Lambda (Java 8) - How to sort and filter array

In this article, I'm going to explain about the way we can sort and filter an array using lambda expressions.

TASK 1

We will assume that, we have a list of students as follows:

 List<Student> students = Arrays.asList(
         new Student("Tharanga", "UOC", 22),
         new Student("Ruwan", "UOM", 24),
         new Student("Ruvini", "UOK", 20),
         new Student("Randika", "TOUC", 25),
         new Student("Amara", "UOR", 23)
);

In this scenario, we will assume that we have to sort the students by name:

Initially, how we cater this without lambda expression:

Collections.sort(students, new Comparator<Student>() {
       @Override
       public int compare(Student o1, Student o2) {
           return o1.getName().compareTo(o2.getName());
       }
});

Now will cater this requirement with lambda expressions.

 Collections.sort(students, (o1, o2) -> o1.getName().compareTo(o2.getName()));

Note: Since 'Comparator' interface is a functional interface we can sort the object list as above.

TASK 2

In here, we will assume that we need to filter the students, name starting with 'R'

Without lambda expression we can achieve this as below.

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class StudentApplication {
    public static void main(String[] args) {
        List<Student> students = Arrays.asList(
                new Student("Tharanga", "UOC", 22),
                new Student("Ruwan", "UOM", 24),
                new Student("Ruvini", "UOK", 20),
                new Student("Randika", "TOUC", 25),
                new Student("Amara", "UOR", 23)
        );

        Collections.sort(students, (o1, o2) -> o1.getName().compareTo(o2.getName()));

        printConditionally(students, new Condition() {
            @Override
            public boolean test(Student student) {
                return student.getName().startsWith("R");
            }
        });

    }

    private static void printConditionally(List<Student> students, Condition condition) {
        for (Student student: students) {
            if(condition.test(student)) {
                System.out.println(student);
            }
        }
    }
}

interface Condition {
    boolean test(Student student);
} 

How we achieve this using lambda expression.

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.function.Predicate;

public class StudentApplication {
    public static void main(String[] args) {
        List<Student> students = Arrays.asList(
                new Student("Tharanga", "UOC", 22),
                new Student("Ruwan", "UOM", 24),
                new Student("Ruvini", "UOK", 20),
                new Student("Randika", "TOUC", 25),
                new Student("Amara", "UOR", 23)
        );

        Collections.sort(students, (o1, o2) -> o1.getName().compareTo(o2.getName()));

        printConditionally(students, student -> student.getName().startsWith("R"));

    }

    private static void printConditionally(List<Student> students, Predicate<Student> predicate) {
        for (Student student: students) {
            if(predicate.test(student)) {
                System.out.println(student);
            }
        }
    }
}

Note: Java 8, OFTB support many functional interfaces such as "Predicate". You can find more on here.

Note: Find more information about funcational interfaces here.