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 Ca         


        

Answer 1

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.