what the line sum+= do?

Answer 1

No it's not like that....

look...

if you are writing '
int sum=0;
sum+=3;
//that means.: sum=sum + 3;
//sum=0+3; I.e. sum = 3 ( sum will get new value as 3)

whatever the answer comes it will be stored into the sum again and sum will get new value as an answer.

Answer 2

sum+= x;
is the same like
sum = sum + x;

You can also use: -=, /=, *=, %=

Answer 3

Ok 😂

Answer 4

it's same as
sum=sum+

example:
int sum=0,
sum+=3;

System.out.print(sum); //3