We Keep Coding

Does Java API of Rocks DB support prefix scan?

I have huge data set(key-value) in Rocks DB and I have to search for key based on prefix of key in hand. I do not want to scan whole data set to filter out key based on key-prefix. is there any way to do that?

You can use something like this.
Using RocksIterator there is a api exposed where you can seek to the key substring and if your key starts with the prefix then consider that key.
Please find the sample code.
List<String> result = new ArrayList<String>();
RocksIterator iterator = db.newIterator();
for (iterator.seek(prefix.getBytes()); iterator.isValid(); iterator
.next()) {
String key = new String(iterator.key());
if (!key.startsWith(prefix))
break;
result.add(String.format("%s", new String(iterator.key())));
}
Hope it will help you.

The answer of #Pramatha V works pretty well, although I made some improvements to the code. I am not deserializing the iterator key in every iteration. I am using the Bytes.increment() from the Kafka common utils (you can extract this class and use it in your code directly). This function increments the underlying byte array by adding 1. With this approach, I can find the next bigger key than my prefix key. I am using the BYTES_LEXICO_COMPARATOR (also from the same class) to make the comparison, but you are free to implement and use your comparator. Moreover, the function returns a map of byte arrays, which you can deserialize later in your code.
public Map<byte[], byte[]> prefixScan(final byte[] prefix) {
final Map<byte[], byte[]> result = new HashMap<>();
RocksIterator iterator = db.newIterator();
byte[] rawLastKey = increment(prefix);
for (iterator.seek(prefix); iterator.isValid(); iterator.next()) {
if (Bytes.BYTES_LEXICO_COMPARATOR.compare(iterator.key(), rawLastKey) > 0
|| Bytes.BYTES_LEXICO_COMPARATOR.compare(iterator.key(), rawLastKey) == 0) {
break;
}
result.put(iterator.key(), iterator.value());
}
iterator.close();
return result;
}

Seek is working very slow. 5.35 Seconds on SSD disk , 1 billion records.
The size of the Keys are fixed 16 bytes. Searched for 8 bytes.
2 Long bytes [xx,xx]
Searched for 1 Long as 8 bytes.
Use ColumnFamily for mapping keys.

Access to the key-value pair of a Map with one element in Java

A method of mine returns a Map<A,B>. In some clearly identified cases, the map only contains one key-value pair, effectively only being a wrapper for the two objects.
Is there an efficient / elegant / clear way to access both the key and the value? It seems overkill to iterate over the one-element entry set. I'm looking for somehing that would lower the brain power required for people who will maintain this, along the lines of:
(...)
// Only one result.
else {
A leKey = map.getKey(whicheverYouWantThereIsOnlyOne); // Is there something like this?
B leValue = map.get(leKey); // This actually exists. Any Daft Punk reference was non-intentional.
}
Edit: I ended up going with #akoskm solution's below. In the end, the only satisfying way of doing this without iteration was with a TreeMap, and the overhead made that unreasonable.
It turns out there is not always a silver bullet, especially as this would be a very small rabbit to kill with it.

If you need both key/value then try something like this:
Entry<Long, AccessPermission> onlyEntry = map.entrySet().iterator().next();
onlyEntry.getKey();
onlyEntry.getValue();

You can use TreeMap or ConcurrentSkipListMap.
TreeMap<String, String> myMap = new TreeMap<String, String>();
String firstKey = myMap.firstEntry().getKey();
String firstValue = myMap.firstEntry().getValue();
Another way to use this:
String firstKey = myMap.firstKey();
String firstValue = myMap.get(myMap.firstKey());
This can work as an alternate solution.

There is a method called keySet() to get set of keys. read this thread.
else {
A leKey=map.keySet().iterator().next();
B leValue; = map.get(leKey); // This actually exists. Any Daft Punk reference was non-intentional.
}

Using for-each loop and var :
for(var entry : map.entrySet()){
A key = entry.getKey();
B value = entry.getValue();
}

How to split next entryset iterator?

Hello I'm trying to split next iterator entryset from hashmap but I can't get it to work.
I have an hashmap in which I put two things, first one is sender, second one is channel:
channelList = HashMap()
channelList.put(playername, channelname) #have on mind that those can be changed, depending on what user types in
I have this iterator:
it = channelList.entrySet().iterator()
next = it.next()
But when I print next out it has "=" between arguments from hashmap. For example, if playername is PLAYER and channel name is balkan I get as result: PLAYER=balkan. Question is, how do I get ONLY PLAYERNAME on every next. I tried splitting it like this, but it's not working:
next = it.next()
realnext = next.split("=")
realrealnext = realnext.split("=")[0]
Have on mind that I check for every next using this while loop:
while it.hasNext():
Thanks in advance, Amar!
P.S. I'm jython/python programmer.

