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Java Java Data Structures 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/