Develop Reference

Add array to newly created file in Java

I'm passing an array of array to a java method and I need to add that data to a new file (which will be loaded into an s3 bucket)
How do I do this? I haven't been able to find an example of this
Also, I'm sure "object" is not the correct data type this attribute should be. Array doesn't seem to be the correct one.
Java method -
public void uploadStreamToS3Bucket(String[][] locations) {
try {
AmazonS3 s3Client = AmazonS3ClientBuilder.standard()
.withRegion(String.valueOf(awsRegion))
.build();
String fileName = connectionRequestRepository.findStream() +".json";
String bucketName = "downloadable-cases";
File locationData = new File(?????) // Convert locations attribute to a file and load it to putObject
s3Client.putObject(new PutObjectRequest(bucketName, fileName, locationData));
} catch (AmazonServiceException ex) {
System.out.println("Error: " + ex.getMessage());
}
}

You're trying to use PutObjectRequest(String,String,File)
but you don't have a file. So you can either:
Write your object to a file and then pass that file
or
Use the PutObjectRequest(String,String,InputStream,ObjectMetadata) version instead.
The later is better as you save the intermediate step.
As for how to write an object to a stream you may ask: Check this How can I convert an Object to Inputstream
Bear in mind to read it you have to use the same format.
It might be worth to think about what kind of format you want to save your information, because it might be needed to be read for another program, or maybe by another human directly from the bucket and there might be other formats / serializers that area easy to read (if you write JSON for instance) or more efficient (if you use another serializer that takes less space).
As for the type of array of array you can use the [][] syntax. For instance an array of array of Strings would be:
String [][] arrayOfStringArrays;
I hope this helps.

How can I convert JSON file into List of objects?

I'm trying to do a loader for my program and the saver into json works, but the reader doesn't. I'm basically converting my list of objects, let's say ArrayList ProjectArr into json which is working, now I'm gonna close terminal and load the info into my initially blank ProjectArr list so I can add the saved data loading feature. here is my reader :
public static ArrayList<Project> MapReaderforProject() {
ObjectMapper objectMapperforProject = new ObjectMapper();
String jsonfile = "C:/Users/elmm/Desktop/DS Project/DsProject/target/test1.json";
File file = new File("C:/Users/elmm/Desktop/DS Project/DsProject/target/test1.json");
try {
ProjectArr = objectMapperforProject.readValue(jsonfile,new TypeReference<ArrayList<Project>>(){});
}
catch (JsonMappingException e) {
} catch (JsonProcessingException e) {
}
System.out.println(ProjectArr.toString());
return ProjectArr;
}
But when I call this even though the JSON file is working and has the info, it's not reading anything and just prints [], empty list basically meaning it's not reading the data from JSON back into my list. Can someone please guide me?

There is some context information missing to give you more help. For example, what is ObjectMapper? Does it come from a library (and from which one) or have you written it yourself?
But what I have recognized so far: In line 3 you create a String which contains the file name. In line 4 you create a File object with the same file name. You could use the String there which you created the line above.
In line 6 you call the readValue method on the ObjectMapper instance and pass the String created in line 3, which contains the file name. Does this method really want a String there with the file name? If this is the ObjectMapper from the Jackson library, you need to parse the JSON code as the first parameter to the readValue call, not the file name.
So it looks like after line 4 you need code to read the content of the file which you should pass to the readValue method.
Check out this short tutorial: https://www.baeldung.com/jackson-object-mapper-tutorial
hth
McPringle

Cannot read the first line of a JSON file in Java

I am trying to read some data from a JSON file that I generated from a MongoDB document. But when trying to read the first entry in the document, i get an exception:
org.json.JSONException: JSONObject["Uhrzeit"] not found.
This only happens with the first entry, reading other entrys does not cause an exception.
Using jsonObject.getString("") on any entry that is not the first returns the values as expected.
//Initiate Mongodb and declare the database and collection
MongoClient mongoClient = new MongoClient(new MongoClientURI("mongodb://localhost:27017"));
MongoDatabase feedbackDb = mongoClient.getDatabase("local");
MongoCollection<Document> feedback = feedbackDb.getCollection("RückmeldungenShort");
//gets all documents in a collection. "new Document()" is the filter, that returns all Documents
FindIterable<Document> documents = feedback.find(new Document());
//Iterates over all documents and converts them to JSONObjects for further use
for(Document doc : documents) {
JSONObject jsonObject = new JSONObject(doc.toJson());
System.out.print(jsonObject.toString());
System.out.print(jsonObject.getString("Uhrzeit"));
}
Printing jsonObject.toString() produces the JSON String for testing purposes (in one line):
{
"Ort":"Elsterwerda",
"Wetter-Keyword":"Anderes",
"Feedback\r":"Test Gelb\r",
"Betrag":"Gelb",
"Datum":"18.05.2018",
"Abweichung":"",
"Typ":"Vorhersage",
"_id":{
"$oid":"5b33453b75ef3c23f80fc416"
},
"Uhrzeit":"05:00"
}
Note, that the order in which the entries appear is mixed up and the first one appearing in the database was "Uhrzeit".
This is how it looks like:
The JSON file is valid according to https://jsonformatter.curiousconcept.com/ .
The "Uhrzeit" is even recognized within the JSONObject while in debug mode:
I assumed it might have something to do with the entries themselves, so I switched "Datum" and "Ort" to the first place in the document but that produced the same results.
There are lots of others that have posted on this error message, but it seems to me like they all had slightly different problems.

