How I can get the third value for the first key in this map? Is this possible?
This question is related to
java
Thinking about a Map with 2 keys immediately compelled me to use a user-defined key, and that would probably be a Class. Following is the key Class:
public class MapKey {
private Object key1;
private Object key2;
public Object getKey1() {
return key1;
}
public void setKey1(Object key1) {
this.key1 = key1;
}
public Object getKey2() {
return key2;
}
public void setKey2(Object key2) {
this.key2 = key2;
}
}
// Create first map entry with key <A,B>.
MapKey mapKey1 = new MapKey();
mapKey1.setKey1("A");
mapKey1.setKey2("B");
I found the blog on random search, i think this will help for doing this: http://tomjefferys.blogspot.com.tr/2011/09/multimaps-google-guava.html
public class MutliMapTest {
public static void main(String... args) {
Multimap<String, String> myMultimap = ArrayListMultimap.create();
// Adding some key/value
myMultimap.put("Fruits", "Bannana");
myMultimap.put("Fruits", "Apple");
myMultimap.put("Fruits", "Pear");
myMultimap.put("Vegetables", "Carrot");
// Getting the size
int size = myMultimap.size();
System.out.println(size); // 4
// Getting values
Collection<String> fruits = myMultimap.get("Fruits");
System.out.println(fruits); // [Bannana, Apple, Pear]
Collection<string> vegetables = myMultimap.get("Vegetables");
System.out.println(vegetables); // [Carrot]
// Iterating over entire Mutlimap
for(String value : myMultimap.values()) {
System.out.println(value);
}
// Removing a single value
myMultimap.remove("Fruits","Pear");
System.out.println(myMultimap.get("Fruits")); // [Bannana, Pear]
// Remove all values for a key
myMultimap.removeAll("Fruits");
System.out.println(myMultimap.get("Fruits")); // [] (Empty Collection!)
}
}
Try using collections to store the values of a key:
Map<Key, Collection<Value>>
you have to maintain the value list yourself
A standard Java HashMap cannot store multiple values per key, any new entry you add will overwrite the previous one.
It sounds like you're looking for a multimap. Guava has various Multimap
implementations, usually created via the Multimaps
class.
I would suggest that using that implementation is likely to be simpler than rolling your own, working out what the API should look like, carefully checking for an existing list when adding a value etc. If your situation has a particular aversion to third party libraries it may be worth doing that, but otherwise Guava is a fabulous library which will probably help you with other code too :)
Write a new class that holds all the values that you need and use the new class's object as the value in your HashMap
HashMap<String, MyObject>
class MyObject {
public String value1;
public int value2;
public List<String> value3;
}
Have you got something like this?
HashMap<String, ArrayList<String>>
If so, you can iterate through your ArrayList and get the item you like with arrayList.get(i).
For example:
Map<Object,Pair<Integer,String>> multiMap = new HashMap<Object,Pair<Integer,String>>();
where the Pair
is a parametric class
public class Pair<A, B> {
A first = null;
B second = null;
Pair(A first, B second) {
this.first = first;
this.second = second;
}
public A getFirst() {
return first;
}
public void setFirst(A first) {
this.first = first;
}
public B getSecond() {
return second;
}
public void setSecond(B second) {
this.second = second;
}
}
You can do something like this (add access modifiers as required):
Map<String,Map<String,String>> complexMap=new HashMap<String,Map<String,String>>();
You can insert data like this:
Map<String,String> componentMap = new HashMap<String,String>();
componentMap.put("foo","bar");
componentMap.put("secondFoo","secondBar");
complexMap.put("superFoo",componentMap);
The Generated Data Structure would be:
{superFoo={secondFoo=secondBar, foo=bar}}
This way each value for the key should have a unique identifier. Also gives O(1) for fetches,if keys are known.
Here is the code how to get extract the hashmap into arrays, hashmap that contains arraylist
Map<String, List<String>> country_hashmap = new HashMap<String, List<String>>();
//Creating two lists and inserting some data in it
List<String> list_1 = new ArrayList<String>();
list_1.add("16873538.webp");
list_1.add("16873539.webp");
List<String> list_2 = new ArrayList<String>();
list_2.add("16873540.webp");
list_2.add("16873541.webp");
//Inserting both the lists and key to the Map
country_hashmap.put("Malaysia", list_1);
country_hashmap.put("Japanese", list_2);
for(Map.Entry<String, List<String>> hashmap_data : country_hashmap.entrySet()){
String key = hashmap_data.getKey(); // contains the keys
List<String> val = hashmap_data.getValue(); // contains arraylists
// print all the key and values in the hashmap
System.out.println(key + ": " +val);
// using interator to get the specific values arraylists
Iterator<String> itr = val.iterator();
int i = 0;
String[] data = new String[val.size()];
while (itr.hasNext()){
String array = itr.next();
data[i] = array;
System.out.println(data[i]); // GET THE VALUE
i++;
}
}
This is what i found in a similar question's answer
Map<String, List<String>> hm = new HashMap<String, List<String>>();
List<String> values = new ArrayList<String>();
values.add("Value 1");
values.add("Value 2");
hm.put("Key1", values);
// to get the arraylist
System.out.println(hm.get("key1"));
RESULT: [Value 1, Value 2]
HashMap – Single Key and Multiple Values Using List
Map<String, List<String>> map = new HashMap<String, List<String>>();
// create list one and store values
List<String> One = new ArrayList<String>();
One.add("Apple");
One.add("Aeroplane");
// create list two and store values
List<String> Two = new ArrayList<String>();
Two.add("Bat");
Two.add("Banana");
// put values into map
map.put("A", One);
map.put("B", Two);
map.put("C", Three);
Apache Commons collection classes is the solution.
MultiMap multiMapDemo = new MultiValueMap();
multiMapDemo .put("fruit", "Mango");
multiMapDemo .put("fruit", "Orange");
multiMapDemo.put("fruit", "Blueberry");
System.out.println(multiMap.get("fruit"));
// Mango Orange Blueberry
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>
Source: Stackoverflow.com