Sometimes you want to store multiple values for the same hash key.
The following code examples show you 2 different ways to do this.
Scenario:
HashMap can be used to store key-value pairs.
But sometimes you may want to store multiple values for the same key.
For example:
For Key A, you want to store - Apple, Aeroplane
For Key B, you want to store - Bat, Banana
For Key C, you want to store – Cat, Car
The following code snippets will show you at least 2
different ways of storing key-value pairs with a distinction of Single
Key and Multiple Values.
HashMap - Single Key and Multiple Values Using List
Sample Code Snippet:
private Map<String, List<String>> mapList;
mapList = new HashMap<String, List<String>>();
public void puStringList(String key, List stringList) {
mapList.put(key, stringList);
}
public List getStringList(String key) {
return mapList.get(key);
}
HashMap – Single Key and Multiple Values Using Google Guava Collections
Maven dependency for Google Guava Collections:
<!-- https://mvnrepository.com/artifact/com.google.guava/guava -->
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>19.0</version>
</dependency>
Sample Code Snippet:
private Multimap<String, String> multiMap;
multiMap = ArrayListMultimap.create();
public void putMultiMap(String key, String value) {
multiMap.put(key, value);
}
public List<String> getMultiMap(String key) {
return (List<String>) multiMap.get(key);
}
Full Code using a class
Map2Multiple combining both 2 ways.
package com.map.utility;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Multimap;
public class Map2Multiple {
private Map<String, List<String>> mapList;
private Multimap<String, String> multiMap;
public Map2Multiple() {
mapList = new HashMap<String, List<String>>();
multiMap = ArrayListMultimap.create();
}
public void puStringList(String key, List<String> stringList) {
mapList.put(key, stringList);
}
public List<String> getStringList(String key) {
return mapList.get(key);
}
public void putMultiMap(String key, String value) {
multiMap.put(key, value);
}
public List<String> getMultiMap(String key) {
return (List<String>) multiMap.get(key);
}
public void printAllMapList() {
for(Map.Entry<String,List<String>> entry : mapList.entrySet())
{
System.out.println("key: " + entry.getKey());
System.out.println("Value: " + entry.getValue());
}
}
public void printAllMultiMap() {
for(String key : multiMap.keySet())
{
System.out.println("key: " + key);
System.out.println("Value: " + multiMap.get(key));
}
}
}
Test
Map2Multiple from main method of
Map2MultipleMain.
package com.map.utility;
import java.util.Arrays;
public class Map2MultipleMain {
public static void main(String[] args) {
Map2Multiple maps = new Map2Multiple();
maps.puStringList("A", Arrays.asList("Apple", "Aeroplane", "Army"));
maps.puStringList("A", Arrays.asList("Angry", "Aeroplane", "Army"));
System.out.println(maps.getStringList("A"));
maps.putMultiMap("B", "Banana");
maps.putMultiMap("B", "Bat");
maps.putMultiMap("B", "Ball");
maps.putMultiMap("B", "Ballon");
maps.putMultiMap("C", "Cat");
maps.putMultiMap("C", "Car");
maps.putMultiMap("C", "Canada");
System.out.println(maps.getMultiMap("Z"));
maps.printAllMapList();
maps.printAllMultiMap();
}
}
