Constraints a and b are non-negative integers and can have maximum 200 digits.

public class Solution {

public static void main(String[] args) {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    Scanner sc = new Scanner(System.in);
    BigInteger x = BigInteger.valueOf(0);
    BigInteger a = new BigInteger(sc.next());
    BigInteger b = new BigInteger(sc.next());

    if(a.bitLength() < 200 && b.bitLength() < 200 && x.compareTo(a) < 0 && x.compareTo(b) < 0) 
        System.out.println(a.add(b));
        System.out.println(a.multiply(b));

}

}

public static void main(String[] args) { /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */ Scanner scan = new Scanner(System.in);

    String a = scan.next();
    String b = scan.next();

    BigInteger bia = new BigInteger(a);
    BigInteger bib = new BigInteger(b);
    BigInteger suma,multy;

    suma = bia.add(bib);
    multy = bia.multiply(bib);

    System.out.println(suma);
    System.out.println(multy);
}

Consider this solution as reference,

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BigInteger first = new BigInteger(sc.next());
        BigInteger second = new BigInteger(sc.next());
        System.out.println(first.add(second));
        System.out.println(first.multiply(second));
   }

import java.io.; import java.util.; import java.util.Scanner; import java.math.BigInteger; public class Solution {

public static void main(String[] args) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
        Scanner sc= new Scanner(System.in);
        BigInteger a = sc.nextBigInteger();
        BigInteger b = sc.nextBigInteger();
        BigInteger sum, mul;
        sum = a.add(b);
        mul = a.multiply(b);
        System.out.println(sum + "\n" + mul);
}

}****

import java.util.; import java.text.; import java.math.; import java.util.regex.;

public class Solution {

public static void main(String[] args) {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    Scanner sc = new Scanner(System.in);
    BigInteger a = new BigInteger(sc.nextLine());
    BigInteger b = new BigInteger(sc.nextLine());
     BigInteger sum = a.add(b);
      BigInteger prod = a.multiply(b);

    System.out.println(sum+"\n"+prod);
}

}