Nested generic type : Generics Basics « Generics « Java Tutorial

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3. Generics Basics

A generic type is itself a type and can be used as a type variable. For example, if you want your List to store lists of strings:

List<List<String>> myListOfListsOfStrings;

To retrieve the first string from the first list in myList, you would use:

String s = myListOfListsOfStrings.get(0).get(0);

Working with List of Lists

import java.util.ArrayList;
import java.util.List;

public class MainClass {
  public static void main(String[] args) {
    List<String> listOfStrings = new ArrayList<String>();
    listOfStrings.add("Hello again");
    List<List<String>> listOfLists = new ArrayList<List<String>>();
    listOfLists.add(listOfStrings);
    String s = listOfLists.get(0).get(0);
    System.out.println(s); // prints "Hello again"
  }
}