lambdaj errors explained

java.lang.IllegalAccessException: Class ch.lambdaj.proxy.ProxyIterator can not access a member of class XYZ with modifiers "public" at line forEach(products_byname).cropDefects();
assuming List products_byname = new List();
means that class Product should be made public.

java.lang.IllegalAccessException: Class ch.lambdaj.proxy.ProxyIterator can not access a member of class XYZ with modifiers "" at line forEach(products_byname).cropDefects();
means that method cropDefects() should be made public.

java.lang.RuntimeException: It is not possible to create a placeholder for class: java.lang.reflect.Field at line List l_null_fields = select(l_fields,having(on(Field.class).get(this),nullValue())); means that you encountered a Limitation of lambdaj caused by the Java language specification because Field is a final class.


java.lang.IllegalArgumentException: forEach() is unable to introspect on an empty iterator. Use the overloaded method accepting a class instead at line forEach(defects).updateBugActivity(stateArray); means that you encountered a Limitation of lambdaj caused by the Java language specification because defects is an empty list.

Java program using lambdaj

A while ago I wrote a program in Java doing some filtering and displaying without using for while statements and an other one using haskell functional language.

Today, I did the same thing (i.e. no for or while statements), but using lambdaj java library.

There is still a for statement but if you make country as an object and not a string then you can use lambdaj features.

1 Have a list of people belonging to various countries:
European people: [Eric from France, Martine from France, John from Great-Britain, Martha from Great-Britain, Carine from France, Gerd from Deutschland, Giuseppe from Italia, Martha from Deutschland]

2 Get the different countries:
European countries: [Great-Britain, France, Italia, Deutschland]

3 List the people that belong to each country:
People from: [France]: [Carine from France, Eric from France, Martine from France]
People from: [Deutschland]: [Gerd from Deutschland, Martha from Deutschland]
People from: [Great-Britain]: [John from Great-Britain, Martha from Great-Britain]
People from: [Italia]: [Giuseppe from Italia]

Here is the code:

   1 import static ch.lambdaj.Lambda.*;

   2 import ch.lambdaj.group.*;

   3 import java.util.List;

   4 import java.util.Arrays;

   5 import java.util.Set;

   6 

   7 

   8 public class Europeans1 {

   9 

  10         static List<People> l_europeans = Arrays.asList(

  11                         new People("Eric", "France"),

  12                         new People("Martine", "France"),

  13                         new People("John", "Great-Britain"),

  14                         new People("Martha", "Great-Britain"),

  15                         new People("Carine", "France"),

  16                         new People("Gerd", "Deutschland"),

  17                         new People("Giuseppe", "Italia"),

  18                         new People("Martha", "Deutschland"));

  19 

  20         public static void main(String[] args) {

  21                 System.out.println("European people: "+l_europeans);

  22                 Group<People> g_countries = Groups.group(l_europeans, 

  23                         Groups.by(on(People.class).getNationality()));

  24                 Set<String> set_countries = g_countries.keySet();

  25                 System.out.println("European countries: "+set_countries);

  26                 for(String s_country:set_countries) {

  27                         print_inhabitants(s_country);

  28                 }

  29         }

  30 

  31         static void print_inhabitants(String s_country)  {

  32                 System.out.print("People from "+s_country+": ");

  33                 List<People> l_inhabitants = select(l_europeans,

  34                         having(on(People.class).getNationality(),

  35                         org.hamcrest.Matchers.equalTo(s_country)));

  36                 forEach(l_inhabitants).printFirstName();

  37                 System.out.println("");

  38         }

  39 }