OnJava8-Examples/arrays/ArrayOfGenerics.java

// arrays/ArrayOfGenerics.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
// It is possible to create arrays of generics.
import java.util.*;

public class ArrayOfGenerics {
  @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    List<String>[] ls;
    List[] la = new List[10];
    ls = (List<String>[])la; // "Unchecked" warning
    ls[0] = new ArrayList<>();
    // Compile-time checking produces an error:
    //! ls[1] = new ArrayList<Integer>();

    // The problem: List<String> is a subtype of Object
    Object[] objects = ls; // So assignment is OK
    // Compiles and runs without complaint:
    objects[1] = new ArrayList<>();

    // However, if your needs are straightforward it is
    // possible to create an array of generics, albeit
    // with an "unchecked" warning:
    List<BerylliumSphere>[] spheres =
      (List<BerylliumSphere>[])new List[10];
    for(int i = 0; i < spheres.length; i++)
      spheres[i] = new ArrayList<>();
  }
}