Lambda expressions allow passing functions as arguments. This document discusses two approaches to filtering a list of Person objects by criteria using lambda expressions. The first approach defines a PersonCriteria interface to test Person objects. A service class uses the criteria to filter and print matching persons. The second approach uses lambda expressions with stream aggregate operations like filter and map to directly operate on the person list.

1. LambdaExpression
What is Lambda Expression?
Lambda expression is a functional programming
where functionality or function implementation is
passed as an argument.
*Functional programming: In this technique pure
logic is executed without changing the Object
state.

2. Why or when to use Lambda?
Approach 1 : Compare the object on various
criterias.
Example : Person with attributes<Name, Age,
Gender, City, State>
Problem Stmt :Filter List of Person for
various criterias as follows
Get all female persons.
All persons having age greater than 30.
List all Persons who belong to one city.
In this we can have different solutions
possible as creating seperate methods for
each condition. Or one generic method

3. Approach 1 : Compare the object on various
criterias.
Create Class
public class Person {
private String name;
private String gender;
private Integer age;
private String city;
private String state;
}
Create criteria interface
public interface PersonCriteria {
boolean testPerson(Person p);
}
This class is responsible to adding the filter

4. Approach 1 : Compare the object on various
criterias.
Create a service class
public class PersonFilterService {
public void printPerson(List<Person> persons,PersonCriteria
pc ){
for(Person p : persons){
if(pc.testPerson(p))
System.out.println("Name :"+p.getName());
}
}
}
Create class to get filtered Person objects
public static void main (String args[]){
Person p1 = new Person("Sumit", "female", 34, "Gurgaon",
"Haryana");
Person p2 = new Person("Niren", "male", 40, "Gurgaon",
"Haryana");

5. Approach 2: Performaing operations on the
elements using Aggregate Operations.
To print elements of an array. Use Aggregate
Operations that Accept Lambda Expressions
as Parameters.
Example : Print the name of persons who have
gender Female.
persons.stream().filter((Person p)-
>p.getGender().equals("female")).map(p->p.getName()
).forEach(name -> System.out.println(name));