Java – map() and filter() methods – Ameerpet Technologies

map():

It takes a lambda expression as its only argument, and uses it to change every individual element in the stream.
Its return value is a new Stream object containing the changed elements.

Creating a Stream:

List<Integer> myList = new ArrayList<Integer>();
myList.add(1);
myList.add(5);
myList.add(8);
Stream<Integer> myStream = myList.stream();

We can create streams for arrays as follows:

Integer[] myArray = {1, 5, 8};
Stream<Integer> myStream = Arrays.stream(myArray);

map to convert all elements in an array of strings to uppercase:

String[] myArray = new String[]{“harin”, “satya”, “annie”, “amar”};
Stream<String> myStream = Arrays.stream(myArray);
Stream<String> myNewStream = myStream.map(s -> s.toUpperCase());

To convert it into an array, you use its toArray method:

String[] myNewArray = myNewStream.toArray(String[]::new);

filter():

filter() method takes lambda expression as input and return boolean value.
If filter() method returns true, then the element enter into resultant stream.
filter() method returns stream after filtering the data.

Program to filter the Strings starts with “s” in a list:

import java.util.*;
import java.util.stream.*;
class Main
{
            public static void main(String[] args)
            {
                        List<String> list = new ArrayList<String>();
                        list.add(“Java”);
                        list.add(“JDBC”);
                        list.add(“Servlets”);
                        list.add(“JSP”);
                        list.add(“Spring”);
                        list.add(“Hibernate”);

                        Stream<String> st = list.stream();
                        Stream<String> res = st.filter(s->s.startsWith(“S”));
                        res.forEach(System.out::println);
            }
}

Write the above filtering logic in single line:

list.stream().filter(s->s.startsWith(“S”)).forEach(System.out::println);

Display string whose length is 4:

import java.util.*;
import java.util.stream.*;
class Main
{
            public static void main(String[] args)
            {
                        String[] arr = {“Java”, “JDBC”, “Servlets”, “JSP”, “Spring”};
                        List<String> list = new ArrayList<String>();
                        for(int i=0 ; i<arr.length ; i++)
                        {
                                    list.add(arr[i]);
                        }
                        list.stream().filter(s->s.length()==4).forEach(System.out::println);
            }
}

Program to display only even numbers in the list using stream api:

import java.util.*;
import java.util.stream.*;
class Main
{
            public static void main(String[] args) {
                        int[] arr = {5, 2, 8, 9, 3, 7, 1, 4, 6};
                        List<Integer> list = new ArrayList<Integer>();
                        for(int x : arr){
                                    list.add(x);
                        }
                        System.out.println(“List is : ” + list);
                        System.out.println(“Even numbers list is : “);
                        list.stream().filter(s->s%2==0).forEach(System.out::println);
            }
}

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