[1.7.2] [Unsolved]Problems with shooting entity

hello guys,

I was making an entity that has to shoot on passing mobs.

But I have 2 problems now.

problem 1: My entity disappears when I leave the world.

problem 2: The entity shoot always and ignores my mob checker.

here is the whole class:

public class EntityTowerGun extends Entity{

private String side;

public EntityTowerGun(World world, double x, double y, double z, String side) {
	super(world);
	setSize(1.0F, 1.0F);
	posX = x;
	posY = y;
	posZ = z;
	this.side = side;
}
public void onUpdate(){
	super.onUpdate();
	this.lookForTarget();
}

private void lookForTarget() {
	//north = negative Z
	//south = positive Z
	//west = negative X
	//east = positive X
	if(side == "north" && this.worldObj.getEntitiesWithinAABB(EntityMob.class, AxisAlignedBB.getBoundingBox(posX - 5, posY - 3, posZ - 2, posX -1, posY - 1, posZ + 2)) != null){
		this.worldObj.spawnEntityInWorld(new EntityGunBullet(this.worldObj, posX - 1, posY, posZ, posX - 3, posY - 3, posZ));
	}
	if(side == "south" && this.worldObj.getEntitiesWithinAABB(EntityMob.class, AxisAlignedBB.getBoundingBox(posX + 5, posY - 3, posZ - 2, posX + 1, posY - 1, posZ + 2)) != null){
		this.worldObj.spawnEntityInWorld(new EntityGunBullet(this.worldObj, posX + 1, posY, posZ, posX + 3, posY - 3, posZ));
	}
}
@Override
protected void entityInit() {
}

@Override
protected void readEntityFromNBT(NBTTagCompound var1) {
	super.readFromNBT(var1);
}

@Override
protected void writeEntityToNBT(NBTTagCompound var1) {
	super.writeToNBT(var1);
}

}

And here is the part where it checks for mobs, (but is shoot always...):

if(side == "north" && this.worldObj.getEntitiesWithinAABB(EntityMob.class, AxisAlignedBB.getBoundingBox(posX - 5, posY - 3, posZ - 2, posX -1, posY - 1, posZ + 2)) != null){
		this.worldObj.spawnEntityInWorld(new EntityGunBullet(this.worldObj, posX - 1, posY, posZ, posX - 3, posY - 3, posZ));
	}
	if(side == "south" && this.worldObj.getEntitiesWithinAABB(EntityMob.class, AxisAlignedBB.getBoundingBox(posX + 5, posY - 3, posZ - 2, posX + 1, posY - 1, posZ + 2)) != null){
		this.worldObj.spawnEntityInWorld(new EntityGunBullet(this.worldObj, posX + 1, posY, posZ, posX + 3, posY - 3, posZ));
	}

The east and west sides are not finished yet.

Does anybody understand why it ignores the check?

I have no errors by the way.