Is it possible to implement a HashMap with one key and two values?
Just as HashMap<userId, clientID,timeStamp>?
If not, is there any other way to implement the storage of multiple values e.g. one key and two values?
Is it possible to implement a HashMap with one key and two values?
Just as HashMap<userId, clientID,timeStamp>?
If not, is there any other way to implement the storage of multiple values e.g. one key and two values?
You could:
Map<KeyType, List<ValueType>>
.Map<KeyType, WrapperType>
.Map<KeyType, Tuple<Value1Type, Value2Type>>
.1. Map with list as the value
// create our map
Map<String, List<Person>> peopleByForename = new HashMap<>();
// populate it
List<Person> people = new ArrayList<>();
people.add(new Person("Bob Smith"));
people.add(new Person("Bob Jones"));
peopleByForename.put("Bob", people);
// read from it
List<Person> bobs = peopleByForename["Bob"];
Person bob1 = bobs[0];
Person bob2 = bobs[1];
The disadvantage with this approach is that the list is not bound to exactly two values.
2. Using wrapper class
// define our wrapper
class Wrapper {
public Wrapper(Person person1, Person person2) {
this.person1 = person1;
this.person2 = person2;
}
public Person getPerson1() { return this.person1; }
public Person getPerson2() { return this.person2; }
private Person person1;
private Person person2;
}
// create our map
Map<String, Wrapper> peopleByForename = new HashMap<>();
// populate it
peopleByForename.put("Bob", new Wrapper(new Person("Bob Smith"),
new Person("Bob Jones"));
// read from it
Wrapper bobs = peopleByForename.get("Bob");
Person bob1 = bobs.getPerson1();
Person bob2 = bobs.getPerson2();
The disadvantage to this approach is that you have to write a lot of boiler-plate code for all of these very simple container classes.
3. Using a tuple
// you'll have to write or download a Tuple class in Java, (.NET ships with one)
// create our map
Map<String, Tuple2<Person, Person> peopleByForename = new HashMap<>();
// populate it
peopleByForename.put("Bob", new Tuple2(new Person("Bob Smith",
new Person("Bob Jones"));
// read from it
Tuple<Person, Person> bobs = peopleByForename["Bob"];
Person bob1 = bobs.Item1;
Person bob2 = bobs.Item2;
This is the best solution in my opinion.
4. Multiple maps
// create our maps
Map<String, Person> firstPersonByForename = new HashMap<>();
Map<String, Person> secondPersonByForename = new HashMap<>();
// populate them
firstPersonByForename.put("Bob", new Person("Bob Smith"));
secondPersonByForename.put("Bob", new Person("Bob Jones"));
// read from them
Person bob1 = firstPersonByForename["Bob"];
Person bob2 = secondPersonByForename["Bob"];
The disadvantage of this solution is that it's not obvious that the two maps are related, a programmatic error could see the two maps get out of sync.
No, not just as a HashMap
. You'd basically need a HashMap
from a key to a collection of values.
If you're happy to use external libraries, Guava has exactly this concept in Multimap
with implementations such as ArrayListMultimap
, HashMultimap
, LinkedHashMultimap
etc.
Multimap<String, Integer> nameToNumbers = HashMultimap.create();
System.out.println(nameToNumbers.put("Ann", 5)); // true
System.out.println(nameToNumbers.put("Ann", 5)); // false
nameToNumbers.put("Ann", 6);
nameToNumbers.put("Sam", 7);
System.out.println(nameToNumbers.size()); // 3
System.out.println(nameToNumbers.keySet().size()); // 2
ArrayListMultimap
yourself... or just use a HashMap<String, List<Integer>>
or whatever. You'd need to create an empty list any time a value is added for the first time, basically.
Feb 10, 2011 at 18:20
Another nice choice is to use MultiValuedMap from Apache Commons. Take a look at the All Known Implementing Classes at the top of the page for specialized implementations.
