Java using instanceof but based on method argument

Question

I have 3 objects

Car
SmallCar extends Car
LargeCar extends Car

Using this method i want to count cars of specific type in a list:

public int availableCars(String carType) {
    int count = 0;
    for (Car i : fleet) {
        if (i instanceof SmallCar) {
            count++;
        }
    }
    return count;
}

What is the best way to pass the carType given (if it is as String or something else that would be better) to have something like:

if (i instanceof carTypeGiven)

The method returns how many cars of specific type are available.

Then you need two int count variables, smallCarCount and largeCarCount. The total count will be their sum of course. — Hovercraft Full Of Eels, Oct 16 '14 at 20:33

David Conrad · Accepted Answer · 2014-10-16T21:05:07.090

Pass in the class instead of a string indicating the desired type. You can limit it to subclasses of Car using wildcards like this: Class<? extends Car>

public int availableCars(Class<? extends Car> carType) {
    int count = 0;
    for (Car car : fleet) {
        if (carType.isInstance(car)) {
            count++;
        }
    }
    return count;
}

Use Class.isInstance(Object) to determine if a car in the fleet is of the desired type.

It is also possible to do this more concisely with Java 8's streams (assuming fleet is a collection):

public long availableCars(Class<? extends Car> carType) {
    return fleet.stream().filter(carType::isInstance).count();
}

If fleet is an array, you would need to do it slightly differently:

public long availableCars(Class<? extends Car> carType) {
    return Arrays.stream(fleet).filter(carType::isInstance).count();
}

Note that I have changed the return type to long since that's what Stream.count() returns. You could keep the return type as int and cast the result from count() to an int, as well.

+1 for the inclusion of Java 8 streams to do this sort of filtering! — Krease, Oct 16 '14 at 21:06

Java using instanceof but based on method argument

1 Answers1