I have got some troubles converting each value in my HashMap to a String.
private static HashMap<String, List<Music>> musiksammlung = new
HashMap<String, List<Music>>();
This is my constructor for the HashMap. The key represents the album, the value a list of tracks from this album.
Now I want to convert each Music object to a String without creating a new HashMap, is this
possible?
I've tried it with the Iterator scheme, for loop over the entry set and so on but nothing seems to work.
Edit://
My code for the convertmethod:
public HashMap<String, List<String>> generateFormatList() {
HashMap<String, List<String>> formatList = new HashMap<String, List<String>>();
for(String key : musiksammlung.keySet())
formatList.put(key, musiksammlung.get(key).toString());
return musiksammlung;
}
But this always results in an error "is not applicable for the Arguments (String, String) so I have no idea. Do I have to override toString()?
You're on the right path but you need to convert the existing List<Music> to a List<String> and put the List<String> into your new HashMap.
You also then want to return your newly created HashMap<String, List<String>> instead of your original one.
public HashMap<String, List<String>> generateFormatList() {
HashMap<String, List<String>> formatList = new HashMap<String, List<String>>();
for(String key : musiksammlung.keySet()) {
// Value to store in map
List<String> value = new ArrayList<String>();
// Get the List<Music>
List<Music> musicList = musiksammlung.get(key);
for (Music m: musicList) {
// Add String of each Music object to the List
value.add(m.toString);
}
// Add the value to your new map
formatList.put(key, value);
}
// Return the new map
return formatList;
}
So answer your question:
Now I want to convert each Music object to a String without creating a
new HashMap, is this possible?
You need to create a new HashMap, because it's storing different type of value: List<Music> is different from List<String>.
Also as mentioned in my previous answer, make sure you override Music.toString() so that it returns a meaningful String for you instead of the one it inherits from its parent classes, which includes at least java.lang.Object
formatList wants a List<String>, but musiksammlung.get(key).toString() returns a String (not a List<String>). Did you mean this?
HashMap<String, String> formatList = new HashMap<String, String>();
Have you tried something like this:
Iterator<String> it = musiksammlung.keySet().iterator();
while (it.hasNext()) {
List<Music> ml = musiksammlung.get(it.next());
for (Music m : ml)
System.out.println(m.toString());
}
And of course you should override the Music#toString() method with something you could use.
Try to change your HashMap like this:
private static HashMap<String, List<Object>> musiksammlung = new HashMap<String,List<Object>>();
So you can save any kind of objects in this HashMap. Also use instanceof to check the type of the object before using it.
Related
I have method that should return Map<Strings, List<String>> but in the mean time my method gives me a Map<Strings, Object>, I want to transfer the values of object into a List of Strings.
Here is the current code:
#SuppressWarnings("unchecked")
static Map<String, List<String>> getQueryParameters(JsonObject inputJsonObject) {
JsonArray parameters = inputJsonObject.getJsonArray("parameters");
Optional<JsonObject> queryParameters = parameters.stream().
filter(JsonObject.class::isInstance).
map(JsonObject.class::cast).
filter(jsonObject -> jsonObject.getJsonObject("queryParameters") != null).
map(item -> item.getJsonObject("queryParameters")).findFirst();
Map<String, Object> paramMap = queryParameters.get().getMap();
paramMap contains key and value , values could be an arrays of integers
so I want to put them into the map below:
Map<String, List<String>> mystore = new LinkedHashMap<String, List<String>>();
My solution is this which did not work correctly
Map<String, List<String>> mystore = new LinkedHashMap<String, List<String>>();
Map<String, Object> paramMap = queryParameters.get().getMap();
Iterator it = paramMap.entrySet().iterator();
while (it.hasNext()) {
String key = it.next().toString();
if (!mystore.containsKey(key)) ;
mystore.put(key, new LinkedList<String>());
mystore.get(key).add(it.next().toString());
}
I was a key holding another key as value and is just a mix up , any suggestions?
After debuging what happens i see that mystore holds both "key and value" together as a key and value it hold the next "key and value as value
Should be something like this:
while (it.hasNext()) {
Map.Entry<String, Object> next = iterator.next();
String key = next.getKey();
Object value = next.getValue();
if (!mystore.containsKey(key)) mystore.put(key, new LinkedList<String>());
mystore.get(key).add(value.toString());
}
I'm not writing a program for you, but instead help you in finding a problem. You are confused with Entry. If you are using IDE, you should solve it easier. Look for this line :
String key = it.next().toString();
Entry has a K,V pair. The iterator returns an EntrySet and thus usage to get key is it.next().getKey() and it.next().getValue()
Now that you have a correct key, please go on debugging. Instead of putting and getting and manipulating in below lines of your code. Put with correct value instead?
