Question

Below is part of the solution to an homework assignment I had, and I don’t know why he has two for loops. If it was me, I would have set if (carsParked[i] == c) then set carsParked[i]=null
I don’t understand why second for loop statement can anyone explain?
By the way, CarsParked array is a type of Car class which stores car objects that are parked.

public void driveOut(Car c)
{
    for (int i=0; i<carsParked.length; i++) // Loop through the carParked array
    {
        if(carsParked[i] == c) // Find Car c at index i
        {
            //carsParked, remove(c);
            for (int j=i; j<carsIn-1; j++)
            {
                carsParked[j] = carsParked[j+1];
            }
            carsParked[carsIn-1] = null; 
            carsIn = carsIn - 1;
        }
    }
}

This is how cars are parked

public void driveIn(Car c)
{
    if(carsIn < carsParked.length)
    {
        carsParked[carsIn] = c;
        carsIn = carsIn + 1;
    }
    else // error message
    {
         System.out.println("Park " + location + " is full, for " + c);
    }
}

Answer 1

Say you have an array with 5 cars: [Car1, Car2, Car3, Car4, Car5], and you call the method driveOut(Car3).

By setting the value to null when you find it, you end up with [Car1, Car2, null, Car4, Car5]. This practice can lead to NullPointerException type errors, because null values are in the middle of the array, and the data is not compacted.

The solution is basically eliminating the gap by starting from that point and shifting the remaining cars to the left:

Inner loop rev 1: [Car1, Car2, Car4, Car4, Car5]
Inner loop rev 2: [Car1, Car2, Car4, Car5, Car5]

Then outside the loop, the last entry is set to null, leaving you with [Car1, Car2, Car4, Car5, null]