How do I make a sprite jump

Question

Hi everyone,
I recently made a program that can move a sprite around but not jump. It seems that im lost on the logic behind writting a jump. My goal for the sprite is to make it jump off the ground and comback to it. However, my loop just stays still when i use the space bar to jump. I have tried 2 differnt ways.
FIRST:
KeyEvent.VK_SPACE){
            //then jump
            guy_y= 250;
            while(guy_y < 400)
                   guy_y++;
        }SECOND:
for(guy_y = guy_y - 20; guy_y < 400; guy++)
     guy >= 400;
}please help me.

Answer 1

      Allow me to explain in greater detail. First let me start by apologizing for any confusion that I have caused. Ok, moving on, my sprite is standing on 400 so my goal is for when i press the jump button he will go up 20 pixels. in that case he would be at 400 - 20 pixels. then go down in increments of 1 or what have you. I hope that levities any confusion that i may have caused.
As for morgalr i believe that the code does tell it to go back to the ground being 400 that is why told it that :
if(guy_y < 400){      //  If your less than 400 on the y scale
                guy_y++; //  Then add plus one till you are not less than 400
            }