Note: PrintingContainers.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

import java.util.*;
public class PrintingContainers
{
  static Collection fill(Collection c) {
    c.add("dog");
    c.add("dog");
    c.add("cat");
    return c;
  }
  
  static Map fill(Map m) {
    m.put("dog","Bosco");
    m.put("dog","Spot");
    m.put("cat","Rags");
    return m;
  }

  public static void main(String[] args) {
    System.out.println(fill(new ArrayList()));
    System.out.println(fill(new HashSet()));
    System.out.println(fill(new HashMap()));
  }
};