Extracting elements of an ArrayList<String> as part of a Hashmap

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I have the following code:

for (Map.Entry < Integer, ArrayList < String >> entry: data.entrySet()) {
    Integer key = entry.getKey();
    ArrayList < String > value = entry.getValue();
    for (String aString: value) {
        System.out.println("key : " + key + " value : " + aString);
    }
}

Hashmap is declared as

Map<Integer, List<String>> data = new HashMap<>();

before filling it via a loop and put methods.

I now need to iterate the ArrayList of strings or better, retrieve specific elements by index .. for manipulation. I have researched as far as I can and seemed to have found the answer, with the above code. It was copied from an answer given here some time back.

However, when I try to use it complains of "Incompatible Types" with the top line underlined in red up to the colon.

  • Required: Entry <Integer,java.util.List<java.lang.String>>
  • Found: Entry <Integer,java.util.ArrayList<java.lang.String>>

because you have declared value type in Hashmap as List<String> and you have given ArrayList<String> as value type Map.Entry in for loop.

change Map<Integer, List<String>> data = new HashMap<>(); to Map<Integer, ArrayList<String>> data = new HashMap<>();

Java arraylist get method, get() method is used to get the element of a specified position within the list. Package: java.util. Java Platform: Java SE 8. Syntax: get(int index) Parameters: Name. Return Value: The element at the specified position in this list. Throws: Pictorial presentation of ArrayList.get() Method. Example: ArrayList.get Note: Before going through this ArrayList Get Extract Elements Java, it is advised to read first ArrayList Introduction where all the methods, class signature, constructors and history are given. 4 programs are given on ArrayList.


In your case you have the List interface and the ArrayList implementation, you have correctly used the List type in the declaration of the map, but you passed to declaring a implementaion (ArrayList) in the loop.

The problem arrises because of the statement:

Map<Integer, List<String>> data = new HashMap<>();

The empty diamond notation leaves open many options for a implementation of the List<String>. It can be ArrayList, LinkedList etc. So you can't be sure that the compiler will use ArrayList implementation.

To favour code extendibility is advisable to do what is known as programming against interfaces. It is always good to maintain the most generic type as long as you can to avoid coupling your code to a specific implementation, that is you will specifiy the implementation when you need to perform some concrete operation on it.

How to extract elements from ArrayList specifically?, You may use a HashMap<String, String> which stores Key/value The user enter a question, check if it is in the map, if yes print the answer, if not  ArrayList.get (int index) method returns the element at the specified position 'index' in the list. 1.1. get () method syntax. indexOf () method. public Object get ( int index ); 1.2. get () method parameter. index – index of the element to return. A valid index will always be from 0 (inclusive) to size of arraylist (exclusive). For example


Try below code:

for (Map.Entry<Integer, List<String>> entry : data.entrySet()) {
       Integer key = entry.getKey();
       List<String> value = entry.getValue();
       for(String aString : value){
            System.out.println("key : " + key + " value : " + aString);
            }

How to compare two ArrayList for equality in Java?, class, this method accepts a list object as a parameter, compares it with the current object, in case of the match it returns true and if not it returns false. The ArrayList.get () method is used to get the element of a specified position within the list. Package: java.util. Java Platform: Java SE 8. get (int index) index of the element to return. The element at the specified position in this list. IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size ())


Program against the interface rather than against an implementation.

The Map's type is Integer as a key and List<String> as a value. Thus you can loop the entries only with the very same structure.

for (Map.Entry < Integer, List < String >> entry: data.entrySet()) {
    Integer key = entry.getKey();
    List <String> value = entry.getValue();
    for (String aString: value) {
        System.out.println("key : " + key + " value : " + aString);
    }
}

Use Array Lists in Java, How do you remove an element from an ArrayList in Java? In this tutorial we will see how to get a sublist from an existing ArrayList. We will be doing it using the subList method of ArrayList class. Here fromIndex is inclusive and toIndex is exclusive. There are few important points regarding this method which I have shared at the end of this post. The subList method returns a list therefore to


ArrayList get() method, Learn how to get the element from an ArrayList. We will be using ArrayList.get() method to get the object at the specified index of the arraylist. I am trying to loop through an arraylist and gradually remove an element every 3 indices. Once it gets to the end of the arraylist I want to reset the index back to the beginning, and then loop through the arraylist again, again removing an element every 3 indices until there is only one element left in the arraylist.


Java.util.ArrayList.get() Method, ArrayList.get(int index) method returns the element at the specified position in this list. Declaration. Following is the declaration for java.util. Java.util.ArrayList.get() Method - The java.util.ArrayList.get(int index) method returns the element at the specified position in this list.


How to extract some of the elements from given list in java , Program to extract elements from a list in java. import java.util.ArrayList; import java.util.List; public class ExArrayExtract { public static void  The list may contain several elements, so the get method takes an argument : the index of the element you want to retrieve. If you want the first one, then it's 0. The list contains Car instances, so you just have to do