int carType;

public void act() {
    if (carType == 1) {
        //move when key pressed;
    }
    else {
        //do something else;
    }
}

//add this constructor to your car class;
public Car(int carType) {
    this.carType = carType;
    //may some other stuff;
}

//if you now got some code like this:
addObject(new Car(), 100, 100);
addObject(new Car(), 100, 200);

//you need to change it to:
addObject(new Car(1), 100, 100);
addObject(new Car(2), 100, 200);

//add this method to your car class;
public void addedToWorld(World world) {
    carType = getWorld().getObjects(Car.class).size() +1;
    //this will make the first car carType 1, the second carType 2 the third carType 3, ...
}

// class field
static Car controlledCar = null; // to be set by code elsewhere
// act method
public void act()
{
    if (controlledCar == this) checkControls();
}

private void checkControls()
{
    // check for keystrokes and act upon them
}

public void loseControl()
{
    controlledCar = null;
}

public void gainControl()
{
    controlledCar = this;
}