Java 8 -Predicate

In Java 8 , you may notice here and there some new interfaces popping out making us so unfamilar with. Well , Don't be panic. Actually, they are just some normal java syntaxs we learned before with a new name. In this article , I will tell one interface called predicate<T>.

Predicate is just a java interface with one method with return type "boolean". That's it . So , do we really need it  in java api rather than we just randomly create a interface with one method with return type "boolean". Yes, we can , we don't need it at all in java api ,but since java api helps us creating one . So we just use it.

public interface Predicate<T> {
    boolean test(T t);
}

So what is it used for ?

Well, it help us to filter some result based on the Class.  Let's see an example below.

public class MainMethod {
 public static void main(String[] args)  {

            // filter the result == 123
List<String> result123 = filter(new ArrayList<String>(), 

                             (String input) -> input.equals("123")); 

                 
 }
 
 
 public static  List<String> filter(List<String> list, Predicate<String> filter) {
  List<String> result = new ArrayList<>();
  for(String item: list)  {
      if( filter.test(item) )  {
   result.add(item);
             }
  }
       return result;
  }
}

This program filter those results with a (String == 123). So we can call filter(...) function with one paramter called Predicate,  then we specify the filter criteria when we call this function.

Alright , That's so simple to use. Well you may wonder why do we need predicate here rather than use boolean to write this function like filter (List<String> list, boolean filter) ?

Because predicate is to filter & judge a Class's field  not just a simple field :

 public static void main(String[] args)  {
  
            List<Apple> result123 = filter(new ArrayList<Apple>(), 

                     (Apple input) -> input.weight() > 100);

     List<Apple> result123 = filter(new ArrayList<Apple>(), 

                     (Apple input) -> input.color().contain("red"));
  
 }
 
 
 public static  List<String> filter(List<Apple> list, Predicate<Apple> filter) {
  List<Apple> result = new ArrayList<>();
  for(String item: list)  {
     if( filter.test(item) )  {
   result.add(item);
     }
  }
   return result;
  }

Moreover, it is more readable for you as well if you use predicate<Apple> .