I imported a .csv with my data into MongoDB and read the documents from there. Somewhere in the process of reading the data, "\r"s were automatically generated where the line breaks were in my .csv (aka. at the end of each dataset). In this case at the key value pair "Feedback" (as seen in the last picture).
When checking my output again with another JSON validator, I noticed that there was an "invisible" symbol in my JSON file that caused the key not to be found. Now this symbol is located in front of the first key (after the MongoDB-id) when importing a .csv document to my DB. I imported a correct version of the .csv into my MongoDB and exported it again and the symbol reappeared.
The problem was that my .csv was in "Windows" format. Converting it to "Unix" format will get rid of the generated "\r"s. The "invisible" symbol was the UTF-8-BOM code that is added at the beginning of a document. You can reformat your .csv to be just UTF-8 and get rid of it that way.

Parsing RDF items

I have a couple lines of (I think) RDF data
<http://www.test.com/meta#0001> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2002/07/owl#Class>
<http://www.test.com/meta#0002> <http://www.test.com/meta#CONCEPT_hasType> "BEAR"^^<http://www.w3.org/2001/XMLSchema#string>
Each line has 3 items in it. I want to pull out the item before and after the URL. So that would result in:
0001, type, Class
0002, CONCEPT_hasType, (BEAR, string)
Is there a library out there (java or scala) that would do this split for me? Or do I just need to shove string.splits and assumptions in my code?

Most RDF libraries will have something to facilitate this. For example, if you parse your RDF data using Eclipse RDF4J's Rio parser, you will get back each line as a org.eclipse.rdf4j.model.Statement, with a subject, predicate and object value. The subject in both your lines will be an org.eclipse.rdf4j.model.IRI, which has a getLocalName() method you can use to get the part behind the last #. See the Javadocs for more details.
Assuming your data is in N-Triples syntax (which it seems to be given the example you showed us), here's a simple bit of code that does this and prints it out to STDOUT:
// parse the file into a Model object
InputStream in = new FileInputStream(new File("/path/to/rdf-data.nt"));
org.eclipse.rdf4j.model.Model model = Rio.parse(in, RDFFormat.NTRIPLES);
for (org.eclipse.rdf4j.model.Statement st: model) {
org.eclipse.rdf4j.model.Resource subject = st.getSubject();
if (subject instanceof org.eclipse.rdf4j.model.IRI) {
System.out.print(((IRI)subject).getLocalName());
}
else {
System.out.print(subject.stringValue());
}
// ... etc for predicate and object (the 2nd and 3rd elements in each RDF statement)
}
Update if you don't want to read data from a file but simply use a String, you could just use a java.io.StringReader instead of an InputStream:
StringReader r = new StringReader("<http://www.test.com/meta#0001> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2002/07/owl#Class> .");
org.eclipse.rdf4j.model.Model model = Rio.parse(r, RDFFormat.NTRIPLES);
Alternatively, if you don't want to parse the data at all and just want to do String processing, there is a org.eclipse.rdf4j.model,URIUtil class which you can just feed a string and it can give you back the index of the local name part:
String uri = "http://www.test.com/meta#0001";
String localpart = uri.substring(URIUtil.getLocalNameIndex(uri)); // will be "0001"
(disclosure: I am on the RDF4J development team)

Java, Turn Each Row of a CSV into a string

Hello I'm a newbie in Java using BlueJ. I have a csv file that contains a load of data in a table arrangement. I'm trying to find a way to take this information and find out how many comma separated values there are in the first row then regardless of rows put each comma separated value into an array.
Does anyone have any advice on how to do this?
Thanks in advance, Harry.

CSV parsing can be tricky because of the need to support quoted values. I suggest not writing your own CSV parser, but using an existing library such as http://opencsv.sourceforge.net/.

You can use the Scanner file, to read each line of the file, something similar to:
// create a File object by giving the filepath
File file = new File("C:\\data.csv");
try {
// Create a new scanner class that will read the file
Scanner scanner = new Scanner(file);
// while the file has lines to read
while (scanner.hasNextLine()) {
// read the line to a string
String line = scanner.nextLine();
// do what you need with that line
}
// catch the exception if no file can be found
} catch (FileNotFoundException e) {
e.printStackTrace();
}

Here is another library to handle csv file. javacsv
Code example is here