looking for a smart and fast searching algorithm - java

lets say i have 2 arrays of the objects which are mapped to each other in the following schemna:
array1 :
String [] prog_types1 = {"Program1","Program2","Program3","Program4"};
and array2 :
String [] prog_types2 ={"SubProgram1","SubProgram2","SubProgram3","SubProgram4",
"SubProgram5","SubProgram6","SubProgram7","SubProgram8","SubProgram9","SubProgram10"};
as it understood from its names, prog_types2 is an extension for prog_types1, but has some repeated values, so the full mapping between these programs would looks liek this:
prog_types1 prog_types2
ProgramType1 SubProgramType1
ProgramType1 SubProgramType2
ProgramType1 SubProgramType7
ProgramType1 SubProgramType9
ProgramType2 SubProgramType12
ProgramType2 SubProgramType7
ProgramType2 SubProgramType9
ProgramType3 SubProgramType1
ProgramType3 SubProgramType2
ProgramType3 SubProgramType21
ProgramType3 SubProgramType27
ProgramType3 SubProgramType7
ProgramType5 SubProgramType12
ProgramType5 SubProgramType9
my question is : what is the best way to map these arrays to each other, from the perspective of faster processing and reuse?
I have implemented it as :
-- set of classes (class prog1 and prog2 and after put it into vector)...
-- hashtable with hashset
-- possible one more array
the way i am looking for should not consist of creating the same prog2 objects again for prog1 object, as it would be in all of the ways described earlier, but map it by the index position for example or in any other way.
just lookin for a nice algorythmical way to resolve it...
thanks in advance
p.s. it should be used within 1 package only between couple of classes and the main use of it would be a population of the prog2 types values based on the prog1 type value
p.s.2 java7

Using MultiMap from Guava Libraries, you could say:
Multimap<String, String> mmap = ArrayListMultimap.create();
mmap.put("Program1", "SubProgramType1");
mmap.put("Program1", "SubProgramType2");
// etc.
mmap.get("Program1")
would look like:
[SubProgramType1, SubProgramType2, SubProgramType7, SubProgramType9]
BTW, Hashtable is not used now for hashed collections, has been superceded by HashMap :)

IMO the best way would be a:
Map<String, List<String>> programs = new HashMap<String, List<String>>();
with the strings in the first list as keys and the corresponding subprograms composing the value list. Now the mapping is obvious:
ProgramType1 -> [SubProgramType1, SubProgramType2, SubProgramType7, SubProgramType9]
ProgramType2 -> [SubProgramType12, SubProgramType7, SubProgramType9]
ProgramType3 -> [SubProgramType1, SubProgramType2, SubProgramType21, SubProgramType27, SubProgramType7]
ProgramType5 -> [SubProgramType12, SubProgramType9]

Guava ListMultimap, that gives List<E>, not Collection<E> - little more pleasant.
private ListMultimap<String,Something> stuff = ArrayListMultimap.create();
// ...
public void add(String key, Something item) {
stuff.put(key, item);
}
public List<Something> get(String key) {
// might as well use the Lists convenience API while we're at it.
return Lists.newArrayList(stuff.get(key));
}
http://www.coffee-bytes.com/2011/12/22/guava-multimaps

btw, since i need :
-- separately use Program1 values
-- separately use SubProgram1 values
-- populate SubProgram1 values based on Program1 value
the easiest solution here would be to declare a double dimensional array with all the dublicates (as it dysplayed in full map schema) and for 1) and 2) populate data from it using non repeating algorythm and 3) loop cycle from 2nd dimension
so no reason to declare 3 objects, huge memory save and nice approach.
i am giving myself a star for it:)

