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Retrieving value from computation - Callable

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If your computation produces some return value which later is required, a simple Runnable task isn’t sufficient. For such cases you can use ExecutorService.submit(Callable<T>) which returns a value after execution completes.

The Service will return a Future which you can use to retrieve the result of the task execution. 

// Submit a callable for execution
ExecutorService pool = anExecutorService;
Future<Integer> future = pool.submit(new Callable<Integer>() {
    @Override public Integer call() {
        //do some computation
        return new Random().nextInt();
    }
});    
// ... perform other tasks while future is executed in a different thread

When you need to get the result of the future, call future.get() 

try {
    // Blocks current thread until future is completed
    Integer result = future.get(); 
catch (InterruptedException || ExecutionException e) {
    // handle appropriately
}
try {
    // Blocks current thread for a maximum of 500 milliseconds.
    // If the future finishes before that, result is returned,
    // otherwise TimeoutException is thrown.
    Integer result = future.get(500, TimeUnit.MILLISECONDS); 
catch (InterruptedException || ExecutionException || TimeoutException e) {
    // handle appropriately
}

If the result of a scheduled or running task is no longer required, you can call Future.cancel(boolean) to cancel it.

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Executor, ExecutorService and Thread pools:
* Executor ExecutorService and Thread pools
* ThreadPoolExecutor
* Retrieving value from computation - Callable
* submit vs execute exception handling differences
* Fire and Forget - Runnable Tasks
* Handle Rejected Execution
* Use cases for different types of concurrency constructs
* Wait for completion of all tasks in ExecutorService
* Use cases for different types of ExecutorService
* Scheduling tasks to run at a fixed time after a delay or repeatedly
* Using Thread Pools
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0 Inheritance
1 Getting started with Java
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91 Performance Tuning
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93 Common Pitfalls
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95 Java Compiler
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118 Pitfalls - threads and concurrency
119 Splitting a string into fixed length parts
120 Pitfalls - nulls and NullPointerException
121 SecurityManager
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123 super keyword
124 java.util.Objects class
125 Java command: java and javaw
126 Atomic types
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146 Concurrent Collections
147 Using ThreadPoolExecutor in multi-threaded applications
148 Java Editions, versions, releases and distributions
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152 Generating Java Code
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174 Readers and writers
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176 Enum starting with a number
177 SortedMap
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180 StringBuffer
181 Choosing Collections
182 C++ Comparison
183 CompletableFuture
184 Contributors