Java HashMap - variable.getKey - Generic Print Method

Question

I'm looking to create a method which will accept a String as a parameter and then print out the respective HashMap that the String refers too.

sensorMappings = new HashMap<>();
sensorMappings.put(136, "doorNumber");

This is my method to print the HashMap:

void printHashMap(String mapChoice){
  for (Integer ID : mapChoice.keySet()) {
    String key = ID.toString();
    String value = sensorMappings.get(ID);
    System.out.println(key + " " + value);
  }
}

I am getting the error:

Can't resolve method 'keySet()'

EDIT - full code:

Main.class

sensorInfo.printHashMap("mapChoice");

Sensor.class

public class Sensors {
  private HashMap<Integer, String> sensorMappings;

Sensors(){
  sensorMappings = new HashMap<>();
  sensorMappings.put(136, "doorNumber");
}

void printHashMap(String mapChoice){
  for (Integer ID : mapChoice.keySet()) {
    String key = ID.toString();
    String value = sensorMappings.get(ID);
    System.out.println(key + " " + value);
  }
}

I have multiple HashMaps and wish to create a generic method to print them by passing in the required HashMap.

Answer 1

You can achieve this if you register all your various hashMaps in another variable keyed by name. Then you can look up any of the hash maps that you've got registered.

An example would be below.

public class MapRegistry {
  static Map<String,HashMap<Integer,String>> allMaps = new HashMap<>();

  public static void register(String name, HashMap<Integer,String> myMap) {
    allMaps.put(name, myMap);
  }

  public static HashMap<Integer,String> find(String name) {
    return allMaps.get(name);
  }
}

In your sensors class

Sensors(){
  sensorMappings = new HashMap<>();
  MapRegistry.register("sensorMappings", sensorMappings);
  sensorMappings.put(136, "doorNumber");
}

Then to look up the particular map:

void printHashMap(String mapChoice){
  HashMap<Integer,String> map = MapRegistry.find(mapChoice);
  for (Integer ID : map.keySet()) {
    String key = ID.toString();
    String value = map.get(ID);
    System.out.println(key + " " + value);
  }
}

You don't need a static class if you are prepared to pass the registry around or can inject it.

You will need to pay attention if you ever remove maps as the registry will keep hold of them. So you'll need to ensure that the registry is updated as well or you'll get leaks.

Answer 2

No you cannot. Java is a strongly typed Language. You have to pass the map and not String to compile it. Maybe you can have a look at the reflection API (not suggested unless you have a string reason to use it).

Answer 3

You are not passing the hashmap at all. You are passing a string. Maybe you are trying to pass a hashmap than a string.

printHashMap(sensorMappings);

In the print function.

void printHashMap(HashMap mapChoice){
    for (Integer ID : mapChoice.keySet()) {
        String key = ID.toString();
        String value = sensorMappings.get(ID);
        System.out.println(key + " " + value);
    }
}

If you are trying to encode the HashMap and convert that back to the string then you can try this

String value = sensorMappings.toString();
value = value.substring(1, value.length()-1);           //remove curly brackets
String[] keyValuePairs = value.split(",");              //split the string to creat key-value pairs
Map<String,String> map = new HashMap<>();               

for(String pair : keyValuePairs)                        //iterate over the pairs
{
    String[] entry = pair.split("=");                   //split the pairs to get key and value 
    map.put(entry[0].trim(), entry[1].trim());          //add them to the hashmap and trim whitespaces
}