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Specific field of a document in MongoDb to Java Object

I want to select a specific field of a document in Mongodb collection and convert it to java object.
My document is like this:
{
"Name":"Ben",
"template":"A fingerprint template which I extracted"
}
So I wish select this "template field"
My sample code is below:
List<Document> documents = (List<Document>) collection.find().into(new ArrayList<Document>());
for (Document document : documents) {
Document doc = documents.get(document);
FingerprintTemplate template = (FingerprintTemplate) doc.get("template");
And the error is:
java: incompatible types: org.bson.Document cannot be converted to int
Any idea or any suggestions please!!

You can query if the field exists:
collection.find("template": { $exists: true, $ne: null } });
For the mapping part, you can have a look at objectMapper to map directly to a java object: https://www.baeldung.com/jackson-object-mapper-tutorial
Your error is still that you have a Document and Java is expecting an int.

Try this :
BasicDBObject query = new BasicDBObject();
BasicDBObject whereQuery = new BasicDBObject();
basicDBObject.put("template",1);
DBCursor cursor = collection.find(query,whereQuery);
while(cursor.hasNext()) {
System.out.println(cursor.next());
}
This would print the documents that have template but to convert to Object you would need some JSON Object mapper maybe jackson

Implementing Mongodb query using $elemMatch in Java

I am using Mongo java driver to retrieve data from mongo collections. I have the following query which I am trying to implement in java:
Json is:
{
"_id" : ObjectId("56cd284767c74d3d4dd3ec80"),
"comments" : "Hello",
"statusLog" : [
{
"status" : "Submitted",
"startDate" : ISODate("2015-01-14T05:00:00.000Z"),
"endDate" : ISODate("2016-02-29T21:24:24.740Z")
},
{
"status" : "Active",
"startDate" : ISODate("2016-02-29T21:24:24.740Z")
}
],
"createdDate" : ISODate("2015-01-14T05:00:00.000Z")
}
Mongo Query:
db.CollectionName.find({},{_id: 0, createdDate:1, "statusLog": {$elemMatch: {"status":"Submitted"}}});
Here is the query in java that I have written (mongo java driver 3.4.2):
BasicDBObject query = new BasicDBObject(new BasicDBObject("statusLog",
new BasicDBObject("$elemMatch", new BasicDBObject("status", "Submitted"))));
Running the java code returns all status logs and not the one that I am looking for.
Any help would be highly appreciated.

elemMatch should be in find method not in projection bson. For example:
collection.find(elemMatch("statusLog", eq("status", "Submitted"))).projection(projection);

You should use newer Document/MongoCollection api and use helper methods to prepare projection.
import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Projections.*;
MongoClient mongoClient = new MongoClient();
MongoDatabase database = mongoClient.getDatabase("db"); // Get DB
MongoCollection<Document> collection = database.getCollection("collection"); // Get Collection
Bson projection = Projections.fields( excludeId(), include("createdDate"), elemMatch("statusLog", eq("status", "Submitted"))); // Add Projections
FindIterable<Document> iterable = collection.find().projection(projection);
Using old BasicDBObject/DBCollection api
MongoClient mongoClient = new MongoClient();
DB database = mongoClient.getDB("db");
DBCollection collection = database.getCollection("collection");
BasicDBObject projection = new BasicDBObject(new BasicDBObject("statusLog",new BasicDBObject("$elemMatch", new BasicDBObject("status", "Submitted"))));
collection.find(new BasicDBObject(), projection);

To find an item in a list of document with multiple objects, using multiple filtrer criteria:
"array_of_data": {$elemMatch: {"AAA": "xxxx", "BBB": "xxxxx"}}

How to return documents of a collection without objectId in MongoDB?

DB db = mongo.getDB("sample");
DBCollection table = db.getCollection("samplecollection");
DBCursor cursor2 = table.find();
gives the result as:
{ "_id" : { "$oid" : "58bfbcff1d30d8a8c1194328"} , "ID" : 1 , "NAME" : "dgsdg"}
How to get the document without objectid?

You can use projection for that.
Try this:
DBCursor cursor2 = table.find().projection(excludeId())
Read this MongoDB documentation to know more.

I have done this and worked:
MongoDatabase database = mongo.getDatabase("db");
MongoCollection<org.bson.Document> contCol = database.getCollection("test");
FindIterable<org.bson.Document> it = contCol.find().projection(excludeId());
ArrayList<Document> docs = new ArrayList();
for (org.bson.Document its : it) {
System.out.println(its);
}

First you will be import this, import com.mongodb.client.model.Projections;
Then again, u try this one also..
MongoClient client = new MongoClient("hostName");
MongoDatabase db = client.getDatabase("dbName");
MongoCollection<Document> collection = db.getCollection("collectionName");
collection.withWriteConcern(new WriteConcern(1));
/* whatever field you dosn`t need.. you will run this following code,
FindIterable<Document> findDoc = collection.find().projection(new Document().append("_id",0)); //if you need mean, you put _id value is 1.
*/
FindIterable<Document> findDoc = collection.find().projection(Projections.excludeId()) ;
for (Document doc : findDoc) {
System.out.println(doc);
}

You can use the aggregation framework to achieve what you want:
db.getCollection('samplecollection').aggregate([{$project: { field1:1 ,field2: 1.....,'_id': 0} }])
https://docs.mongodb.com/manual/reference/operator/aggregation/project/

Updating value of a subfield in MongoDB with Java driver?

