learning Java and have figured out how to store a hashmap in an array. But I can't figure out how to get to the stored data. Here is a simplified version of what I'm doing. I've got as far as displaying the specific array items, but how do I access the hash map stored in the array?
import java.util.*;
public class HelloWorld {
public static void main(String[] args) {
Map<String, String> custOrder = new HashMap<String, String>();
List ordersPlaced = new ArrayList();
custOrder.put("colour", "blue");
custOrder.put("manu", "bmw");
custOrder.put("body", "4x4");
ordersPlaced.add(custOrder);
custOrder = new HashMap();
custOrder.put("colour", "green");
custOrder.put("manu", "merc");
custOrder.put("body", "saloon");
ordersPlaced.add(custOrder);
System.out.println(ordersPlaced.get(0).toString());
}
}
Hope that makes sense. Thanks in advance
Neil
You're already accessing it.
In order to get the iterate on the map's items, you can:
ordersPlaced.get(0).forEach((key, value) -> {
System.out.println("Key is: " + key + ", Value is: " + value);
});
Or, earlier to Java 8, something like:
for (Map.Entry<String, String> entry : ordersPlaced.get(0).entrySet()) {
String key = entry.getKey();
String value = entry.getValue();
}
Please note that you should avoid using raw type list. Instead, you should have:
List<Map<String, String>> ordersPlaced = new ArrayList<>();
And then:
Map<String, String> m = ordersPlaced.get(0);
You know it already.
You can get back the stored map by writing
Map<String, String> placedCustOrder = ordersPlaced.get(0);
And avoid using raw types while using List. Declare your list as
List<Map<String, String>> ordersPlaced = new ArrayList<>();
I would like to know how to access the colour of the data stored in the array at location 0
Since you got the map as I said in the line 1
Map<String, String> placedCustOrder = ordersPlaced.get(0);
String colorVal = placedCustOrder.get("colour");
I strongly suggest you to look through Map documentation before proceeding further.
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
I have a populated hashmap in the form Hashmap<String , ArrayList<Double>>. There are many cases in which the values (Arraylist) are the same. What I want tro do is if the value is same , the two separate entries should be changed to one with a change in the key. For eg:
HashMap<String, ArrayList<Double>> map = new HashMap<>();
ArrayList<Double> arr = new ArrayList<>();
arr.add(1);
arr.add(2)
map.put("a",arr);
map.put("b",arr);
map.put("c",arr);
This should result in a hashmap with a single entry :
{a, b , c=[1,2]}
I have tried to solve this for hours but am still unable to do so. A million thanks in advance.
Creating your custom HashMap by overriding put() should work, performance-wise it is not really efficient though.
Something like this:
public class CustomMap extends HashMap<String, ArrayList<Double>> {
#Override
public ArrayList<Double> put(String key, ArrayList<Double> value) {
for (Entry<String, ArrayList<Double>> entry : entrySet()) {
if (entry.getValue().equals(value)) {
String oldKey = entry.getKey();
remove(oldKey);
String newKey = oldKey + ", " + key;
return super.put(newKey, value);
}
}
return super.put(key, value);
}
}
To use it, just replace the following:
HashMap<String, ArrayList<Double>> map = new HashMap<>();
With:
CustomMap map = new CustomMap();
Maps relate keys with values. So even if the values are the same, you can't combine these entries because the keys are different
Here Below, I'm having one list of HashMap's and I wanna store all of these map's in single key of redis but I'm not getting any method to store all these maps in single key. Please help me in this problem.
Jedis jedis = new Jedis("localhost");
List <HashMap<String, String>> listOfMaps = new ArrayList<HashMap<String, String>>();
listOfMaps.add(new HashMap<String,String>);
listOfMaps.add(new HashMap<String,String>);
listOfMaps.add(new HashMap<String,String>);
listOfMaps.add(new HashMap<String,String>);
.
.
.
and so on lets take upto 10 values
Now, I wanna store these maps in a key like this:
for(int i=0;i<listOfMaps.size();i++){
jedis.hmset("mykey",listofMaps[i]);
}
But in his case hmset overwrites all older values to write new values.
Please tell me any alternative to store all these map values in single key mykey.
You can use Multimap object provided by Redisson framework. It allows to store multiple values per map key as list or set. Here is the example:
RMultimap<String, Integer> multimap = redisson.getListMultimap("myMultimap");
for (int i = 0; i < 10; i++) {
myMultimap.put("someKey", i);
}
// returns Redis list object
RList list = myMultimap.get("someKey");
You can use something like below
public static void main(String[] args) {
Map<String, List<String>> map = new HashMap<String, List<String>>();
List<String> valSetOne = new ArrayList<String>();
valSetOne.add("ABC");
valSetOne.add("BCD");
valSetOne.add("DEF");
List<String> valSetTwo = new ArrayList<String>();
valSetTwo.add("CBA");
valSetTwo.add("DCB");
map.put("FirstKey", valSetOne);
map.put("SecondKey", valSetTwo);
for (Map.Entry<String, List<String>> entry : map.entrySet()) {
String key = entry.getKey();
List<String> values = entry.getValue();
System.out.println("Value of " + key + " is " + values);
}
}
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 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.