In swift I can declare a dictionary (key value pairs) like so
var toppings = [
"Onions":"Red",
"Peppers":"Green",
]
What is the equivalent (declare key value pairs) in Java?
I have tried modifying an array i.e. changing...
public String[] couplets = {
"Onions",
"Peppers",
};
...to...
public String[] toppings = {
"Onions":"Red",
"Peppers":"Green",
};
...but it does not work.
I appreciate they are different languages so likely I am oversimplifying this by trying to do a straight like for like.
Essentially I would like to create a static list of key value pairs in Java.
I have googled for suggestions but all the answers seem overly complicated compared to what I can do in swift so I am wondering if there is a straightforward way - Perhaps there isn't...
Any advice is much appreciated
Thanks
http://docs.oracle.com/javase/tutorial/collections/interfaces/map.html
A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value.
Map<String,String> toppings = new HashMap<>();
toppings.put("Onions","Red");
toppings.put("Peppers","Green");
Maps are great, you will end up using them a lot :)
Try the Map.
Instanciate new Map :
Map<String, String> map = new HashMap<String, String>();
Put values :
map.put("Onions","Red");
Read values :
map.get("Onions"); will return "Red"
If you need work with a String key you should use an HashMap as suggested in other answers.
If you can use an Integer key, I suggest you using a SparseArray
SparseArray<String> values = new SparseArray<String>();
values.put(1,"value1");
values.put(2,"value2");
Related
Given that I have the array of :
List<CustEnt> bulkList= CustRepo.fetchData();
//System.out.println(bulkList) -->
gives me :
CustEct(name:"kasis",age:24,surname:"kumar"),CustEct(name:"samika",age:50,surname:"sharma"),CustEct(name:"manoj",age:84surname:"kumar")
OR
bulkList.get(1) --> CustEct(name:"kasis",age:24,surname:"kumar")
I want to create a new array which is grouped by the 3rd parameter of surname object.
So that my array becomes
ArrayFinal = [CustEct(name:"kasis",age:24,surname:"kumar"),CustEct(name:"samika",age:50,surname:"sharma")],CustEct(name:"manoj",age:84surname:"kumar")
So that when we do .get(1) we would get object of kasis and samika.
Need the help in respective to java 8.
I heard that we can use the Map ,but can anyone give the small code sample or any other implementation guide.
A Map tracks key-value pairs.
Your key is the surname string.
Your value is a list of the CustEnt objects carrying that surname.
Map<String, List<CustEnt>>
Modern syntax with streams and lambdas makes for brief code to place your objects in a map.
Something like:
Map<String, List<CustEnt>> map = originalList.stream.collect(Collectors.groupingBy(CustEnt::getSurename));
Map<String, List<CustEntity>> NamesList
= bulkList.stream()
.collect(Collectors.groupingBy(CustEntity::getSurames));
for (Map.Entry<String, List<CustEntity>> entry: NamesList.entrySet()) {
ExcelGenerationService exp = new ExcelGenerationService( entry.getValue());
//service call
exp.export(entry.getKey());
}
I cannot seem to figure out how to access the values of my hashmap
What I am basically trying to do is create a hashmap with an array as one of the values like json style.. If that makes sense?
So I want something like hash{key: value1, value2, value3, [number1,number2]}
and be able to access it like (pseudocode:) hash.get(3).get(1)
public class WebSearch {
readFile.ReadFile xfile = new readFile.ReadFile("inputgraph.txt");
HashMap webSearchHash = new HashMap();
ArrayList belongsTo = new ArrayList();
ArrayList keyphrase = new ArrayList();
public WebSearch() {
}
public void createGraph()
{
HashMap <Object, ArrayList<Integer> > outlinks = new HashMap <Object, ArrayList<Integer>>();
for (int i = 0; i < xfile.getNumberOfWebpages(); i++ )
{
keyphrase.add(i,xfile.getKeyPhrases(i));
outlinks.put(keyphrase.get(i), xfile.getOutLinks(i));
}
}
keyphrases is an ArrayList
this is my output of System.out.print(outlinks);
{[education, news, internet]=[0, 3], [power, news]=[1, 4], [computer, internet, device, ipod]=[2], [university, education]=[5]}
How would I go about getting say just this: [education, news, internet]=[0, 3]
I have tried:
outlinks.get(xfile.getKeyPhrases(i))
xfile.getKeyPhrases(0) would for example return [education, news, internet]
You can get the key set (Map.keySet()) of the map first; outlinks.keySet()
Then you can use these keys on your map to get your entries (values of the keys)
You haven't posted enough of the surrounding code for your question to be entirely clear, but look at the Javadocs for Map. You will probably get what you want by iterating over outlinks.values().