The problem is you're casting java.util.Map.Entry to a String. Try this instead
#!/usr/bin/jython
import java.util.HashMap
channelList = java.util.HashMap()
channelList.put("Hello", "World")
it = channelList.entrySet().iterator()
while (it.hasNext()):
e = it.next()
print("key = " + e.getKey())
print("value = " + e.getValue())
Which on my system runs as follows -
$ ./test.py
key = Hello
value = World
$

You shouldn't name reference to Map ....List, it is confusing. You should name it channelMap.
Next, your Maps should use generic types to set up elements they are using, like for example
Map<String, Channel> channelMap = new HashMap<>();
This way you would be able to safely use
Iterator<Entry<String, Channel>> it = channelMap.entrySet().iterator();
and have access to it.next().getKey() (notice that order of elements in HashMap is based on hashCode if its Key so don't be surprised with order like Player2, Player1, Player 3).
Anyway if you just want to iterate over all keys then maybe
for (String key: channelMap.keySet()){
System.out.println(key);
}
would be better solution.

I have a query in mongodb and the reference key's are in hashmap list , I need to process a simple query using java in mongodb

mongodb query is db.test.find({"col1":{"$ne":""}}).count(), I have tried many sources to find the solution, the "col1" must be populated from list array, please help me
I have pasted a part of my code
`
List<String> likey = new ArrayList<String>();
for (DBObject o : out.results())
{
likey.add(o.get("_id").toString());
}
Iterator<String>itkey = likey.iterator();
DBCursor cursor ;
//cursor = table.find();
HashMap<String, String> hashmap = new HashMap<String, String>();
while (itkey.hasNext())
{
System.out.println((String)itkey.next());
String keys = itkey.next().toString();
//System.out.println("keys --> "+keys);
String nullvalue = "";
Boolean listone = table.distinct(keys).contains(nullvalue);
hashmap.put(keys, listone.toString());
//System.out.println("distinct --> "+keys+" "+listone);
//System.out.println("proper str --- >"+ '"'+keys+'"');
}
Iterator<String> keyIterator = hashmap.keySet().iterator();
Iterator<String> valueIterator = hashmap.values().iterator();
while (keyIterator.hasNext()) {
//System.out.println("key: " + keyIterator.next());
while (valueIterator.hasNext()) {
//System.out.println("value: " + valueIterator.next());
//System.out.println("Key: " + keyIterator.next() +""+"value: "+valueIterator.next());
String hashkey = valueIterator.next();
}
}
`

When you post code, it helps if you indent it, so it is more readable. As I mentioned to you on another forum, you need to go back and review the Java collection classes, since you have multiple usage errors in the above code.
Here are a few things you need to do to clean up your code:
1) You don't need to use the itkey iterator. Instead, use:
for (String key : likey)
and get rid of all the itkey.next calls. Your current code only processes every second element of the List. The other ones are printed out.
2) Your HashMap will map a key to a Boolean. Is that what you want? You said you want to count the number of non-zero values for the key. So, the line:
Boolean listone = table.distinct(keys).contains(nullvalue);
is almost certainly in error.
3) When you iterate over the HashMap, you don't need the valueIterator. Instead, get the key (either from the keyIterator, or a variable you define using the simpler iterator syntax above), then use the key to get the matching value using hashmap.get(key).
This will not make your code work, but it will clean it up somewhat - at the moment it is difficult to understand what you are intending it to do.

My arraylist is only outputting the last value

I created a HashMap to store a text file with the columns of information. I compared the key to a specific name and stored the values of the HashMap into an ArrayList. When I try to println my ArrayList, it only outputs the last value and leaves out all the other values that match that key.
This isn't my entire code just my two loops that read in the text file, stores into the HashMap and then into the ArrayList. I know it has something to do with my loops.
Did some editing and got it to output, but all my values are displayed multiple times.
My output looks like this.
North America:
[ Anguilla, Anguilla, Antigua and Barbuda, Antigua and Barbuda, Antigua and Barbuda, Aruba, Aruba, Aruba,
HashMap<String, String> both = new HashMap<String, String>();
ArrayList<String> sort = new ArrayList<String>();
//ArrayList<String> sort2 = new ArrayList<String>();
// We need a try catch block so we can handle any potential IO errors
try {
try {
inputStream = new BufferedReader(new FileReader(filePath));
String lineContent = null;
// Loop will iterate over each line within the file.
// It will stop when no new lines are found.
while ((lineContent = inputStream.readLine()) != null) {
String column[]= lineContent.split(",");
both.put(column[0], column[1]);
Set set = both.entrySet();
//Get an iterator
Iterator i = set.iterator();
// Display elements
while(i.hasNext()) {
Map.Entry me = (Map.Entry)i.next();
if(me.getKey().equals("North America"))
{
String value= (String) me.getValue();
sort.add(value);
}
}
}
System.out.println("North America:");
System.out.println(sort);
System.out.println("\n");
}

Map keys need to be unique. Your code is working according to spec.

if you need to have many values for a key, you may use
Map<key,List<T>>
here T is String (not only list you can use any collection)

Some things seems wrong with your code :
you are iterating on the Map EntrySet to get just one value (you could just use the following code :
if (both.containsKey("North America"))
sort.add(both.get("North America"));
it seems that you can have "North America" more than one time in your input file, but you are storing it in a Map, so each time you store a new value for "North America" in your Map, it will overwrite the current value
I don't know what the type of sort is, but what is printed by System.out.print(sort); is dependent of the toString() implementation of this type, and the fact that you use print() instead of println() may also create problems depending on how you run your program (some shells may not print the last value for instance).
If you want more help, you may want to provide us with the following things :
sample of the input file
declaration of sort
sample of output
what you want to obtain.