在流联合中进行类型细化，流不识别if语句

import org.apache.flink.streaming.api.datastream.DataStream;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;

public class TypeRefinementExample {
    public static void main(String[] args) throws Exception {
        final StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

        DataStream<String> input = env.fromElements("apple", "banana", "cherry");

        DataStream<String> filtered = input.filter(value -> {
            if (value.startsWith("a")) {
                return true;
            } else {
                return false;
            }
        });

        filtered.print();

        env.execute("Type Refinement Example");
    }
}