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How to create a map with enum values in java?

I need to validate the value of a parameters passed as part of REST API. These parameters are a fixed set of values. I thought of using a map having parameter name as key and enum as value. So I can check if the value sent in REST API is one of the enum keys.
But I am not able to create a Map with String key and enum value in java, tried creating Map and then putting and enum as value, but it fails.
class Validation {
enum Type {
INTERNAL,
EXTERNAL
};
}
Map<String, Object> validationMap = new HashMap<String, Object>();
validationMap.put("type", Validation.Type);
This is throwing an error that type is not defined.

This is probably what you're looking for, Map<String, Object> is changed to Map<String, Validation.type>:
Map<String, Validation.type> validationMap = new HashMap<String, Validation.type>();
validationMap.put("type", Validation.type.INTERNAL);
validationMap.put("type2", Validation.type.EXTERNAL);
Your original code would have worked, if you had changed Validation.type to Validation.type.INTERNAL for example, however your validationMap map allows the storage of any Object, so validationMap.put("type2", 123.123); would also have worked, which is unlikely to be something you want.

Is this what you need?
Map<String, List<Validation.type>> validationMap = new HashMap<>();
validationMap.put("type",
Arrays.asList(Validation.type.EXTERNAL,Validation.type.INTERNAL));

I believe Your code does not work because Validation.Type is not an instance (of an enum). When you put a value in your map, I believe you would want to put in an actual instance as a value. I suppose that is why the above answers suggest adding an actual instance to the Map such as Type.INTERNAL or using a class instance.
This code will run
Map<String, Object> validationMap = new HashMap<String, Object>();
// this works
validationMap.put("type", Validation.Type.EXTERNAL);
// as shown by:
System.out.println(validationMap.toString());
// here is an analogy:
// make an instance of a class
Validation validation = new Validation();
// this works
validationMap.put("classType",validation);
Validation validationGet = (Validation) validationMap.get("classType");
System.out.println(validationMap.toString());
// by analogy this will not work because Validation is not an instance:
// validationMap.put("classType",Validation);
And the output on my machine is:
{type=EXTERNAL}
{type=EXTERNAL, classType=Validation#3feba861}

How to make these simple and beautiful?

When I put a (KEY, VALUE) into a map such as Map<String, List<String>>, and I want to check if the KEY is existed first to decide if I have to make a new List, usually My Java Code looks like this:
Map<String, List<String>> example = new HashMap<>();
public void put(String k, String v){
if(example.containsKey(k)){
example.get(k).add(v);
return;
}
List<String> vs = new ArrayList<>();
vs.add(v);
example.put(k,vs);
}
It doesn't looks very nice. Is there any way to make it more simple and more beautiful?

If you have Java 8 you can write this as one line:
example.computeIfAbsent(k, key -> new ArrayList<>()).add(v);
This uses a lambda, so the new ArrayList is only created if required.
(k and key need to have different names, as they are different variables)

Map<String, List<String>> example = new HashMap<>();
public void put(String k, String v){
if (!example.containsKey(k)){
example.put(k, new ArrayList<>();
}
example.get(k).add(v);
}
Arguably, this is slightly wasteful - requiring you to get the list you just put - but to my eye it is much cleaner and more expressive.

If you can't use other libraries, or java 8, you could wrap the whole map and construct in a class of you own.
With your own class you:
Confine the messiness to one place.
Hide the face you are using a Map behind the scenes.
Have a place to move any additional logic to.

How to pass a multidimensional hash as query parameters using retrofit by Square

I have a map of string id's and points, from which I need to generate a path similar to this:
.../?assessment[id_1][points]=3&assessment[id_2][points]=5
I'm not exactly sure how i would do something like this in retrofit.
I've tried manually generating a string and appending it to the end of a request
#PUT("/path/to/api/{assessment}?{}")
void postScore(#Path("assessment") String rubricAssessment, Callback<Submission> callback);
Which is pretty hacky and results in the error.
java.lang.IllegalArgumentException: Inteface.postScore: URL query string "{assessment}" must not have replace block.
Could someone suggest a better method of accomplishing this in retrofit?
Thank you for reading my question.
EDIT:
One possible solution i've found is to use either a QueryMap or a FieldMap, but that still requires me to create a helper method to manually construct each key value. Something like this:
private static String getFieldMap(Map<String,String> map){
Map<String, String> fieldMap = new HashMap<String, String>();
for (Map.Entry<String, String> entry : map.entrySet())) {
map.put("assessment[" +entry.getCriterionId() +"][points]", String.valueOf(entry.getPoints()));
}
return fieldMap;
}
Then passing that map into something like this?
#put("/things")
void things(#FieldMap Map<String, String> fields);
}
Is that the best way to do this? Any feedback is appreciated.