I recommend to use a customized object and use it inside your collections.
You may create a POJO/Bean class and overwrite the toString method with the details that you want, for instance the a iterate over items inside a array.
When you use it to print or display the toString method will be call.
The following link show you some ideas:
http://www.javapractices.com/topic/TopicAction.do?Id=55
http://en.wikipedia.org/wiki/Plain_Old_Java_Object
You can access the keys of any HashMap using Map.keySet() method.
Also note that java.util.HashMap is unordered. HashMap makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.
You would like to relook at the structure of your HashMap, you are having ArrayList as your key.
Lets say I have hashmap store and it contains for example-(11,name1) (11,name2) and i call HashMap.get(11), it only shows name2 which means it overrides the first input for 11. How can i store both name1 and name2 with ID 11 using hashmap?I know i can use both HashMap and HashSet but i dont want to create every HashSet for HashMap. I just want to use hashSet only. how should I do this? I hope you can help me with it. Thank you.
public void insert(int ID, String key){
int hashKey = Hash(key);
System.out.println("Hash Key" + hashKey);
int node = Find(ID,hashKey);
storeR.put(node, key);
}
You can use:
HashMap<Integer, List<String>>
In HashMap you must put a value with every key. So of course, if you put the same key twice, the value will be override.
The solution is to hold a collection of values for every key.
in your code instead of:
storeR.put(node, key);
you should write:
List<String> nodeValues = storeR.get(node);
if (nodeValues == null) {
nodeValues = new ArrayList<String>();
storeR.put(node, nodeValues );
}
nodeValues.add(key);
And you should also change storeR type to be HashMap<Integer, List<String>>
MultiMap is also a similar solution.
You can probably use MultiMap from Apache Commons Collections.
You will have to either have a HashMap where the value of each key is another collection (list or set) or concatenate the string values together (e.g. comma separated).
Alternatively you may be able to find a data collection that supports multiple values per key.
To store multiple values for a single key, use a HashMap that contains a list as a value. HashMap's implementation overrides values for existing keys.
HashMap<Integer,List<String>>
Also, you could use MultiMap from Apache Commons or, if you're just using Integer I can suggest you use an array directly:
List<String>[] yourList = new List<String>[initCapacity];
So you can access that list like this:
yourList[0].add("A New Value");
As a final note, you can use any collection you deem appropiate, even a HashSet if performance is important for you and you won't store duplicated values for a same index.
I am creating Dictionary and an ArrayList such as this.
Dictionary testDict, testDict2 = null;
ArrayList al = new ArrayList();
testDict.put ("key1", dataVar1);
testDict.put ("key2", dataVar2);
testDict2.put ("key1", dataVar1);
testDict2.put ("key2", dataVar2);
al.add(testDict);
al.add(testDict2);
now my issue here is, how can I access the data within the dictionaries? Like for example how would I retrieve key1 from testDict using al?
Many thanks in advance :)
As you can read in the Java Docs all Dictionary objects (note that e.g. Hashtable is one of them) have a method Object get(Object key) to access it's elements. In your example you could access the value of the entry key1 in textDict like that:
// first access testDict at index 0 in the ArrayList al
// and then it's element with key "key1"
al.get(0).get("key1");
Note, that you nowhere initialize your Dictionary objects and that the Dictionary class is abstract. So you could for example use a Hashtable (or if you don't need synchronized access use a faster HashMap instead) for your purpose like this:
testDict = new Hashtable<String, String>();
And make sure to use the correct generic types (the second one has to be the type that your dataVars have)
Maybe this:
al.get(0).get("key1");
Since testDict is at position 0 (first element of your ArrayList) you can retrieve it with get(0)..
