Please explain what's happening in the code

Question

https://code.sololearn.com/cU1oFgaQPXA8/?ref=app

Answer 1

""+b+a will be converted into string, .toString() or ""+ before variable or value converts value into the string and a+b will be added because they are not converted into string.
a+b+""+b+a
3+1""+1+3
4 1 3
Hence output will be 4 1 3.

For example :
class Main {
public static void main(String[] args) {

// create double variables
double num1 = 347.6D;
double num2 = 86.56D;

// convert double to string
// using + sign
String str1 = "" + num1;
String str2 = "" + num2;

// print string variables
System.out.println(str1); // 347.6
System.out.println(str2); // 86.56
}
}

Notice the line,

String str1 = "" + num1;
where ""+ convert variable to string and print variables in string.

Answer 2

If you add a string and a number the number becomes a string.
So let's cut this into pieces.
a = 3
b = 1
a+b+""+b+a
3+1 = 4 //<- these are two numbers
then
4 + "" // <- number and string
"4" + 1 // <- string and number
"41" + 3 // <- string and number
"413"
I hope it's now more clear what's happening. 😉

Answer 3

a = 3;
b = 1;
a+b = 4;
So the first word is 4.
You have a quotes so it turns to String type.
So the second word is 1, because the value of b is 1.
And the third word is 3, because the value of a is 3.