Yours:
mystore.put(key, new LinkedList<String>());
mystore.get(key).add(it.next().toString());
What about?:
Entry entry = it.next();
//Get key and value here. DO coding using Entry's methods
List<String> ll = new LinkedList<String>();
ll.add(value)
mystore.put(key, ll);
Tip: Always have the Javadoc or reference documentation handy for knowing more. That's how you learn the language. Refer:https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html
Here is what I'm trying to do.
Map<String, List<Address>> mapObj = someService.getSomeAddress();
Using above call I'm getting mapObj of type Map<String, List<Address>>
And I want to send mapObj as a parameter in a another method (that I cannot change) as a LinkedHashMap<String, List<LinkedHashMap>> which does some further processing.
Is there any way that I can solve this problem without affecting data inside mapObj?
You will need to perform a couple of conversion steps.
Convert all Address objects to LinkedHashMap objects.
Put all Map entries into a LinkedHashMap object.
For 1st one, you can write a utility method some where that can do this conversion. For example,
public static List<LinkedHashMap> addresstoMap(List<Address> addresses)
{
List<LinkedHashMap> list = new ArrayList<>();
for(Address a: addresses){
LinkedHashMap map = new LinkedHashMap();
// Add address fields to map here
list.add(map);
}
return list;
}
Then, for the 2nd step, you can do this:
LinkedHashMap<String, List<LinkedHashMap>> map = new LinkedHashMap<?,?>();
iterate through the entry sets of mapObj and put them into the above map object.
for (Map.Entry<String, List<Address>> e : m.entrySet()) {
map.put(e.getKey(), addresstoMap(e.getValue()));
}
The final map object above will contain the correct representation in the LinkedHashMap<String, List<LinkedHashMap>> datatype.
Hope this helps!
I'm trying to retrieve values from a nested hashmap depending on user selection. I'm having trouble getting the values from the nested Hashmap.
I've created the HashMap like this
private Map<String, Map<String, String>> contentTypesMap = new HashMap<String, Map<String, String>>();
If the user selects MDX i want to go to the nested HashMap and get the two string values in the 2nd hashmap.
contentTypesMap.put("MDX", new HashMap<String,String>()).put("HELLO1", "FOO");
And this my function for where to use hashmap,
public void getDatabaseSelectionValues (){
resourceType = (String) FileUtilityScreen.contentTypes.getSelectedItem();
sqlTableName = contentTypesMap.get(resourceType).get(key);
languageSelected = FileUtilityScreen.languagesFromDataBase.get(
FileUtilityScreen.languageDropDown.getSelectedItem());
}
with your exemple, if the selection is "MDX", you get your second map by doing
HashMap<String,String> selection = contentTypesMap.get("MDX")
then you can use entrySet() method to retrieve all entries of your map and iterate over entries.
See the documentation from Map.put(K key, V value)
#return the previous value associated with key, or
null if there was no mapping for key
So calling contentTypesMap.put("MDX", new HashMap<String,String>()) will always return the value that is not in the contentTypesMap map anymore.
Solution :
HashMap<String, String> map = contentTypesMap.get("MDX");
if (map == null) {
map = new HashMap<>();
contentTypesMap.put("MDX", map);
}
map.put("HELLO1", "FOO");
Or you can use guava Multimap instead.
Is the HELLO1 and FOO meant to be the value for MDX?
You are not putting it correct. Try this:
Map<String, String> value = new HashMap<String, String>();
value.put("HELLO1", "FOO");
contentTypesMap.put("MDX", value);
I'd also consider wrapping this outer map in its own object as it may get complex.
What I meant by a wrapper class in my comment above was something along these lines, to make it more readable. I'm assuming the key in the outer map is the table name, and in the inner map is the column name and data type for that column.
public class DatabaseTables {
private Map<String, Map<String, String>> dbTables = new HashMap<String, Map<String, String>>();
public void addColumn(String tableName, String column, String type){
Map<String, String> columns = dbTables.get(tableName);
if(dbTables.get(tableName) == null) {
columns = new HashMap<String, String>();
}
columns.put(column, type);
dbTables.put(tableName, columns);
}
public Map<String,String> getColumnsForTable(String tableName){
return dbTables.get(tableName);
}
}
That still feels ugly though. I think a better solution than this, would be to have a class called Table, with a string property for the table name, and a map for its columns and data types.
And then, elsewhere, you could have a list or a set of type Table.
Edit: Or, in Table.java, rather than a map for the columns, you could have a list or set of type Column. And then create a column class with two fields, one for the column name and the other for the type.
I have this ArrayList
public ArrayList<HashMap<String, String>> xmlFileNames = new ArrayList<>();
and I want to convert this to:
HashMap<String, String> comparemap2 = new HashMap<>();
What I want is: I want all the Items inside the ArrayList and want to put them into the HashMap
My HashMap looks like:
KEY VALUE
job_id 032014091029309130921.xml
job_id 201302149014021492929.xml
job_id 203921904901920952099.xml
EDIT:
Later I want to compare this map with an existing map:
Properties properties = new Properties();
try {
properties.load(openFileInput("comparexml.kx_todo"));
} catch (IOException e) {
e.printStackTrace();
}
for (String key : properties.stringPropertyNames()) {
compareMap.put(key, properties.get(key).toString());
}
HashMap<String, String> oldCompareMap = new HashMap<>();
for (HashMap key : xmlFileNames) {
oldCompareMap.putAll(key);
}
isEqualMaps(oldCompareMap, compareMap);
I only want to compare, if the filename exists in the compareMap. If not, than add it to the xmlFileName Map
I've looked up in StackOverFlow, how I can convert ArrayList to HashMap. But the other Threads treat data types like Item or Product.
