Why is it that the output of this code is 234 and not 23?

So I may not have a very clear understanding of how while loops work. But I expected the output of this code would be 23 instead of 234. I thought that the statement x++; being before the print of x would make it so that the loop would end right when x becomes 4, but it doesn't it just keeps going, executes by printing x which becomes 4 and then it ends.

I guess these are just the rules of Java but it seems sort of counter-intuitive. Maybe a break statement would be in order?

void printNums(){
  int x = 1;
  int y = 0;
  while(x != 4){
    while(y < 0){
      System.out.print(y);
      y++;
    } 
    x++;
    System.out.print(x);
  } 
  System.out.print(" ") 
} 

> I thought that the statement x++; being before the print of x would make it so that the loop would end right when x becomes 4

This is what you misunderstand. The entire body of the loop always runs in each iteration (unless you break or return out of it, or exit it a different way).

while (x != 4) {
    ...
	x++; <-- Java doesn't "watch" x. If x becomes 4, next statement will still run
	System.out.print(x);
}

The loop condition (x != 4) is executed at the beginning of each iteration. It's only when the entire body of the loop is executed that the condition is evaluated again to check whether it should be run once again. x is not being "watched" such that whenever it becomes 4, an automatic break; is executed. If you need that, you should explicitly end the loop yourself.

The loop condition is not constantly checked. It is only checked every time a new iteration is about to start.

When x is 3, and x++ is executed, x does become 4, but the loop condition is not immediately checked. The rest of the loop (the System.out.println line) will still be executed. It is only when the current iteration finishes (} is reached), and a new iteration is about to start, that the loop condition is checked. And at that point, x is found to be 4, and the loop stops.

If you want the loop condition to be checked every time you change x, you need to write the checks manually:

while(x != 4){
    x++;
    if (x == 4) break;
    System.out.print(x);
} 
System.out.print(" ");

Note that y is never less than 0, so the inner while loop is never executed.

Before going to for loop 
x =1
y=0 
Fist iteration -
y= 0 , so never go to internal while loop
x++
x=2 , so print 2 

second iteration -
x =2
y=0 

y= 0 , so never go to internal while loop
x++
x=3 , so print 3

Third iteration -

x =3
y=0 

y= 0 , so never go to internal while loop
x++
x=4 , so print 4

but now x=4 , means 4!=4 false, so outer loop ends...

hence in the console , u will see output 2 3 4.

1. In the first iteration x = 1, since y = 0 the while loop for (y <0) is never executed. Then x++ increments the value of x and now x is equal to 2.

2. In the second iteration x = 2 which is different than 4, the while loop for (y<0) is ignored again. Then x++ increments the value of x and now x is equal to 3.

3. In the third iteration x = 3 which is different than 4, the while loop for (y<0) is ignored again. Then x++ increments the value of x and now x is equal to 4.

Then since x = 4 the while loop (x!=4) evaluates to false and the loop stops.

Therefore you get 234.