Java使用instanceof但基于方法参数

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.

Answer 1

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> 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.