Choosing the data structure for orgaininsng data with list of values in java

Question

I have a map as shown below in which there is a key and values is of type List:

Map newdatamap = new HashMap<&g         


        

Answer 1

I would do this:

Map<Integer, List<String>> dataMap = new HashMap<>();
dataMap.put("B1".hashCode()+"RtyName".hashCode(), Arrays.asList("weepn", "weepfnb", "eedgeft", "xbteehy"));
dataMap.put("B1".hashCode()+"rtyRate".hashCode(), Arrays.asList("deed", "ww", "terrty", "hteetyure"));
dataMap.put("B2".hashCode()+"RtyName".hashCode(), Arrays.asList("SSSweepn", "wpefSSSnb", "GGeGdgeft", "xbteYYYYhy"));
dataMap.put("B2".hashCode()+"rtyRate".hashCode(), Arrays.asList("WWded", "wTeTYw", "YYYYtrerty", "IIIehttyure"));

Which would represent:

B1, RtyName  ----> "weepn", "weepfnb", "eedgeft", "xbteehy"
B1, rtyRate ----->"deed", "ww", "terrty", "hteetyure"

B2, RtyName  ----> "SSSweepn", "wpefSSSnb", "GGeGdgeft", "xbteYYYYhy"
B2, rtyRate ----->"WWded", "wTeTYw", "YYYYtrerty", "IIIehttyure"

Note that hashCode is just a convient function from the String class that meets my needs. You could roll your own that returns a String key (or really anything else) if you preferred.

Actually since your original method didn't require an order independent function, you could really even concatenate the String keys to use as a new key:

dataMap.put("B1"+"RtyName", Arrays.asList(/*your list here*/));

This is a little less convenient (and not as "good" programmatically) than the first method, but still much better than nesting Map classes. (And makes keys much easier to recognize when outputted than hashCode.)

Two-way Mapping

Values as Keys

If you want each List value to map to keys as well as the other way around, you need a second Map:

Map<List<String>, String> valueMap = new HashMap<>(); //New map for value->key 
for(String key: dataMap.keySet()) //Get all keys
    valueMap.put(dataMap.get(key), key); //Create mapping value->key

Each Item in Value as a Key

If you want each String item in the values list to map to keys as well as the other way around, you need a second Map:

Map<String, String> itemMap = new HashMap<>(); //New map for item->key mapping
    for(String key: dataMap.keySet()) //Get all keys and iterate through
        for(String item: dataMap.get(key)) //For each item in your value list
            itemMap.put(item, key); //Create new mapping item->key

Answer 2

As you asked for it:

You simply define a class, ABC in this case as you've suggested that name, which holds two lists, the RtyName and the rtyRate list:

public class ABC {
    private List<String> RtyName;
    private List<String> rtyRate;

    public ABC(List<String> RtyName, List<String> rtyRate) {
        setRtyNames(RtyName);
        setRtyRates(rtyRate);
    }

    public void setRtyNames(List<String> RtyName) {
        this.RtyName = RtyName;
    }

    public List<String> getRtyNames() {
        return this.RtyName;
    }

    public void setRtyRates(List<String> rtyRate) {
        this.rtyRate = rtyRate;
    }

    public List<String> getRtyRates() {
        return this.rtyRate;
    }
}

Once this class is ready, you can change your map definition to:

Map<String, ABC> newdatamap = new HashMap<>();

And assign new values to it similar as you already did before - instead of nesting those lists in a further map and putting this map into the outer map though, you create a new instance of ABC, provide the two lists as input parameter and put the resulting ABC object into the map (formally the outer map):

List<String> RtyName = Arrays.asList("wpn", "wpfnb", "dgeft", "xbthy");
List<String> rtyRate = Arrays.asList("dd", "ww", "trrty", "httyure");
newdatamap.put("B1", new ABC(RtyName, rtyRate));

You could also assign those lists directly as input-parameters:

newdatamap.put("B2", new ABC(Arrays.asList("SSSweepn", "wpefSSSnb", "GGeGdgeft", "xbteYYYYhy"), 
                             Arrays.asList("WWded", "wTeTYw", "YYYYtrerty", "IIIehttyure"));

An entry is now simply retrieved on calling get(String) as you already used to do:

ABC data = newdatamap.get("B1);

You can also retrieve one of the lists directly like:

List<String> RtyNames = newdatamap.get("B1").getRtyNames();

In my opinion this is way easier to manage and to read at the same time. On top of that, you gain also runtime-saftiness as you do not run into eventual typing errors for RtyName or rtyRate.

Update for redesign

If RtyName entries and rtyRate entries always go hand in hand, f.e. wpn is the name and dd is the assigned rate, grouping those fields together would make sense. Therefore you could refactor your code to furthermore reduce one list and introduce a new class:

public class RtyEntry {
    private String name;
    private String rate;

    public RtyEntry(String name, String rate) {
        setName(name);
        setRate(rate);
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }

    public void setRate(String rate) {
        this.rate = rate;
    }

    public String getRate() {
        return this.rate;
    }
}

If you now change ABC to only contain one list of RtyEntry objects like this:

public class ABC {
    private List<RtyEntry> rtyEntries;

    public ABC(List<RtyEntry> rtyEntries) {
        this.rtyEntries = rtyEntries;
    }

    public ABC() {
        this.rtyEntries = new ArrayList<>();
    }

    public void setRtyEntries(List<RtyEntry> rtyEntries) {
        this.rtyEntries = rtyEntries;
    }

    public List<RtyEntry> getRtyEntries() {
        return this.rtyEntries;
    }

    // convenience methods

    public void addRtyEntry(RtyEntry entry) {
        this.rtyEntries.add(entry);
    }
}

The creation would now change to:

RtyEntry entry1 = new RtyEntry("wpn", "dd");
List<RtyEntry> entries = Arrays.asList(entry1, ...);
newdatamap.put("B1", new ABC(entries));

You can then also assign new entries like that:

newdatamap.get("B1").addRtyEntry(new RtyEntry("wpfnb", "ww"));

Retrieving entries also changed to the refactoring:

ABC data = newdatamap.get("B1");
List<RtyEntry> entries = data.getRtyEntries();
for (RtyEntry entry : entries) {
    System.out.println(entry.getName() + " has a rate of: " + entry.getRate());
}

Of course you can also retrieve the list directly via:

List<RtyEntry> entries = newdatamap.get("B1").getRtyEntries();

as before.