Example:
Dictionary firstDict = (Dictionary) al.get(0);
Object key1Data = firstDict.get("key1");
Ps: Generics can greatly improve your code if you are allowed to use it.
Another point is... Why Dictionary and not Map?
Not sure why'd you want keep your dictionaries like that, but you can simply loop over your dictionaries.
public Data getData(String key) {
for(Dictionary dict : al) {
Data result = dict.get(key);
if(result != null)
return result;
}
return null;
}
I've been using a lot Perl hashes due to super flexibility and convenient.
for instance, in Perl I can do the following:
$hash{AREA_CODE}->{PHONE}->{STREET_ADDR}
I wondering how can I accomplish the same thing with Java, I guess it has something to do with HashMap?
Thanks,
I've been using a lot Perl hashes due to super flexibility and convenient. for instance, in Perl I can do the following:
$hash{AREA_CODE}->{PHONE}->{STREET_ADDR}
I wondering how can I accomplish the same thing with Java, I guess it has something to do with HashMap?
The Java code which approximates the following Perl code:
my %hash;
$hash{AREA_CODE}{PHONE}{STREET_ADDR} = "221B Baker Street";
printf "Street address is %s\n", $hash{AREA_CODE}{PHONE}{STREET_ADDR};
is
HashMap<String, HashMap<String, HashMap<String, String>>> hash =
new HashMap<String, HashMap<String, HashMap<String, String>>>();
hash.put("AREA_CODE", new HashMap<String, HashMap<String, String>>());
hash.get("AREA_CODE").put("PHONE", new HashMap<String, String>());
hash.get("AREA_CODE").get("PHONE").put("STREET_ADDR", "221B Baker Street");
System.out.printf("Street address is %s\n",
hash.get("AREA_CODE").get("PHONE").get("STREET_ADDR"));
Isn’t that special? :)
I say ‘approximates’ for many reasons. One of these is that in Java you’ll be frustrated to the point of extreme apoplexy merely for wanting to then do on the next line of Java the equivalent of this perfectly straightforward Perl code:
$hash{AREA_CODE}{PREFIX} = 800;
If you want Perl’s flexibility and convenience in things like this, Java simply isn’t going to give it to you. Even worse, its partisans will often berate you for even expressing such a desire.
First of all, your specific example ($hash{AREA_CODE}->{PHONE}->{STREET_ADDR}), with hard-coded strings as hash keys, is not really a useful data structure in Java as Michael Carman pointed out - it should be stored as a class with attributes (and to be honest it's a bad data structure in concept - data like this is more likely to be an array of phones, not hash of phones).
Second, assuming you actually meant $hash{$AREA_CODE}->{$PHONE}->{$STREET_ADDR}, it looks like everyone's Java code so far was NOT implementing a generic equivalent code - the code all assumed that the Java hash is newly initialized for storing example OR fully populated for retrieval example (in other words, as leonbloy's answer noted, is missing autovivification feature).
The correct code mimiquing autovivification is:
// This method will ensure that hash-of-hash-of-hashes structure exists of a given set of 3 keys.
