CODEWITHSUNDEEP
javaclassreturn
pei wang
pei wang

Reputation: 295

how to return value in a method in Java

I got a question about how to return values when the method invoked in main method, here is my partial code:

 import java.util.ArrayList;


public class Race {

private ArrayList<Car>cars;


 public Race(){
cars=new ArrayList<Car>();
}

public Car find(double position){
for(Car d:cars){
    if(d.getPosition()==position){
        return d;
    }
}
return null;
}
 .................

When I called the find method in main class:

     public class Test {

public static void main(String[] args) {

    Race race=new Race();
    Car mazda=new Car("Mazda",1.0,1.0,2.0,2.0);
    Car honda=new Car("Honda",1.0,1.0,1.0,2.0);
    race.addCars(mazda);
    race.addCars(honda);

     System.out.println(race.find(1.0).getName());

The third values are position, which means mazda and honda have the same position and it is supposed to print both, but when I run it, it only prints mazda, I am confused, please advise, cheers!

Upvotes: 0

Views: 5888

Answers (4)

tbodt
tbodt

Reputation: 16987

You can only return one value from a method. But you can return a List:

public List<Car> find(double position) {
    List<Car> ret = new ArrayList<Car>();
    for (Car c : cars) 
        if (c.getPosition() == position)
            ret.add(c);
    return ret;
}

If this is all you do, you will get this output:

[mazda, toyota]

If you want each one on a separate line, you will need to iterate through the list in main and print each one:

for (Car c : race.find(1.0)) 
    System.out.println(c.getName());

Upvotes: 3

StormeHawke
StormeHawke

Reputation: 6207

Two problems: 1, you can only return one Object from a method, and 2, your method returns immediately when it finds a Car in the requested position. You need to take a look at your return type and make some adjustments if you want to be able to print more than one car.

Instead, do something like this:

public List<Car> find(double position){
List<Car> cars = new ArrayList<Car>();
for(Car d:cars){
    if(d.getPosition()==position){
        cars.add(d);
    }
}
return cars;
}

You would then need to do some iterating back in your main() method to print out each returned car, like so:

for(Car car : race.find(1.0))
{
    System.out.println(car.getName());
}

Upvotes: 0

Jon Newmuis
Jon Newmuis

Reputation: 26502

A couple of things here:

  1. You can only return one thing from a method. Your method returns type Car, which means you will return literally one Car object. If you want to return multiple, you will need to either return an array of cars (Car[]) or a collection of cars (e.g. List<Car>, Set<Car>, etc.).
  2. Using doubles with exact equality is a bad idea. Because of rounding and stuff that's potentially out of the scope of this question, double values may not always be exactly what you'd expect (e.g. 1.0 might not always mean 1.0, it could be 0.999999..., etc). Either use integers, or see this answer (or similar) on how to compare double values in Java.

Upvotes: 2

David says Reinstate Monica
David says Reinstate Monica

Reputation: 20129

You need to change your return type from Car to Set<Car>. The reason is that when you find the first car (in this case the mazda) you execute the line return d;. That means its the end of your function. What you need to do is declare a Set<Car> locally in the function, and intead of writing return d you write Set.add(d);

Upvotes: 0

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