Can't figure out how to retrieve objects in a map - java

I have a
Map<String, List<String>> myMap;
I'm having a hard time retrieving the objects in the List:
For example I can retrieve from the Map
String myString = myMap.get("Toyota").toString();
but how do I retrieve from the List?
Lets say that the List contains all the models for each car brand. How can I retrieve only the item 0 in that that List?
Thanks


myMap.get("Toyota").get(0) - That should do it.
The reason is that you ask myMap the value that is pointed by the key "Toyota". As you defined in your definitions, the values are always of type List<String> which means that the call myMap.get("Toyota") returns a List<String> object. When you do a get(0) call you actually call the get method of List<String>


You need to retrieve the List and then its contents :
//myMap.get(key).get(listIndex);
// myMap.get("Toyota"); // gets the List object
myMap.get("Toyota").get(0);
Hence , you need to use Map#get() and then List#get().


The statement myMap.get("Toyota") will return the List type which is value for the key Toyota. You can get the first element of list using myMap.get("Toyota").get(0)


Basically when you use a Map and want to retrieve values based on certain key, you can iterate over the Map and retrieve them as follows :
for (final Entry<String, List<String>> entry : myMap.entrySet()) {
String key = entry.getKey();
List<String> value = entry.getValue();
}
As you can see, inside the for loop, you have both key and the value. You can do the further processing here after.

Related

Java: How to properly remove a value from a HashMap NOT based on the key

I have a Singleton class that inside has a HashMap. The HashMap is made of String and Set<String>:
private Map<String, Set<String>> mMap = new HashMap<>();
What I want to achieve?
Remove a given item from all the Set values inside the Map. For example:
mMap.put(keyName, new HashSet<String>())
....
mViewsSwipeStates.get(keyName).add("1");
mViewsSwipeStates.get(keyName).add("2");
mViewsSwipeStates.get(keyName).add("3");
....
//Remove an item from the set
mMap.values().remove("3"); //Does not work
What is the correct way to remove an item from inside the Set?

I'm assuming you want to remove productCode from all the values of the Map (and not just from the value of a specific key).
You have to iterate over all the values of the Map, and remove from each of them the required element :
mMap.values().forEach(v->v.remove(productCode));
This code assumes there are no null values in the Map.
EDIT :
In Java 7 you can write:
for (Set<String> value : mMap.values()) {
value.remove(productCode);
}

Iterate over all map entries, mutating the value for each:
map.forEach((k, v) -> v.remove(productCode)};

using java 8 and lambda expressions
map.forEach((key, value) -> value.remove(productCode));

Unable to retrieve multi line value using map.get()

I have a multi-line value that needs to be processed. I'm using the map.get() method to retrieve that value but it seems to be getting only the last line value.
Here's my code:
map = new LinkedHashMap();
updateMap("BUG", parser, map, bugRec);
map.put(nextBuildIdTagName, nextBuildId); // putting the new value in
String value = (String)map.get(nextBuildIdTagName); // This is where it is not working
nextBuildIdTagName already has a value, and the new value gets inserted as a new line. I need to be able to retrieve the existing value as well as the new value.

So from what I understand you want to store multiple values in the map under single key. Easiest way to do it (without using any external lib with multimap impl) is to create a map of lists like this:
Map<String, List<String>> map = new HashMap<>();
then when you add to map you can do sth like this:
if(!map.containsKey(nextBuildIdTagName)) {
map.put(nextBuildIdTagName, new ArrayList<>());
}
map.get(nextBuildIdTagName).add(nextBuildId);
then to get all the items and iterate over them
for(String value : map.get(nextBuildIdTagName)) {
// do sth to each line
}

In the javadoc for HashMap (of which LinkedHashMap is a subclass), it is clearly stated that: "If the map previously contained a mapping for the key, the old value is replaced". To store multiple values (the lines) for the same key, you'd need to do something like:
Map<String, List<String>> map = new Linked HashMap<>();
And the add to the list (which is the map value) the needes lines. Note: here I assumed your key to be a String, change its type as needed.
Source: http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html#put(K,%20V)

Java - Adding another String value to existing HashMap Key without overwriting?

I was wondering if someone would be able to help with regards to adding another String value to an existing key within a HashMap in Java?
I understand that you can add a Key-Value pair using the this.put("String", "String") method. However, it overwrites the existing value, whereas I would like multiple values stored and paired, with the same key?
Thanks for your help.

What are you hoping to achieve here?
A Map (the HashMap) in your case is a direct "mapping" from one "key" to another value.
E.g.
"foo" -> 123
"bar" -> 321
"far" -> 12345
"boo" -> 54321
This means that if you were to try:
myHashMap.get("foo");
It would return the value 123 (of course, the type of the value you return can be anything you want).
Of course, this also means that any changes you make to the value of the key, it overrides the original value you assigned it, just like changing the value of a variable will override the original one assigned.
Say:
myHashMap.put("foo", 42);
The old value of "foo" in the map would be replaced with 42. So it would become:
"foo" -> 42
"bar" -> 321
"far" -> 12345
"boo" -> 54321
However, if you need multiple String objects that are mapped from a single key, you could use a different object which can store multiple objects, such as an Array or a List (or even another HashMap if you wanted.
For example, if you were to be using ArrayLists, when you are assigning a value to the HashMap, (say it is called myHashMap), you would first check if the key has been used before, if it hasn't, then you create a new ArrayList with the value you want to add, if it has, then you just add the value to the list.
(Assume key and value have the values you want)
ArrayList<String> list;
if(myHashMap.containsKey(key)){
// if the key has already been used,
// we'll just grab the array list and add the value to it
list = myHashMap.get(key);
list.add(value);
} else {
// if the key hasn't been used yet,
// we'll create a new ArrayList<String> object, add the value
// and put it in the array list with the new key
list = new ArrayList<String>();
list.add(value);
myHashMap.put(key, list);
}

