Extracting map values as ArrayList from LinkedHashMap

Question

I created a simple class MapExtension to accomodate passing of 4 values to a listview adapter and used LinkedHashmap to add the ArrayList of MapExtension.

public class MapExtension {
private String studname;
private String studnumber;
private String schedule;
public MapExtension(String studname, String studnumber, String schedule) {
 this.studname = studname;
 this.studnumber= studnumber;
 this.schedule= schedule;
}
public String getStudname () {
 return studname;
}
public String getStudnumber() {
 return studnumber;
}
public String getSchedule() {
 return schedule;
}

}

Whenever I try to extract the ArrayList<MapExtension> from LinkedHashMap returning Collections, I get these errors from different trials(in comments):

ListViewAdapter(Context context, LinkedHashMap<Integer, ArrayList<MapExtension>> mValues) {
 super(context, R.layout.listview_layout, mValues.keySet().toArray());
 this.context = context;
 //java.lang.ClassCastException: java.util.HashMap$Values 
 //cannot be cast to java.util.ArrayList
 ArrayList mValues = (ArrayList) mValues.values();
 // says incompatible as it will become ArrayList<ArrayList<MapExtension>>
 ArrayList<MapExtension> mValues = new ArrayList<>(mValues.values()); 
}

How can I successfully retrieve it and place in its compatible type?

Thanks in advance.

Answer 1

Every value of mValues is an ArrayList<MapExtensions>, and values returns an Collection<V>, so you should be able to do..

Collection<ArrayList<MapExtensions>> localVar = mValues.values();

If you want to flatten your nested arrays, you can look at the streams flatMap() method. Here is one person's blog on that. BTW, Kotlin collections have a flatMap() extension method that works on collections.

• Mr. Nacho: This leads me to Usage of API documented as @since 1.8+. I can't make it work yet as Android Studio does not allow me to use version 1.8 but only until 1.7.

Answer 2

If I understand you correcly, given LinkedHashMap<Integer, ArrayList<MapExtension>> mValues, you want ArrayList<MapExtension> flattening the values of the input map. With Java 8, you could easily do it like this:

ArrayList<MapExtension> extensions = mValues .values() .stream() .collect(ArrayList::new, ArrayList::addAll, ArrayList::addAll);

A small tip: you should program against interfaces so instead of using ArrayList in your types, think about using List so that you are not tied to one particular concrete implementation everywhere.

• charles-allen: Given that the SO would likely be better off using List<MapExtension>, is it better to flapMap(List::stream) then collect(Collectors.toList())?
• Marimuthu Madasamy: Yes, it does get simpler in that case.
• Mr. Nacho: Thanks for the tip. I'm having Usage of API documented as @since 1.8+ by using that code and AS only allows me to use 1.7.