Related

Javascript Objects but for Java

How do I write this in Java?
//js
const hello = {
foo: "bar",
test: "world",
name: "david"
}
I want have a very long object, then refer it back like hello[test] or hello[foo]
I've heard of hashmaps, but you can only create an empty one and then add elements into it.
I've got a really long list like that in js. How can I copy those into Java? Doing .put() one by one would take forever, and I don't think that's efficient.
And even if someone wrote a script to turn uwu: "owo" into hello.put("uwu", "owo");, it'd be ugly in the code with a big block of hello.put()s.
I also don't want to create a new file for that (it only has around 34 lines) and want to keep it in the code. Also, because I have three more like these with 20-40 keys and values in each of them, I don't want to create three extra files with just 30 lines in them. I also don't want to go into complexity of reading them.
Oh and also, I won't be changing the hashmap btw, just reading data like a constant.
In summary, can I do something like this in Java for long lists without doing .put()?
public HashMap<String, String> hello = new HashMap<String, String>(
"foo": "bar",
"test": "world",
"name": "david",
"uwu": "owo"
);
And refer to them like hello["name"]? I also don't want this thing.
public HashMap<String, String> hello = new HashMap<String, String>();
hello.put("foo", "bar");
hello.put("test", "world");
hello.put("name", "david");
hello.put("uwu", "owo");
//for 25 more lines
public HashMap<String, String> hello2 = new HashMap<String, String>();
hello2.put("stuff", "thing");
//... for around 20 more lines
//repeat for 3 more hashmaps
In modern Java (14 and later) you can use a record:
record Hello(String foo, String test, String world) { }
and create an instance like this:
final Hello hello = new Hello("bar", "world", "david");
You access the values like:
System.out.print(hello.foo());
Using a record has the advantage that your data is statically typed -- you can't mistype a key, or forget to remove usages of a key you've removed from the record.
IN Java 14 and beyond, I would recommand using a record, as explained in the other answer.
It's the safest and also probably the most efficient way.
For Java 9 to 14, you may use Map.of("hello", "world", "foo", "bar");.
But you may not be able to go beyond a certain number of key/value pairs.
For java 8 and below, or if you exceed the number of arguments allowed with Map.of, you don't have other choice than create an empty map and put key/value pairs one by one.
Note however that, performances aren't necessarily going to be worse.
You can of course reimplement your own version of Map.of with variable number of arguments.
Since you need something constant like, you can save those values in files and read from those files. For example save data in file in json format:
{
"foo": "bar",
"test": "world",
"name": "david"
}
Then parse this file to a Map.
public class Main {
public static void main(String[] args) throws IOException {
ObjectMapper mapper = new ObjectMapper();
Map<String, Object> map = mapper.readValue(ClassLoader.getSystemResourceAsStream("constants.json"), Map.class);
map.forEach((k, v) -> System.out.println(k + " -> " + v));
}
}
This example uses reading file as project resource and uses ObjectMapper to parse json to Map, but you can use any other tool for the same effect. If the data format is simple enough(string key to string value, no nested arrays, objects and such) you can save it in even simpler format and do the read, parse, add to map manually.

Dynamically nesting group by inside Collector

I have this piece of code which is working fine:
Collector nestedCollector = Collectors.groupingBy((Map map) -> map.get("PROCESS_NAME"),
Collectors.groupingBy((Map map) -> map.get("PROCESS_INSTANCE")));
However, I can't use fixed map keys in my code (PROCESS_NAME, PROCESS_INSTANCE).
What I need is to pass the list of keys via parameter and generate a nested collector, something like this:
List<String> keys = new ArrayList();
keys.add("PROCESS_NAME");
keys.add("PROCESS_INSTANCE");
keys.add("...some other key...");
Collector nestedCollector= Collectors.groupingBy(...nested groupingBy based on keys...)
I'm struggling with the syntax.
Can anybody help? Thank you.

Converting Linq queries to Java 8

Im traslating a old enterprise App who uses C# with Linq queries to Java 8. I have some of those queries who I'm not able to reproduce using Lambdas as I dont know how C# works with those.
For example, in this Linq:
from register in registers
group register by register.muleID into groups
select new Petition
{
Data = new PetitionData
{
UUID = groups.Key
},
Registers = groups.ToList<AuditRegister>()
}).ToList<Petition>()
I undestand this as a GroupingBy on Java 8 Lambda, but what's the "select new PetitionData" inside of the query? I don't know how to code it in Java.
I have this at this moment:
Map<String, List<AuditRegister>> groupByMuleId =
registers.stream().collect(Collectors.groupingBy(AuditRegister::getMuleID));
Thank you and regards!
The select LINQ operation is similar to the map method of Stream in Java. They both transform each element of the sequence into something else.
collect(Collectors.groupingBy(AuditRegister::getMuleID)) returns a Map<String, List<AuditRegister>> as you know. But the groups variable in the C# version is an IEnumerable<IGrouping<string, AuditRegister>>. They are quite different data structures.
What you need is the entrySet method of Map. It turns the map into a Set<Map.Entry<String, List<AuditRegister>>>. Now, this data structure is more similar to IEnumerable<IGrouping<string, AuditRegister>>. This means that you can create a stream from the return value of entry, call map, and transform each element into a Petition.
groups.Key is simply x.getKey(), groups.ToList() is simply x.getValue(). It should be easy.
I suggest you to create a separate method to pass into the map method:
// you can probably came up with a more meaningful name
public static Petition mapEntryToPetition(Map.Entry<String, List<AuditRegister>> entry) {
Petition petition = new Petition();
PetitionData data = new PetitionData();
data.setUUID(entry.getKey());
petition.setData(data);
petition.setRegisters(entry.getValue());
return petition;
}

