如何将JavaRDD<List<String>>转换为JavaRDD<String>并在不使用"[“和"]”的情况下写入文件

import org.apache.spark.api.java.JavaRDD;
import org.apache.spark.api.java.function.Function;
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

Function<List<String>, String> listToString = new Function<List<String>, String>() {
    @Override
    public String call(List<String> list) throws Exception {
        StringBuilder sb = new StringBuilder();
        for (String str : list) {
            sb.append(str).append(" ");
        }
        return sb.toString().trim();
    }
};

JavaRDD<String> stringRDD = listRDD.map(listToString);

try {
    BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"));
    for (String str : stringRDD.collect()) {
        writer.write(str);
        writer.newLine();
    }
    writer.close();
} catch (IOException e) {
    e.printStackTrace();
}