What is LinkedHashMap and what is it used for?

Question

When I was going through a example code which has ListViews I came up with LinkedHashMap. What is a LinkedHashMap and where can we use it and how? I went through several articles but did not understand fully. Is it necessary when creating ListView. What is the connection between ListViews and LinkedHashMaps? Thank you.

Answer 1

For Simplicity, let us understand what is the difference between HashMap and LinkedHashMap.

HashMap: It gives output in Random orders means there is no proper sequence how we have inserted values.

whereas

LinkedHashMap: It gives output in sequential order.

Let us see a small example: with HashMap

    // suppose we have written a program
    .
    .
    // now use HashMap
    HashMap map = new HashMap();  // create object
    map.put(1,"Rohit");          // insert values
    map.put(2,"Rahul");
    map.put(3,"Ajay");

    System.out.println("MAP=" +map); //print the output using concatenation


    //So the output may be in any order like we can say the output may be as:
    Map={3=Ajay,2=Rahul,1=Rohit}

but this is not the case in LinkedHashMap Just replace the "HashMap" with "LinkedHashMap" in the above code and see it will display the output in Sequential order like 1=Rohit will be displayed first then the others in sequence.

Answer 2

The docs are here. But its basically a HashMap that also has a linked list, so you can have a consistently ordered iteration through it. Note that this means removals may be O(n) time because you need to remove it from both data structures.