I'm developing a game. I'm really new in java and researched google: why does an int when added, keeps on adding java

Most of the times, it always adds even:

STOP_DAMAGE+=1;

Even sometimes, it subtract till negative. I'm annoyed. Sometimes it doesn't add too much. I do not understand. Here is my code:

if(isPressed==true)
    {


            if(STOP_DAMAGE<=5)
            {
            if(WAIT_DAMAGE>=3000)
            {
                ENEMY_SHIP_HEALTH-=SHIP_DAMAGE_ENEMY;
                STOP_DAMAGE+=1;
            }
            }
    }
 for(;WAIT_DAMAGE>=3003;)
    {

        WAIT_DAMAGE-=WAIT_DAMAGE;//time is deducted
    }

//WAIT_DAMAGE is a time int