I hope you can help me!
Kind Regards
Given...
public ArrayList<HashMap<String, String>> xmlFileNames = new ArrayList<>();
then something like this should do it.
HashMap<String, String> nhm = new HashMap<>();
for (HashMap xmlFileHm : xmlFileNames ) {
nhm.putAll(xmlFileHm);
}
but be aware if you have duplicate keys in your hashmaps they will get overwritten.
You should also think about coding to interfaces. Take a look at Map and List rather than typing your collections to implementations (ArrayList and HashMap). Take a look at this thread which is quite interesting What does it mean to "program to an interface"?
Depending on what you are trying to do as well you might consider a MultiMap as this might server your purposes better
Edit After update to the question...
A multimap would be better here with one key and multiple values. Although arguably if the key never changes then you could just store the values in a list. For multiamps you can use Google's guava library or do one yourself. For example (not checked for compilation errors as Im doing this from my head)
Map<String, List<String>> m = new HashMap<>();
if (m.containsKey("key")) {
m.get("key").add("new value");
}
else {
List<String> l = new ArrayList<>();
l.add("new value");
m.put("key", l);
}
You can create a new HashMap, then iterate through the list and put all elements from the map from the list to the main map.
List<Map<String, String>> list = new ArrayList<>();
Map<String, String> map = new HashMap<>();
for (Map<String, String> mapFromList : list) {
map.putAll(mapFromList);
}
You can try something like this..
ArrayList<HashMap<String, String>> xmlFileNames = new ArrayList<>();
HashMap<String, String> comparemap2 = new HashMap<>();
for(HashMap<String, String> i:xmlFileNames){
comparemap2.putAll(i);
}
You may need to consider the case of duplicate keys. else they will get override.
Create a new map and put All each element of arrayList to the map.
But in that case if you have same keys in two element of arrayList (hashmap) then it will override the previous one.
I am trying to create a dictionnary in a <K, List<V>> format.
private static Map<String, Collection<String>> dict = new HashMap<String, Collection<String>>();
Using new HashMap<>(); or new HashMap<String, ArrayList<String>>(); throws incompatible data types error
I need a dictionary similar to the one below.
a: apple, ajar, axe, azure
b: ball, bat, box
d: dam, door, dish, drown, deer, dare
u: urn, umbrella
y: yolk
To do this, I worte below code. put() returns incompatible paramters compilation error. What is the right way to use put() for this example?
dict.put("a", "apple");
dict.put("a", "ajar");
.
.
.
dict.put("u", "umbrella");
dict.put("y", "yolk");
You need to place a List as the value to the map, for example:
List<String> listA = Arrays.asList("apple", "ajar", "axe", "azure");
dict.put("a", listA);
Alternatively, you can use guava Multimap which allows more than one value to be mapped to a given key.
This is because you need to put an arrayList in the value as your Map declaration is Map<String, Collection<String>> so it cannot take Map<String, String>.
ArrayList<String> list = new ArrayList<String>();
list.add("apple");
dict.put("a",list );
As per java 7 you can do it using diamond operator so you can create a map as,
List<String, List<String>> = new ArrayList<>();
What you need is this;
List al = new ArrayList<String>();
al.add("apple");
al.add("ajar");
HashMap<String, List<String>> hm = new HashMap<String, List<String>>();
hm.put("a", al);
System.out.println(hm.get("a"));
This is because, when you use;
private static Map<String, Collection<String>>
You need a Collection like a List. DoNOT insert Objects as Strings
You can only follow the definition you have done :
Map<String, Collection<String>> implies you use dict.put(a,b) with a being a String and b a Collection.
You're trying to put a String as a value that's your problem.
You may want to do something like that :
Collection col = dict.get("a");
if (col == null) {
col = new ArrayList();
}
col.add("apple");
dict.put("a",col);
I would first change the type of the dictionary to
private static Map<Character, ArrayList<String>> dict = new HashMap<>();
It'll allow easier putting of array lists as generics are not covariant.
For each letter, create:
ArrayList<String> myList=new ArrayList<>();
and put() it to dict with
dict.put(myList);
Then you can add words with:
dict.get(letter).put(word);
Your exact need is MultiMap feature of apache-commons
MultiMap dict = new MultiHashMap();
dict.put("a", "apple");
dict.put("a", "ajar");
.
.
.
dict.put("u", "umbrella");
dict.put("y", "yolk");