Example:
HashMap<K, ArrayList<String>> map = new HashMap<K, ArrayList<String>>()
could be replaced with
MultiValuedMap<K, String> map = new MultiValuedHashMap<K, String>();
So,
map.put(key, "A");
map.put(key, "B");
map.put(key, "C");
Collection<String> coll = map.get(key);
would result in collection coll
containing "A", "B", and "C".
Take a look at Multimap
from the guava-libraries and its implementation - HashMultimap
A collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values.
I use Map<KeyType, Object[]>
for associating multiple values with a key in a Map. This way, I can store multiple values of different types associated with a key. You have to take care by maintaining proper order of inserting and retrieving from Object[].
Example: Consider, we want to store Student information. Key is id, while we would like to store name, address and email associated to the student.
//To make entry into Map
Map<Integer, String[]> studenMap = new HashMap<Integer, String[]>();
String[] studentInformationArray = new String[]{"name", "address", "email"};
int studenId = 1;
studenMap.put(studenId, studentInformationArray);
//To retrieve values from Map
String name = studenMap.get(studenId)[1];
String address = studenMap.get(studenId)[2];
String email = studenMap.get(studenId)[3];
HashMap<Integer,ArrayList<String>> map = new HashMap<Integer,ArrayList<String>>();
ArrayList<String> list = new ArrayList<String>();
list.add("abc");
list.add("xyz");
map.put(100,list);
If you use Spring Framework. There is: org.springframework.util.MultiValueMap
.
To create unmodifiable multi value map:
Map<String,List<String>> map = ...
MultiValueMap<String, String> multiValueMap = CollectionUtils.toMultiValueMap(map);
Or use org.springframework.util.LinkedMultiValueMap
The easiest way would be to use a google collection library:
import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Multimap;
public class Test {
public static void main(final String[] args) {
// multimap can handle one key with a list of values
final Multimap<String, String> cars = ArrayListMultimap.create();
cars.put("Nissan", "Qashqai");
cars.put("Nissan", "Juke");
cars.put("Bmw", "M3");
cars.put("Bmw", "330E");
cars.put("Bmw", "X6");
cars.put("Bmw", "X5");
cars.get("Bmw").forEach(System.out::println);
// It will print the:
// M3
// 330E
// X6
// X5
}
}
maven link: https://mvnrepository.com/artifact/com.google.collections/google-collections/1.0-rc2
more on this: http://tomjefferys.blogspot.be/2011/09/multimaps-google-guava.html
Just for the record, the pure JDK8 solution would be to use Map::compute
method:
map.compute(key, (s, strings) -> strings == null ? new ArrayList<>() : strings).add(value);
Such as
public static void main(String[] args) {
Map<String, List<String>> map = new HashMap<>();
put(map, "first", "hello");
put(map, "first", "foo");
put(map, "bar", "foo");
put(map, "first", "hello");
map.forEach((s, strings) -> {
System.out.print(s + ": ");
System.out.println(strings.stream().collect(Collectors.joining(", ")));
});
}
private static <KEY, VALUE> void put(Map<KEY, List<VALUE>> map, KEY key, VALUE value) {
map.compute(key, (s, strings) -> strings == null ? new ArrayList<>() : strings).add(value);
}
with output:
bar: foo
first: hello, foo, hello
Note that to ensure consistency in case multiple threads access this data structure, ConcurrentHashMap
and CopyOnWriteArrayList
for instance need to be used.
computeIfAbsent
. map.computeIfAbsent(key, k -> new ArrayList<>()).add(value);
Yes and no. The solution is to build a Wrapper clas for your values that contains the 2 (3, or more) values that correspond to your key.
Yes, this is frequently called a multimap
.
I prefer the following to store any number of variables without having to create a separate class.
final public static Map<String, Map<String, Float>> myMap = new HashMap<String, Map<String, Float>>();
Using Java Collectors
// Group employees by department
Map<Department, List<Employee>> byDept = employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment));
where Department is your key
String key= "services_servicename"
ArrayList<String> data;
for(int i = 0; i lessthen data.size(); i++) {
HashMap<String, String> servicesNameHashmap = new HashMap<String, String>();
servicesNameHashmap.put(key,data.get(i).getServiceName());
mServiceNameArray.add(i,servicesNameHashmap);
}
I have got the Best Results.