You are correct. You can indeed use a QueryMap. To be honest never tried using a FieldMap since QueryMap works perfectly fine for me.
If you want to use queryMap it is very simple. Example:
Map testMap = new HashMap<String,String>();
testMap.put("key1", "value1");
testMap.put("key2", "value2");
And pass it to you postScore call.
something.postScore("yourStringValue",testMap);
Your retrofit call should look something like this:
void postScore(#Path("assessment") String rubricAssessment, #QueryMap Map<String,String> callback)
Hope this helps.

How to pass in initialized HashMap as param?

How can I pass in a new HashMap in the most canonical (simplest, shortest hand) form?
// 1. ? (of course this doesn't work)
passMyHashMap(new HashMap<String, String>().put("key", "val"));
// 2. ? (of course this doesn't work)
passMyHashMap(new HashMap<String, String>(){"key", "val"});
void passMyHashMap(HashMap<?, ?> hm) {
// do stuff witih my hashMap
}

Create it, initialize it, then pass it:
Map<String,String> myMap = new HashMap<String,String>();
myMap.put("key", "val");
passMyHashMap(myMap);
You could use the "double curly" style that David Wallace mentions in a comment, I suppose:
passMyHashMap(new HashMap<String,String>(){{
put("x", "y");
put("a", "b");
}});
This essentially derives a new class from HashMap and sets up values in the initializer block. I don't particularly care for it (hence originally not mentioning it), but it doesn't really cause any problems per se, it's more of a style preference (it does spit out an extra .class file, although in most cases that's not a big deal). You could compress it all to one line if you'd like, but readability will suffer.

You can't call put and pass the HashMap into the method at the same time, because the put method doesn't return the HashMap. It returns the old value from the old mapping, if it existed.
You must create the map, populate it separately, then pass it in. It's more readable that way anyway.
HashMap<String, String> map = new HashMap<>();
map.put("key", "val");
passMyHashMap(map);

HashMap< K,V>.put
public **V** put(K key,V value)
Associates the specified value with the specified key in this map. If
the map previously contained a mapping for the key, the old value is
replaced.
Returns the previous value associated with key, or null if there was
no mapping for key. (A null return can also indicate that the map
previously associated null with key.)
As you can see, it does not return the type HashMap<?, ?>

You can't do that. What you can do is create a factory that allow you to do so.
public class MapFactory{
public static Map<String, String> put(final Map<String, String> map, final String key, final String valeu){
map.put(key, value);
return map;
}
}
passMyHashMap(MapFactory.put(new HashMap<String, String>(),"key", "value"));
Although I can't image a approach that would need such implementation, also I kinda don't like it. I would recommend you to create your map, pass the values and just then send to your method.
Map<String, String> map = new HashMap<String, String>();
map.put("key","value");
passMyHashMap(map);

java reflection to create field/value hashmap

I need to create a Hashmap of field/values contained in an Entity, so I can can use them to replace them in a String containing tags with the field names.
I have this code:
public static String replaceTags(String message, Map<String, String> tags) ...
Which replaces all tags found in message for the equivalent values in tags, but in order to build the Map table I need to take "any" Entity and be able to create a Map from the Entity. So, how could I make that possible? to get a routine where I send the Entity and get as return a Map with all the fields and values.
public static Map<String, String> getMapFromEntity(Object entity){
Map<String, String> map = new HashMap<String, String>();
...?????
return map;
}
I know I could use reflection and this is the only approach I have found to get this done, but is there any other way to accomplish the same?, I mean a more efficient way.
Thanks.

Field[] fields = entity.getClass().getFields();
Map<String, String> map = new HashMap<String, String>();
for(Field f : fields)
map.put(f.getName(),(String) f.get(entity));
O, and your entity should be an object of your class, not your class itself.
If your fields are private and you have getters for them, you should use getMethods() and check if method name starts with "get" prefix.Like this:
Method[] methods = entity.getClass().getMethods();
Map<String, String> map = new HashMap<String, String>();
for(Method m : methods)
{
if(m.getName().startsWith("get"))
{
String value = (String) m.invoke(entity);
map.put(m.getName().substring(3), value);
}
}

If all you want to get out of this is a map, why not use a literary like Jackson, then you can simply convert it like this
Map map = new ObjectMapper().convertValue(object, Map.class);

I know I could use reflection and this is the only approach I have
found to get this done, but is there any other way to accomplish the
same?
As far as I know, reflection is the only way to accomplish this, unless the class(es) you want to build the map from implement some interface, and your code extracting the map is aware of this interface.