Lambda Expression Example In Java

Java 8 added support for Lambda Expression . So what exactly are Lambda Expressions ?


Lambda Expressions are nothing but Anonymous Functions . Anonymous functions are methods which does not have any names , they make the code more concise and readable ,Suppose you want to sort a list of person based on their age , you can write following code using anonymous class syntax :

Collections.sort(list, new Comparator() {
   @Override
   public int compare(Person p1, Person p2) {
    return (p1.getAge() > p2.getAge()) ? 1 : 0 ; 
   }});

Now with the help of lambda expressions , this code can be reduced to one single line of code :

 Collections.sort(list,(p1,p2 ) -> (p1.getAge() > p2.getAge()) ? 1 : 0 );

its syntax is : (${Parameters}) -> {${method body}}

1 ) If the method takes only one parameter , then we do not need to use parenthesis.
2 ) We do not need to specify the type of parameter , it will be inferred automatically by java . so we do not need to write (Person p1 , Person p2 ) instead of (p1,p2).
3 ) The Interface must be Functional Interface . A functional interface is a kind of interface which contains only one single abstract method .In this example , Collections.sort method takes a Comparator as parameter , Comparator Interface has only one single method , so you can directly implement it without specifying its name using lambda .
4 ) You can write method body with multiple statements also like this :

Collections.sort(list,(p1,p2 ) ->{
   if ( p1.getAge() > p2.getAge())
    return 1 ;
   else 
    return 0 ;
  });

5 ) If you want to use variable declared in the enclosing method , you have to declare that as final .
6 ) You can create a Runnable using Lambda like this :

Runnable r = () -> System.out.println("Hello World !! ");

Hers's the complete java program example for Lamda Expressions in java .

package java8Test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class LambdaTest {
 
 public static void main(String[] args) {
  
  List<Person> list = generateList();
  //try to sort with age using anonymous classes way 
  Collections.sort(list, new Comparator<Person>() {
   @Override
   public int compare(Person p1, Person p2) {
    return (p1.getAge() > p2.getAge()) ? 1 : 0 ; 
   }
  });
                // method variable must be final to be used in lambda expression 
  final int x = 0 ;
                 //you can create a Runnable like this also using lambda expression
  //Runnable r = () -> System.out.println("thread running ");
  
  //sorting with Age using lambda expression  
  Collections.sort(list,(p1,p2 ) -> (p1.getAge() > p2.getAge()) ? 1 : 0 );
  Collections.sort(list,(p1,p2 ) ->{
   System.out.println(x);
   if ( p1.getAge() > p2.getAge())
    return 1 ;
   else 
    return 0 ;
  });
   
  for(Person p : list)
   System.out.println(p.getName() + " " + p.getAge() + "  " + p.getSalary());
  
  Collections.sort(list,(p1,p2 ) -> (p1.getSalary() > p2.getSalary()) ? 1 : 0 );
  for(Person p : list)
   System.out.println(p.getName() + " " + p.getAge() + "  " + p.getSalary());
  Collections.sort(list,(p1,p2 ) -> p1.getName().compareTo(p2.getName()));
  for(Person p : list)
   System.out.println(p.getName() + " " + p.getAge() + "  " + p.getSalary());
  
 }

 private static List<Person> generateList() {
  List<Person> mylist = new ArrayList<>();
  Person p1 = new Person();
  p1.setAge(25);
  p1.setName("ABHishek");
  p1.setSalary(5250);
  
  Person p2 = new Person();
  p2.setAge(26);
  p2.setName("pqww");
  p2.setSalary(5252);
  
  Person p3 = new Person();
  p3.setAge(29);
  p3.setName("ewewe");
  p3.setSalary(6525);
  
  mylist.add(p1);
  mylist.add(p2);
  mylist.add(p3);
  return mylist;
 }

}

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