Foreach through different objects but all implement same interface possible?

Question

Suppose I have this

interface Movable
{//some stuff}

And I have

class Car implements Movable
{//some stuff}

And also maybe I have

class Bike implements Movable
{//some stuff}

I noticed that if I had this:

ArrayList<Movable> movableThings = new ArrayList<Movable>();
movableThings.add(some kind of Car)
movableThings.add(some kind of Bike)
movableThings.add(some kind of Bike)

This can be called:

for(Movable m: movableThings)

But if I call this I get incompatible types:

for(Bike b: movableThings)

Can someone explain, and maybe offer a better way? I know I can use foreach Movable m: movableThings and then use instanceof to check for Bikes but is there another way?

Edit: alright thanks for clarifying guys... so I guess I either use instanceof or redesign my game

Community · Accepted Answer · 2017-05-23 10:28:03Z

11

I would not recommend using instanceof. The whole point of two types implementing a common interface is that, when using the interface, consumer code shouldn't be concerned with the specific implementation. I tend to get very suspicious when I see instanceof outside of equals().

Use polymorphic dispatch instead of instanceof if you want different behaviors from different implementations:

interface Movable
{
    void move();
}

class Bike implements Movable
{
    public void move()
    {
        // bike-specific implementation of how to move
    }
}

class Car implements Movable
{
    public void move()
    {
        // car-specific implementation of how to move
    }
}

The implementation-specific method will be called on each type:

for (Movable m : movableThings)
{
    m.move();
}

If you only want to iterate over Bike types, create a collection that only consists of Bikes:

List<Bike> bikes = new ArrayList<Bike>();
// etc...

for (Bike bike : bikes)
{
    // do stuff with bikes
}

N.B. You should almost always declare the collection as a List (an interface) rather than an ArrayList (an implementation of the interface).

1 Comment

Heisenbug Over a year ago

+1 yes, of course you are right. @some Java amateur the common methods of derived classes should be put in the interface.

Heisenbug · Accepted Answer · 2011-06-09 16:57:00Z

2

Because your ArrayList is defined to be

 ArrayList<Movable>

the the get method of your ArrayList will return a type Movable. You need to cast it manually if you are sure about the runtime type (use instanceof to check it).

for(Movable m: movableThings){
      if (m instanceof Car){
            Car car = (Car)m;
      }
}

answered Jun 9, 2011 at 16:57

Heisenbug

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9 Comments

Matt Ball Over a year ago

That is a strong code smell, however. If you only want to work with Bikes, this is not the right way to do it.

Heisenbug Over a year ago

I think he is using an ArrayList<Movable> to store both Bikes, Car and so on.

some Java amateur Over a year ago

Yes my arraylist of movable things has both bikes and cars because at some point i use foreach(moveable m: movableThings) to call m.move()

Heisenbug Over a year ago

@Matt Ball: Could you explain me a better solution, if ArrayList<Movable> stores both Bikes and Car?

some Java amateur Over a year ago

it is defined in the interface however, sometimes i might want to call move for only bikes from the list

|

John Gietzen · Accepted Answer · 2011-06-09 17:00:37Z

2

You do need to use instanceof. You may want to write a filter function to make it reusable.

That being said, this is probably a case where you want to use inheritance to allow the same method to be called on both classes in the same way.

edited Jun 9, 2011 at 17:00

answered Jun 9, 2011 at 16:55

John Gietzen

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3 Comments

some Java amateur Over a year ago

that is unfortunate... I didnt want to resort to instanceof because I have nested foreach loops and nested ifs inside those nested foreach loops will make it loop quite messy well thanks for the confirmation

John Gietzen Over a year ago

@someJavaamateur, do you mind posting a little bit more of what would be in that //somestuff section? Maybe there is a better way, altogether.

Matt Ball Over a year ago

The second sentence nails it - that's the whole point of the two types implementing a common interface.

Mark Peters · Accepted Answer · 2011-06-09 17:39:46Z

0

This is where Google Guava or a similar collection library can be really beneficial. You'll find tons of other uses for it so it's not a hard thing to justify including in your project.

In Guava:

for (Bike bike : Iterables.filter(movableThings, Bike.class) ) {
    //now you only have bikes
}

answered Jun 9, 2011 at 17:39

Mark Peters

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