java program is not able to read a string

You are not reading the string as you want, just use below code to make it work as you want

s1 = scan.next();

For more information please see this link

EDIT :
If your input consist a string with spaces than you can do something like:

scan.nextLine();
s1 = scan.nextLine();

This is because of nextDouble(), this method does not consume the last newline character of your input, and thus that newline is consumed in the next call to nextLine(). Adding newLine() before reading String will consume the new Line and you will be able to input your String.

Change this line

s1 = scan.nextLine();

to

s1 = scan.next();

this is because of you enters: , so till get new line it can not go to next line so for non string use example:

int j = scan.nextInt(); scan.nextLine();

import java.util.*;

class DataTypes {

    public static void main(String args[]) {

        int i = 4;
        double d = 4.0;
        String s = "Hackerrank";
        Scanner scan = new Scanner(System.in);

//      System.out.print("Enter j: ");
        int j = scan.nextInt();
        scan.nextLine();
//      System.out.print("Enter dl: ");
        double d1 = scan.nextDouble();
        scan.nextLine();

        String s1 = new String();
//      System.out.print("Enter s1: ");
        s1 = scan.nextLine();
        j = j + i;
        System.out.println(j);
        d1 = d1 + d;
        System.out.println(d1);
        System.out.println(s + " " + s1);
        scan.close();
    }
}

Replace this lines as

double d1 = Double.parseDouble(scan.nextLine());

Changing the line

 s1 = scan.nextLine();

to below line

 s1 = scan.next();

will work.

try this:

import java.util.*;
class Demo{
public static void main(String args[]){
  int i = 4;
  double d= 4.0;
  String s = "Hackerrank";
  Scanner scan = new Scanner(System.in);
  int j = scan.nextInt();
  double d1 = scan.nextDouble();
  String s1  = scan.next();
  j = j+i;
  System.out.println(j);
  d1 = d1+d;
  System.out.println(d1);
  System.out.println(s+" "+s1);
  scan.close();
 }
}

This problem is appearing because a call to nextDouble only consumes the number itself, and not only newlines after it.

This happens:

1. Your program asks the scanner for a new double
2. Your give the console 4 characters: 0.1\n
3. Scanner reads from the console, and gets 0.1\n
4. Since it now can calculate where the double ends, it pushes back \n to its buffer
5. Your program asks the scanner for a new newline
6. The scanner reads from its buffers: \n
7. Since it found a valid line ending, it returns the string in front of it without the trailing newline.

There are different solutions for this:

Prepend the string directly after the number in the input:

1 3.5I'm a short string

Consume the unneeded line character in your code manually:

        double d1 = scan.nextDouble();
        scan.nextLine();
        String s1 = scan.nextLine();

Call next instead of nextLine:

Notice, this will only consume 1 word at a time

        double d1 = scan.nextDouble();
        String s1 = scan.next();

Read the double using a call to nextline:

This will limit your program to newline separated numbers, instead of allowing any seperator:

int j = Integer.parseInteger(scan.nextLine());
double d1 = Double.parseDouble(scan.nextLine());
String s1 =scan.nextLine();

It happens because after inputing the double value you press the enter key and it scans the "enter key" value which is considered to be new line for that reason it is showing you the output as soon as you hit enter.

Instead what you can do is read the enter key and ignore and then read the next line like this.

import java.util.*;
public class Main
{
    public static void main(String[] args) {
        int i = 4;
        double d= 4.0;
        String s = "Hackerrank";
        Scanner scan = new Scanner(System.in);
        int j = scan.nextInt();
        double d1 = scan.nextDouble();
        String s1 = new String();
        scan.nextLine(); // add this to read enter key 
        s1 = scan.nextLine();
        j = j+i;
        System.out.println(j);
        d1 = d1+d;
        System.out.println(d1);
        System.out.println(s+" "+s1);
        scan.close();
    }
}