CustomFunc.java

import scala.runtime.AbstractFunction1; 

// somewhere
sc = ScalaClass.fromCustomFunc(new AbstractFunction1<String, List<String>>() {
    @Override
    public List<String> apply(String fn) {
        BasicAWSCredentials AWSCredentials = new BasicAWSCredentials(AWS_ACCESS_KEY, AWS_SECRET_KEY);
        AmazonS3Client amazonS3Client = new AmazonS3Client(AWSCredentials);
        return getData(amazonS3Client.getObject(BUCKET, PATH + fn).getObjectContent());
    }
});