所以我一直在做一些Java多线程项目来更好地理解这个问题。我写了一些代码来解决一个停车场问题。基本上，线程形式的多辆汽车应该随机进入和退出一组停车位。我想我还没有完全理解Java同步的概念。
下面是我的代码：

public class CarPark
{
 
    private int slots;

    public CarPark(int slots) 
    { 
        if(slots < 0)
            slots = 0;
        this.slots = slots; 
    } 

    // Try to enter the car park
    public void tryParking(String car)
    {
        
        if (slots >= 1) // At least one free slot to park in the car park
        {
            park(car);  
        }
        else if (slots <= 0) // If there is no free slot then the car has to wait
        {
            System.out.println(car + ": is waiting for a slot. Free slots: " + slots);
        }

    }
    
    // Parking in the car park
    private void park(String car)
    { 
        slots--;
        System.out.println(car + ": is parking. Free slots: " + slots);
    } 

    // Leaving the car park
    public void leave(String car) 
    { 
        slots++;
        System.out.println(car + ": is leaving the car park. Free slots: " + slots);
    }

    public static void main( String[] args)
    {
        // 5 slots
        CarPark carpark = new CarPark(5);

        // 20 Cars
        for (int i=1; i<=20; i++) {
            Car a = new Car("Car " + i, carpark);
        }
    }
}

class Car extends Thread 
{ 
    private CarPark carpark; 

    public Car(String name, CarPark carpark) 
    { 
        super(name); 
        this.carpark = carpark;
        start(); 
    } 

    public void run() 
    {
        try {
            sleep((int)(Math.random() * 1000)); 
        } catch( InterruptedException e) { }
        
        carpark.tryParking(getName()); 
        
        try {
            sleep((int)(Math.random() * 2000)); 
        }
        catch( InterruptedException e) { } 
        
        carpark.leave(getName());
    } 
}

所以现在我面临的问题是，汽车正在退出插槽（并"生产"免费插槽），从来没有停在那里摆在首位。我如何让汽车只退出，如果他们停在停车场第一？