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Java Java Data Structures - Retired Exploring the Java Collection Framework Maps

Matthew Francis
Matthew Francis
6,967 Points
Posted by Matthew Francis
Matthew Francis
6,967 Points

How can this interface(Maps) be casted into another interface(List)?

//Treet is a class by the way

       Treet[] treets = {treet, secondTreet};
        Map<String, List<Treet>> treetsByAuthor = new HashMap<String, List<Treet>>();

        for(Treet innerTreets : treets){
         List<Treet> authoredTreets = treetsByAuthor.get(innerTreets.getAuthor());
       ..other code...
       }

Could someone explain to me why the Map is able to be casted to List? shouldn't the console say "Object(Maps.get(), retruns an Object) cannot be casted to List";

And what is the purpose of casting it? what good does it do?

1 Answer

Alexander Nikiforov
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Alexander Nikiforov
Java Web Development Techdegree Graduate 22,175 Points
Alexander Nikiforov
.a{fill-rule:evenodd;}techdegree seal-36
Alexander Nikiforov
Java Web Development Techdegree Graduate 22,175 Points

First of all there is no casting at all.

Map<String, List<Treet>> is mapping String to List<Treet>, somewhat like this:

{ key -> value}

{ "string 1" -> { Treet1, Treet2}, "string 2 " -> { Treet3, Treet4} }

When you execute get("string 1") method: it is looking for keys. In this particular case, when he finds key with "string 1", he returns List<Treet> in this case will be {Treet1, Treet2}

For more on casting please take a look somewhat here, e.g. https://howtoprogramwithjava.com/java-cast/