Java mongodb 3.0 driver query without filter

How do I query using the Java mongodb 3.0 driver?

I tried to query unique category records from the location collection in mongodb In the Mongo shell, this is simple: dB locations. distinct(“categories”);

In Java, it is different

MongoClient client = new MongoClient();
MongoDatabase db = client.getDatabase("yelp");

//this will not compile,although examples from before 3.0 do it this way
MongoCursor<Document> c = 
    db.getCollection("locations").distinct("categories").iterator();@H_502_16@

Solution

To avoid casting with distinct, the mongocollection API allows you to provide different expected value types for fields Therefore, if you know that they are all strings, you can write:

MongoCursor<String> c = 
   db.getCollection("locations").distinct("categories",String.class).iterator();@H_502_16@ 
 
或所有数字：
 
 
 
 
MongoCursor<Number> c = 
   db.getCollection("locations").distinct("categories",Number.class).iterator();@H_502_16@ 
 
你仍然可以这样做：
 
 
 
 
MongoCursor<Object> c = 
   db.getCollection("locations").distinct("categories",Object.class).iterator();@H_502_16@ 
 
如果你不能保证你正在查询的字段值的类型.