How to define the map as a constant object instead of a method which will be created every time when it is called

Question

This is my method which used to populate a map, and there's other data will be added in the future.

public class myClass{
  public Map populateMap(client: RedissonClient) {
   Map map = client.getMap("map");
   map.put(k1, v1);
   map.put(k2, v2);
   ...
   
   return map;
 }
}

The problem is, each time when I calling this method in my main function, it will re-create the same map content again and again, is there a way to define it as a constant map object, not in a method.

Map map = myClass.populateMap(client);
...

Answer 1

From your question, it's not completely clear whether the map should belong to the instance or to the class.

To create a class constant map, use Map::of :

class MyClass {

    static final Map<String, String> MAP = Map.of("k1", "v1", "k2", "v2");

}

The abovementioned map is an unmodifiable map : it is guaranteed that no modifications could ever be made to the map ¹ . If you want to get the contents of this map into a new map, in order to add additional data to it, just use

Map<String, String> newMap = new HashMap<>(MyClass.MAP);
newMap.put(...);

---

^{1 Well, perhaps it is possible with reflection. But for reflection applies: only use it if you know what you're doing.}

Answer 2

if you make the method static, you can initialize a static map:

public class MyClass{
  public static Map<String, String> map = populateMap();

  private static Map<String, String> populateMap() {
   Map<String, String> map = new HashMap<>();
   map.put("k1", "v1");
   map.put("k2", "v2");
   ...
  
   return map;
 }
}

now the populated MyClass.map is accessible to all the other classes

Answer 3

It is actually already said, make the map a global constant, or in java terms: a static final field .

public class myClass {
    private static final Map<String, String> MAP = new HashMap<>();
    static {
        MAP.put(k1, v1);
        MAP.put(k2, v2);
    }

Or since java 9 easier done as (thanks to @GhostCat):

    private static final Map<String, String> MAP =
        Map.of(k1, v1,
               k2, v2);

Use typing, like the interface Map<Integer, Object> . And for the creation pick the implementing class.

static {... } is called a static initializer .

Answer 4

You can make it as a global variable like mentioned above.

public final static Map<String, String> MAP = populateMap();