Callable接口使用以及计算斐波那契数字的数值总和

一、简单使用

Runnable是执行工作的独立任务，但是它不返回任何值。如果你希望任务完成的时能够返回一个值，那么可以实现一个Callable接口。在Java SE5中引入的Callable是一种具有类型参数的泛型，它的类型参数表示的是从call()方法中返回的值，并且必须用ExecutorService.submit()方法调用它。下面是一个简单的例子（摘自Java编程思想）

class TaskWithResult implements Callable<String> {
    private int id;

    public TaskWithResult(int id) {
        this.id = id;
    }

    @Override
    public String call() throws Exception {
        return "Result of TaskWithResult" + id;
    }
}

public class CallableDemo {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        ArrayList<Future<String>> results = new ArrayList<>();
        for (int i = 0;i<10;i++){
            results.add(executorService.submit(new TaskWithResult(i)));
        }
        for (Future<String> f: results){
            try {
                System.out.println(f.get());
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }finally {
                executorService.shutdown();
            }
        }
    }
}

执行的结果如下

Result of TaskWithResult0
Result of TaskWithResult1
Result of TaskWithResult2
Result of TaskWithResult3
Result of TaskWithResult4
Result of TaskWithResult5
Result of TaskWithResult6
Result of TaskWithResult7
Result of TaskWithResult8
Result of TaskWithResult9

二、计算斐波那契数字的数值总和

public class FibonacciSumDemo {
    public static void main(String[] args) {
        ExecutorService exec = Executors.newCachedThreadPool();
        ArrayList<Future<Integer>> results = new ArrayList<Future<Integer>>();
        for (int i = 1; i <= 6; i++) results.add(exec.submit(new FibonacciSum(i)));
        Thread.yield();
        exec.shutdown();
        for (Future<Integer> fi : results)
            try {
                System.out.println(fi.get());
            } catch (Exception e) {
                e.printStackTrace();
            }
    }
}

class FibonacciSum implements Generator<Integer>, Callable<Integer> {
    private int count;
    private final int n;

    public FibonacciSum(int n) {
        this.n = n;
    }

    public Integer next() {
        return fib(count++);
    }

    private int fib(int n) {
        if (n < 2) return 1;
        return fib(n - 2) + fib(n - 1);
    }

    public Integer call() {
        int sum = 0;
        for (int i = 0; i < n; i++)
            sum += next();
        return sum;
    }
}

Generator接口定义如下

public interface Generator<T> {
    T next();
}

执行结果如下：

1
2
4
7
12
20