I have recently been having this problem within my code (java) where subclasses of certain classes share variables with every other instance.

Ex.

public abstract class Entity
{
protected int x, y;
protected int xvel, yvel;

public void setVel(int x, int y)
{
xvel = x;
yvel = y;
}
}

public class Grass extends Entity
{
public void update()
{
x += xvel;
y += yvel;
}
}

in my level class I have a list of Grass objects and so I update them in a for loop, but what I notice is that if I do the following:

entities.get(0).setVel(10, 10);

every single grass object has its velocity set to 10, 10

Please help. I have been struggling with this for 2 days.