Create a stream of the values in maps that are values in another map in Java

Sorry about the title of the question; it was kind of hard for me to make sense of it. If you guys have a better title, let me know and I can change it.
I have two types of objects, Bookmark and Revision. I have one large Map, like so:
Map<Long, Bookmark> mapOfBookmarks;
it contains key: value pairs like so:
1L: Bookmark1,
2L: Bookmark2,
...
Each Bookmark has a 'getRevisions()' method that returns a Map
public Map<Long, Revision> getRevisions();
I want to create a Stream that contains all revisions that exist under mapOfBookmarks. Essentially I want to do this:
List<Revision> revisions = new ArrayList<>();
for (Bookmark bookmark : mapOfBookmarks.values()) { // loop through each bookmark in the map of bookmarks ( Map<Long, Bookmark> )
for (Revision revision : bookmark.getRevisions().values()) { // loop through each revision in the map of revisions ( Map<Long, Revision> )
revisions.add(revision); // add each revision of each map to the revisions list
}
}
return revisions.stream(); // return a stream of revisions
However, I'd like to do it using the functionality of Stream, so more like:
return mapOfBookmarks.values().stream().everythingElseThatIsNeeded();
Which would essentially be like saying:
return Stream.of(revision1, revision2, revision3, revision4, ...);
How would I write that out? Something to note is that the dataset that it is looping through can be huge, making the list method a poor approach.
I'm using Windows 7 and Java 8
A flatmap is what you looking for. When you have streams contained within a stream that you wish to flatten, then flatmap is the answer,
List<Revision> all =
mapOfBookmarks.values().stream()
.flatMap(c -> c.getRevisions().values().stream())
.collect(Collectors.toList());
You are looking for the flatMap(mapper) operation:
Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.
In this case, we're making a Stream<Bookmark> by calling stream(), flat mapping it to the revisions of each bookmark and, finally, collecting that into a list with toList().
List<Revision> revisions =
mapOfBookmarks.values()
.stream()
.flatMap(bookmark -> boormark.getRevisions().values().stream())
.collect(Collectors.toList());
Note that your current code could also be improved by calling addAll instead of looping over each revisions:
for (Bookmark bookmark : mapOfBookmarks.values()) { // loop through each bookmark in the map of bookmarks ( Map<Long, Bookmark> )
revisions.addAll(bookmark.getRevisions().values());
}

How to maintain JSON's order in Groovy's JsonSlurper?

I am reading a simple JSON....
{"A":0,"B":0,"C":2,"D":0,"F":5}
into a map using JsonSlurper in Groovy...
Map gradeDistributon = jsonSlurper.parseText(jsonString)
But when iterating over this map with a closure..
gradeDistributon.each{ entry ->
println "From map got key ${entry.key}"
I am seeing the keys are not in the order they were in the original JSON, for example 'C' comes first. I think this is because Map does not maintain insertion order in Java. Is there a way I can keep the order of the original JSON?
If it means reading the JSON in a different way (instead of into a Map with JsonSlurper) then I am fine with that if you can show me how.
You can set JVM system property jdk.map.althashing.threshold to make JsonSlurper to use a LinkedHashMap instead of TreeMap as the internal Map implementation, e.g. -Djdk.map.althashing.threshold=512.
The reason is in source code of groovy.json.internal.LazyMap used by JsonSlurper.
private static final String JDK_MAP_ALTHASHING_SYSPROP = System.getProperty("jdk.map.althashing.threshold");
private void buildIfNeeded() {
if (map == null) {
/** added to avoid hash collision attack. */
if (Sys.is1_7OrLater() && JDK_MAP_ALTHASHING_SYSPROP != null) {
map = new LinkedHashMap<String, Object>(size, 0.01f);
} else {
map = new TreeMap<String, Object>();
}
}
}
Please note this solution should be used as a hack as it depends on Groovy's internal implementation details. So this behavior may change in future version of Groovy.
See my blog post for details.
So it was just a matter of sorting the keys after JsonSlurper built the Map, for that I just read into a TreeMap which sorts the keys by default..
TreeMap gradeDistributon = jsonSlurper.parseText(jsonString)
I can't reproduce your behaviour with groovy 2.4.5 but you can try using LinkedHashMap (allow to iterate over map keys maintaining the order in which the entries were inserted):
import groovy.json.*
def jsonText = '''
{"A":0,"B":0,"C":2,"D":0,"F":5,"G":7,"H":9}
'''
LinkedHashMap json = new JsonSlurper().parseText(jsonText)
json.each{ entry ->
println "${entry.key}"
}
NOTE: as stated by #XenoN the JsonSlurper() sort the json keys during the parsing process so independently of the input order (ie. {"H":0,"B":0,"A":2,"D":0,"G":5,"F":7,"C":9}) the output of JsonSlurper will be always: {"A":2,"B":0,"C":9,"D":0,"F":7,"G":5,"H":0}.
Using the LinkedHashMap instead of a HashMap we preserve the order given by JsonSlurper.
I run the same code on Groovy 2.4.x and on 3.0.x.
On 2.4 the order is preserved,but on 3.0 is sorted asc by default.
use the JsonSluperClassic().parse() instead it will preserve the order

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