You can do like this!
Map<String,List<String>> map = new HashMap<>();
.
.
if(map.containsKey(key)){
map.get(key).add(value);
} else {
List<String> list = new ArrayList<>();
list.add(value);
map.put(key, list);
}
Or you can do the same thing by one line code in Java 8 style .
map.computeIfAbsent(key, k ->new ArrayList<>()).add(value);

Would you like a concatenation of the two strings?
map.put(key, val);
if (map.containsKey(key)) {
map.put(key, map.get(key) + newVal);
}
Or would you like a list of all the values for that key?
HashMap<String,List<String>> map = new HashMap<String,List<String>>();
String key = "key";
String val = "val";
String newVal = "newVal";
List<String> list = new ArrayList<String>();
list.add(val);
map.put(key, list);
if (map.containsKey(key)) {
map.get(key).add(newVal);
}

As others pointed, Map by specification can have only one value for a given key. You have 2 solutions:
Use HashMap<String, List<String>> to store the data
Use Multimap which is provided by 3rd party Google Collections lib

As described in Map interface documentation Map contains a set of keys, so it is not capable of containing multiple non-unique keys.
I suggest you to use lists as values for this map.

Store value as list under map So if key is test and there are two values say val1 and val2 then key will be test and value will be list containing val1 and val2
But if your intention is to have two separate entries for same key, then this is not Map is designed for. Think if you do map.get("key"), which value you expects

You could use Map<String, Collection<String>> but adding and removing values would be cumbersome . Better way is using guava Multimap - a container that allows storing multiple values for each key.

You can't directly store multiple values under a single key, but the value associated with a key can be any type of object, such as an ArrayList, which will hold multiple values. For example:
import java.util.ArrayList;
import java.util.HashMap;
public class HashMapList {
HashMap<String, ArrayList<String>> strings = new HashMap<String, ArrayList<String>>();
public void add(String key, String value) {
ArrayList<String> values = strings.get(key);
if (values == null) {
values = new ArrayList<String>();
strings.put(key, values);
}
values.add(value);
}
public ArrayList<String> get(String key) {
return strings.get(key);
}
public static void main(String[] argv) {
HashMapList mymap = new HashMapList();
mymap.add("key", "value1");
mymap.add("key", "value2");
ArrayList<String> values = mymap.get("key");
for (String value : values) {
System.out.println(value);
}
}
}

it's impossible,because String is immutable if you use the String as the key of map the same key's value has the same hashcode value.

Java Iterate through a HashMap<?, List<?>> and for each Key count how many Values the Key has

I'm having problems on how to tackle this since I know a map has no specific order. I would think that you would iterate over the map's Keys and then check the value count by getting the size of the LinkedList since the Values to the Key are held in a LinkedList I can just call a size or length call for the LinkedList, but my main question is how to get inside the HashMap with an iterator first to do this?

An iterator over all the entries of a map can be done as follows in Java:
for (Map.Entry<Key, Value> entry : map.entrySet()) {
Key k = entry.getKey();
Value v = entry.getValue();
// do something with k,v
}
However, a map can only contain at most one value associated with a key. So, if using a map of lists to associate multiple values, the list would be accessed simply through get.

I suppose you have something like this :
Map<Integer,List<Integer>> map = new HashMap<>();
You can just iterate very simply through all the values you have.
for (List<Integer> values : map.values()){
System.out.println(values.size());
}

How to add HashMap values to List and print the same

I am trying to add map values to list for my scenario as below.
I have select statement which returns n-no of columns and row, I am storing them into List of Hash Map of type String and pass it to some other method to produce an EXCEL file out of the result.
i am unable to see any data in the list
Please advice where i am going wrong.
while (result.next()) {
resultValues.put("PARTC_ID",result.getString("PARTC_ID"));
resultValues.put("FILE_NME",result.getString("FILE_NME"));
resultValues.put("LOC_ID",result.getString("LOC_ID"));
resultValues.put("CRTE_DTE",result.getString("CRTE_DTE"));
resultValues.put("CRTE_BY",result.getString("CRTE_BY"));
value.add(resultValues); resultValues.clear(); System.out.println(value);
}

You're clearing your Map after adding it to the List. The Map references are all thus the same (and empty)... I think you want to make this change -
// resultValues.clear(); // No, if you need another Map... do this
resultValues = new HashMap<String, String>();
Then to iterate your value List try
for (HashMap<String, String> map : value) {
for (String key : map.keySet()) {
System.out.printf("key[%s] = %s\n", key, map.get(key));
}
}

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