public HashMap<String, HashMap<String, HashMap<String, Object>>>
autovivification_3rd_level (
HashMap<String, HashMap<String, HashMap<String, Object>>> hash
, String AREA_CODE, String PHONE, String STREET_ADDR) {
if (hash == null) {
hash = new HashMap<String, HashMap<String, HashMap<String, Object>>>();
}
if (!hash.contains(AREA_CODE) || hash.get(AREA_CODE) == null) {
hash.put(new HashMap<String, HashMap<String, Object>>());
}
HashMap<String, HashMap<String, Object>> AREA_CODE_hash
= (HashMap<String, HashMap<String, Object>>) hash.get(AREA_CODE);
if (!AREA_CODE_hash.contains(PHONE) || AREA_CODE_hash.get(PHONE) == null) {
AREA_CODE_hash.put(new HashMap<String, Object>());
}
return hash;
}
////////////////////////////////////////////////////////////////////////////////////
// Equivalent to Perl's "$hash{$AREA_CODE}->{$PHONE}->{$STREET_ADDR} = value;"
public Object put_3d_level_hash(
HashMap<String, HashMap<String, HashMap<String, Object>>> hash
, String AREA_CODE, String PHONE, String STREET_ADDR,
, Object value) {
hash = autovivification_3rd_level(hash, AREA_CODE, PHONE, STREET_ADDR);
return hash.get(AREA_CODE).get(PHONE).put(STREET_ADDR, value);
}
put_3d_level_hash(hash, AREA_CODE, PHONE, STREET_ADDR, obj);
////////////////////////////////////////////////////////////////////////////////////
// Equivalent to Perl's "$var = $hash{$AREA_CODE}->{$PHONE}->{$STREET_ADDR}"
public Object get_3d_level_hash(HashMap<String, HashMap<String, HashMap<String, Object>>> hash
, String AREA_CODE, String PHONE, String STREET_ADDR) {
hash = autovivification_3rd_level(hash, AREA_CODE, PHONE, STREET_ADDR);
return hash.get(AREA_CODE).get(PHONE).get(STREET_ADDR);
}
Object obj = get_3d_level_hash(hash, AREA_CODE, PHONE, STREET_ADDR);
See the Map interface and its implementations, specially HashMap.
Beware that Java doesn't have Perl's auto-vivification (handy but dangerous feature) so that
hash.get("areaCode").get("phone").get("streetAdr")
will throw an exception if, eg, get(phone) returns null.
Beware also that you should not uses hashes for things that have fixed names ("properties"), you should define your own classes with its getters and setters.
Java has hashes, but because of strong typing they're not quite as flexible as hashes in Perl. Multidimensional hashes are harder to work with. In Perl, you can just declare a hash and let autovivification create the nested hashes on demand.
my %hash;
$hash{a}{b} = 1;
In Java, you have to declare it to be a hash-of-hashes up-front.
Map<String,Map<String,Integer>> hash = new HashMap<String,HashMap<String,Integer>>();
hash.put("a", new HashMap<String, Integer>());
hash.get("a").put("b", new Integer(1));
For every extra dimension you need to add another nesting of Map<K,V> to the declaration. Aside from being tedious, this isn't very OO.
If the hash keys are constant, why won't hash.getAreaCode().getPhone().getStreetAddr() do? Keep in mind that either your getters or your constructors will need to handle default value generation.
You can easily subclass your hash to add a method that'll autovivify for you.
From: $hash{AREA_CODE}->{PHONE}->{STREET_ADDR}
To: hash.vivifyingGet(areaCode).put(phone, streetAddr).
Assuming I've created the hash with:
/**
* A two-level autovivifying hashmap of X and Y to Z. Provides
* a new method #vivifyingGet(X) which creates the next level of hash.
*/
Map<AreaCode, Map<Phone, StreetAddr>> hash =
new HashMap<AreaCode, Map<Phone, StreetAddr>>() {
/**
* Convenience method to get or create the next level of hash.
* #param key the first level key
* #return the next level map
*/
public Map<Phone, StreetAddr> vivifyingGet(Phone key) {
if (containsKey(key)) {
return get(key);
} else {
Map<Phone, StreetAddr> = hash = new HashMap<Phone, StreetAddr>();
put(key, hash);
return hash;
}
}
};
I missed the perl hashes a lot in my work and made some ugly workarounds with hash classes.
Last week I had an idea to implement the whole thing in one PerlMap class which use delimiters to access objects and foremost the Lists zu access subsets.
It works fine with map.get(code:street:phone) and map.put(code:street:phone,"123456789"). To get a list of phonenumber you just use map.getList(code:street).
I've just started but use in my project now. It has no limitations of complexity :-) and you can choose the delimiter free. I put the whole stuff under http://www.jdeer.org. Have fun.
You're probably going to want to go with Groovy if you want this sort of flexibility but still run within the JVM. tchrist likes to ignore the point that Java is strong-typed as opposed to dynamic-typed languages like Perl or PHP - and also likes to ignore that Java is an order of magnitude faster at running, but that's just me being a "partisan", apparently.