使用java.util.Function函数接口给出“找不到符号”错误

Question

我正在使用下面的代码创建java.util.Function Function>instance并使用Function instance的返回值将其传递给ExecutorService.Submit（）方法。

   //Approach-1
    Function<Integer, Callable<Integer>> doubleIt_1 = (index) -> {return () -> {return index * 2;};};
    //Approach-2
    Function<Integer, Callable<Integer>> doubleIt_2 = (index) -> () -> {return  index * 2;};
    //Approach 3
    Function<Integer, Callable<Integer>> doubleIt_3 = (index) -> () -> index * 2;

    //Use the "doubleIt" lambda function defined above to pass as a Lambda function to ExecutorService threadpool's submit method.
    Function<Integer, Future<Integer>> task = (Integer index) ->  pool.submit(doubleIt_1(index));

编译器抛出错误:java:找不到符号symbol:method doubleIt_1(java.lang.integer)location:class declarative.l12legacytoFunctionalInterface_4

请帮忙...

Answer 1

您必须调用函数的方法来调用doubleIt_1实现，如下所示：

//Use the "doubleIt" lambda function defined above to pass as a Lambda function to ExecutorService threadpool's submit method.
Function<Integer, Future<Integer>> task = (Integer index) ->  pool.submit(doubleIt_1.apply(index));

函数接口只有4个方法，核心方法是apply(T):R，在这里传递T值，它返回R响应。示例：

public static void main(String[] args) {
    Function<Integer, Double> t = new Function<Integer, Double>() {

        @Override
        public Double apply(Integer t) {
            return t.doubleValue();
        }
    };

    callFunction(t, 10);

    t = (i) -> i.doubleValue();
    callFunction(t, 10);

    t = (Integer i) -> {return i.doubleValue();};
    callFunction(t, 10);

    callFunction((i) -> i.doubleValue(), 10);

}

private static Double callFunction(Function<Integer, Double> f, Integer i){
    return f.apply(i);
}