Java Executors Callable() Method

The Callable() method of Executors class returns a Callable object that, when called, runs the given task and returns null.

Syntax

public static Callable<Object> callable(Runnable task)
public static <T> Callable<T> callable(Runnable task, T result)
public static Callable<Object> callable(PrivilegeAction<?> action)
public static Callable<Object> callable(PrivilegedExceptionAction<?> action)

Parameters

task - the task to run

result - the result to return

action - the privileged action to run

Return

a callable object

Throw

NullPointerException

Example 1

    import java.util.ArrayList;  
    import java.util.Date;  
    import java.util.List;  
    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 ExecutorCallableExample1 implements Callable<String> {  
      
        @Override  
        public String call() throws Exception {  
            Thread.sleep(1000);  
            return Thread.currentThread().getName();  
        }  
          
        public static void main(String args[]){  
      
            ExecutorService executor = Executors.newFixedThreadPool(20);  
            List<Future<String>> list = new ArrayList<Future<String>>();  
            Callable<String> callable = new ExecutorCallableExample1();  
      
            for(int i=0; i< 10; i++){  
                Future<String> ftr = executor.submit(callable);  
                list.add(ftr);  
            }  
            for(Future<String> fut : list){  
                try {  
                    System.out.println(new Date()+ "::"+fut.get());  
      
                } catch (InterruptedException | ExecutionException e) {  
                    e.printStackTrace();  
                }  
            }  
            executor.shutdown();  
        }  
      
    } 

Test it Now

Output:

Sun Mar 17 02:29:46 IST 2019::pool-1-thread-1
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-2
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-3
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-4
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-5
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-6
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-7
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-8
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-9
Sun Mar 17 02:29:48 IST 2019::pool-1-thread-10

Example 2

    import java.util.ArrayList;  
    import java.util.Date;  
    import java.util.List;  
    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 ExecutorCallableExample2 implements Callable<String> {  
      
        @Override  
        public String call() throws Exception {  
            Thread.sleep(500);  
            return Thread.currentThread().getName();  
        }     
        public static void main(String args[]){  
      
            ExecutorService executor = Executors.newFixedThreadPool(20);  
            List<Future<String>> lst = new ArrayList<Future<String>>();  
            Callable<String> clbl = new ExecutorCallableExample1();  
               int i =0;
            while( i < 5){  
                Future<String> ftr = executor.submit(clbl);  
                lst.add(ftr);
                i++;
            }  
            for(Future<String> fut : lst){  
                try {  
                    System.out.println( "::"+fut.get());  
      
                } catch (InterruptedException | ExecutionException e) {  
                    e.printStackTrace();  
                }  
            }  
            executor.shutdown();  
        }  
      
    } 

Test it Now

Output:

::pool-1-thread-1
::pool-1-thread-2
::pool-1-thread-3
::pool-1-thread-4
::pool-1-thread-5

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