadWithCallable {
public static void main(String[] args) {
// TODO Auto-generated method stub
Callable<String> c1 = new Callable<String>() {
@Override
public String call() throws Exception {
// TODO Auto-generated method stub
System.out.println("Callable started");
for(int i=1; i<=10; i++) {
System.out.println(i);
Thread.sleep(100);
}
return "Completed";
}
};
ExecutorService executorService = Executors.newFixedThreadPool(3);
Future<String> result = executorService.submit(c1);
try {
System.out.println(result.get());
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
}
Result
Callable started
1
2
3
4
5
6
7
8
9
10
Completed
Conclusion
Now, you should know what is callable interface and you should able to write some basic java code using the callable interface.