You just have to create new HashMap
like
HashMap<String, String> servicesNameHashmap = new HashMap<String, String>();
in your for
loop. It will have same effect like same key and multiple values.
import java.io.*;
import java.util.*;
import com.google.common.collect.*;
class finTech{
public static void main(String args[]){
Multimap<String, String> multimap = ArrayListMultimap.create();
multimap.put("1","11");
multimap.put("1","14");
multimap.put("1","12");
multimap.put("1","13");
multimap.put("11","111");
multimap.put("12","121");
System.out.println(multimap);
System.out.println(multimap.get("11"));
}
}
Output:
{"1"=["11","12","13","14"],"11"=["111"],"12"=["121"]}
["111"]
This is Google-Guava library for utility functionalities. This is the required solution.
I could not post a reply on Paul's comment so I am creating new comment for Vidhya here:
Wrapper will be a SuperClass
for the two classes which we want to store as a value.
and inside wrapper class, we can put the associations as the instance variable objects for the two class objects.
e.g.
class MyWrapper {
Class1 class1obj = new Class1();
Class2 class2obj = new Class2();
...
}
and in HashMap we can put in this way,
Map<KeyObject, WrapperObject>
WrapperObj will have class variables: class1Obj, class2Obj
You can do it implicitly.
// Create the map. There is no restriction to the size that the array String can have
HashMap<Integer, String[]> map = new HashMap<Integer, String[]>();
//initialize a key chosing the array of String you want for your values
map.put(1, new String[] { "name1", "name2" });
//edit value of a key
map.get(1)[0] = "othername";
This is very simple and effective. If you want values of diferent classes instead, you can do the following:
HashMap<Integer, Object[]> map = new HashMap<Integer, Object[]>();
Can be done using an identityHashMap, subjected to the condition that the keys comparison will be done by == operator and not equals().
I am so used to just doing this with a Data Dictionary in Objective C. It was harder to get a similar result in Java for Android. I ended up creating a custom class, and then just doing a hashmap of my custom class.
public class Test1 {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.addview);
//create the datastring
HashMap<Integer, myClass> hm = new HashMap<Integer, myClass>();
hm.put(1, new myClass("Car", "Small", 3000));
hm.put(2, new myClass("Truck", "Large", 4000));
hm.put(3, new myClass("Motorcycle", "Small", 1000));
//pull the datastring back for a specific item.
//also can edit the data using the set methods. this just shows getting it for display.
myClass test1 = hm.get(1);
String testitem = test1.getItem();
int testprice = test1.getPrice();
Log.i("Class Info Example",testitem+Integer.toString(testprice));
}
}
//custom class. You could make it public to use on several activities, or just include in the activity if using only here
class myClass{
private String item;
private String type;
private int price;
public myClass(String itm, String ty, int pr){
this.item = itm;
this.price = pr;
this.type = ty;
}
public String getItem() {
return item;
}
public void setItem(String item) {
this.item = item;
}
public String getType() {
return item;
}
public void setType(String type) {
this.type = type;
}
public int getPrice() {
return price;
}
public void setPrice(int price) {
this.price = price;
}
}
We can create a class to have multiple keys or values and the object of this class can be used as a parameter in map. You can refer to https://stackoverflow.com/a/44181931/8065321
Apache Commons collection classes can implement multiple values under same key.
MultiMap multiMapDemo = new MultiValueMap();
multiMapDemo .put("fruit", "Mango");
multiMapDemo .put("fruit", "Orange");
multiMapDemo.put("fruit", "Blueberry");
System.out.println(multiMapDemo.get("fruit"));
Maven Dependency
<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-collections4 -->
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-collections4</artifactId>
<version>4.4</version>
</dependency>
MultiValueMap
is deprecated since 4.1. Use MultiValuedMap
instead
May 24, 2021 at 20:38