How to filter an array in Java?

Question

How can I filter an array in Java?

I have an array of objects, for example cars:

Class:

public class Car{
    public int doors;
    public Car(int d){
        this.doors = d;
    }
}

Use:

Car [] cars = new Cars[4];
cars[0] = new Car(3);
cars[1] = new Car(2);
cars[2] = new Car(4);
cars[3] = new Car(6);

Now I want to filter the array of cars, keeping only 4 doors and more:

for(int i = 0; i<cars.length; i++){
    if(cars[i].doors > 4)
         //add cars[i] to a new array
    }
}

How should I do this?

Before I did it with a Vector:

Vector subset = new Vector();
for(int i = 0; i<cars.length; i++){
    if(cars[i].doors > 4)
         //add cars[i] to a new array
        subset.addElement(cars[i]);
    }
}

And then I would make a new array with the size of the Vector. Then I would loop over the vector again and fill the new array. I know this is a very large procedure for something simple.

I'm using J2ME.

Answer 1

EDIT: saw that ArrayList is not in J2ME, but based on documentation, it does have a Vector. If that Vector class is different than J2SE Vector (as this documentation indicates), then perhaps the following code would work:

Vector carList = new Vector();
for(int i = 0; i<cars.length; i++){
    if(cars[i].doors > 4)
         carList.addElement(cars[i]);
    }
}
Car[] carArray = new Car[carList.size()];
carList.copyInto(carArray);

Answer 2

The most efficient way to do this--if the predicate you're filtering on is inexpensive and you're accessing it with a single thread--is usually to traverse the list twice:

public Car[] getFourDoors(Car[] all_cars) {
  int n = 0;
  for (Car c : all_cars) if (c.doorCount()==4) n++;
  Car[] cars_4d = new Car[n];
  n = 0;
  for (Car c : all_cars) if (c.doorCount()==4) cars_4d[n++] = c;
  return cars_4d;
}

This traverses the list twice and calls the test twice, but has no extra allocations or copying. The Vector-style methods traverse the list once, but allocates about twice the memory it needs (transiently) and copies every good element about twice. So if you are filtering a tiny fraction of the list (or performance isn't an issue, which very often it isn't), then the Vector method is good. Otherwise, the version above performs better.

Answer 3

If you really need a plain array as the result, I think your way is the way to go: you don't know the number of resulting elements before you filter, and you can't construct a new array without knowing the number of elements.

However, if you don't need thread-safety, consider using ArrayList instead of a Vector. It ought to be somewhat faster. Then use ArrayList's toArray method to get the array.