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Guava: Function, Predicate and Ordering



Function and Predicate are just interfaces but plays key roll in many use-cases.

Function: Determines an output value based on an input value
Predicate: Determines a true or false value for a given input.
Ordering: It's a comparator with added methods to support common functions.

Assume we have some User Object List
User{firstName=userA, lastName=B, salary=1234567.0}
User{firstName=userB, lastName=C, salary=1234577.0}
User{firstName=userB, lastName=D, salary=1234777.0}
User{firstName=userE, lastName=A, salary=1237777.0}
User{firstName=userD, lastName=A, salary=1237778.0}
User{firstName=userA, lastName=D, salary=1237788.0}


User Function Class
public enum UserFunctions implements Function<User, String>{
 FIRSTNAME{
  @Override
  public String apply(User user){
   return user == null ? "" : user.getFirstName();
  }
 },

 LASTNAME{
  @Override
  public String apply(User user){
   return user == null ? "" : user.getLastName();
  }
 },

 FIRSTNAME_LASTNAME{
  @Override
  public String apply(User user){
   return user == null ? "" : user.getFirstName() + " " + user.getLastName();
  }
 };
}


User Predicates Class
public enum UserPredicates implements Predicate<User>{
 FIRSTNAME_END_WITH_VOWEL {
  @Override
  public boolean apply(User user) {
   if(user == null){return false;}
   String firstName = user.getFirstName();
   return asList('a', 'e', 'i', 'o', 'u')
    .contains(firstName.toLowerCase().charAt(firstName.length()-1));
  }
 };
}


User Ordering Class
public class UserOrdering { 
 public static Ordering<User> BY_FIRST_NAME = 
  Ordering.natural().onResultOf(UserFunctions.FIRSTNAME);
  
 public static Ordering<User> BY_LAST_NAME = 
  Ordering.natural().onResultOf(UserFunctions.LASTNAME);
  
 public static Ordering<User> BY_FIRSTNAME_THEN_LASTNAME = 
  BY_FIRST_NAME.compound(BY_LAST_NAME);
 
 public static Ordering<User> BY_SALARY = 
  Ordering.natural().onResultOf(
   new Function<User, Float>(){
    @Override
    public Float apply(User user){
     if(user == null){return 0F;}
     return user.getSalary();
    }
   }
  );
}


One of the best part about the predicate, function and ordering is usability. It separate the repetitive code in well encapsulated classes which can be used in many diff. places. So you don't have to write the boilerplate code each time and it make your code much smaller and readable.

Here are few useful scenarios

User with maximum salary 
User user = Collections.max(userList, UserOrdering.BY_SALARY);

Output:->
User{firstName=userA, lastName=D, salary=1237788.0}


Retrieve the list of "firstName lastName" from the user list
Collection<String> userList = Collections2.transform(
                          userList, UserFunctions.FIRSTNAME_LASTNAME);

Output:->
userA B
userB C
userB D
userE A
userD A
userA D


Divide the user list in group of 3 and extract the second group
int pageSize = 3;
int pageIndex = 1;
List<User> userList = Lists.partition(userList, pageSize).get(pageIndex); 

Output:->
User{firstName=userE, lastName=A, salary=1237777.0}
User{firstName=userD, lastName=A, salary=1237778.0}
User{firstName=userA, lastName=D, salary=1237788.0}


Filter the user where first name ends with vowel
Predicate<User> allPredicate = Predicates.and(
 Predicates.<User>notNull(), 
 UserPredicates.FIRSTNAME_END_WITH_VOWEL);
  
Collection<User> userList = Collections2.filter(userList, allPredicate);

Output:->
User{firstName=userA, lastName=B, salary=1234567.0}
User{firstName=userE, lastName=A, salary=1237777.0}
User{firstName=userA, lastName=D, salary=1237788.0}


Sort the user list with first name then last name
Collections.sort(userList, UserOrdering.BY_FIRSTNAME_THEN_LASTNAME);

Output:->
User{firstName=userA, lastName=B, salary=1234567.0}
User{firstName=userA, lastName=D, salary=1237788.0}
User{firstName=userB, lastName=C, salary=1234577.0}
User{firstName=userB, lastName=D, salary=1234777.0}
User{firstName=userD, lastName=A, salary=1237778.0}
User{firstName=userE, lastName=A, salary=1237777.0}




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