15:44 java:不兼容的类型：org。阿帕奇。梁sdk。价值观POutput无法转换为组织。阿帕奇。梁sdk。价值观P收集

Question

下面的代码大多是从apache束文档复制粘贴的。仍然无法找出这个错误。此外，我正在尝试学习束。有人能推荐一些好的教程/文章来看看吗？

import org.apache.beam.sdk.io.TextIO;
import org.apache.beam.sdk.transforms.MapElements;
import org.apache.beam.sdk.transforms.SimpleFunction;
import org.apache.beam.sdk.values.PCollection;


public class reports {
    public static void main(String[] args) {
        Pipeline p = Pipeline.create();
        PCollection<String> students = p.apply(TextIO.read().from("./students.csv"));
// error line below
        PCollection output = students.apply(MapElements.via(new calculatePercentage()));
        output.apply(TextIO.write().to("./reports.csv"));
        p.run();
    }
}
class calculatePercentage extends SimpleFunction{
    @Override
    public Object apply(Object input) {
        return input;
    }
}```