I've quite new to MongoDB and it's Java driver.
I need to update the value of a subfield, but I can't find any examples online.
The document:
{
"_id" : ObjectId("45678942342"),
"user" : "me",
"aStruct" : {
"subfield_1" : true,
"subfield_2" : true
}
}
How do I update the value of subfield subfield_1 to false, for every document that has user = me ?
Thank you.

You can do it as follows :
db.collection.update({user : "me"},{$set:{"aStruct.subfield_1" : false}}, false, true)
In Java you can do it as follows :
DBCollection coll = // Define your collection here
DBObject query = new BasicDBObject();
query.put("user", "me");
DBObject updateObj = new BasicDBObject();
updateObj.put("aStruct.subfield_1", false);
coll.updateMulti(query, new BasicDBObject("$set", updateObj));
For more information read the following document.
Update document in MongoDB

How to update value of specific embedded document, inside an array, of a specific document in MongoDB?

I have the following structure in my document:
{
_id : ObjectId("43jh4j343j4j"),
array : [
{
_arrayId : ObjectId("dsd87dsa9d87s9d7"),
someField : "something",
someField2 : "something2"
},
{
_arrayId : ObjectId("sds9a0d9da0d9sa0"),
someField : "somethingElse",
someField2 : "somethingElse2"
}
]
}
I want to update someField and someField2 but only for one of the items in the array, the one that matches _arrayId (e.g. _arrayId : ObjectId("dsd87dsa9d87s9d7"); and only for this document (e.g. _id : ObjectId("43jh4j343j4j") ) and no other.
The arrayIds are not unique to the document that's why I need it to be for a specific document. I could use the $ positional operator if I wanted to update that value within the array for every document it exists in, but that's not what I want.
I am trying to accomplish this in java but a command line solution would work as well.

Here is RameshVel's solution translated to java:
DB db = conn.getDB( "yourDB" );
DBCollection coll = db.getCollection( "yourCollection" );
ObjectId _id = new ObjectId("4e71b07ff391f2b283be2f95");
ObjectId arrayId = new ObjectId("4e639a918dca838d4575979c");
BasicDBObject query = new BasicDBObject();
query.put("_id", _id);
query.put("array._arrayId", arrayId);
BasicDBObject data = new BasicDBObject();
data.put("array.$.someField", "updated");
BasicDBObject command = new BasicDBObject();
command.put("$set", data);
coll.update(query, command);

You could still use $ positional operator to accomplish this. But you need to specify the objectid of the parent doc along with the _arrayid filter. The below command line query works fine
db.so.update({_id:ObjectId("4e719eb07f1d878c5cf7333c"),
"array._arrayId":ObjectId("dsd87dsa9d87s9d7")},
{$set:{"array.$.someField":"updated"}})

...and this is how to do it with mongo-driver version >= 3.1 (mine is 3.2.2):
final MongoClient mongoClient = new MongoClient(new MongoClientURI(mongoURIString));
final MongoDatabase blogDatabase = mongoClient.getDatabase("yourDB");
MongoCollection<Document> postsCollection = blogDatabase.getCollection("yourCollection");
ObjectId _id = new ObjectId("4e71b07ff391f2b283be2f95");
ObjectId arrayId = new ObjectId("4e639a918dca838d4575979c");
Bson filter = Filters.and(Filters.eq( "_id", id ), Filters.eq("array._arrayId", arrayId));
Bson setUpdate = Updates.set("array.$.someField", "updated");
postsCollection.updateOne(postFilter, setUpdate);

Seeing as none of the answers actually explain how to do this a) in Java and b) for multiple fields in a nested array item, here is the solution for mongo-java-driver 3.12.3.
import com.mongodb.client.MongoCollection;
import com.mongodb.client.model.Filters;
import com.mongodb.client.model.Updates;
import org.bson.Document;
import org.bson.types.ObjectId;
MongoClient mongoClient = MongoClients.create(...);
MongoDatabase db = mongoClient.getDatabase("testDb");
MongoCollection<Document> collection = db.getCollection("testCollection");
collection.updateOne(
Filters.and(
Filters.eq("_id", new ObjectId("43jh4j343j4j")),
Filters.eq("array._arrayId", new ObjectId("dsd87dsa9d87s9d7"))
),
Updates.combine(
Updates.set("array.$.someField", "new value 1"),
Updates.set("array.$.someField2", "new value 2")
)
);
This thread has helped me towards the right solution, but I had to do more research for the full solution, so hoping that someone else will benefit from my answer too.