Best way to iterate through ArrayList?

Newbie & Debugging Questions
#21

I’ve kind of wondered about that ever since I started doing Java a couple of years ago and looked through people’s code. I’d noticed LinkedLists weren’t really used but never really thought about why. This post made me look into it and now I know it’s because array processing is more cache friendly! Well I’ll be… ::slight_smile: Guess I’m a bit behind the cutting edge here. The C++ code I’ve seen and worked on over the years uses plenty of linked lists, but I guess was mostly written before cache architectures became common.

Anyway I took a look at the only place I’ve used a LinkedList in my Java code so far (as the A* open list) and swapped it out for an ArrayList just for kicks. Who knew, the ArrayList is straight away quicker a lot of the time, even though the code was specifically written to work with a LinkedList!!! Haha, what a waste of time those data structure courses were :smiley:

Having said that, the linked list comes back into its own with a less aggressive heuristic function, and if I put more obstacles in the way, because it gets costly to insert all the new nodes into the array. So now I have to decide if I can be bothered to rewrite or not - if I do I’ll probably home bake something in between using some of the suggestions people have made in this thread.

Either way it’s been a good learning day :slight_smile:

#22

For me it’s either regular Array[], ArrayList or ArrayDeque: :persecutioncomplex:
http://docs.oracle.com/javase/8/docs/api/java/util/ArrayDeque.html

#23

What about: for (Enemy e : getEnemies()) enemies.add(e); :stuck_out_tongue:
or even 133t: enemies.addAll( java.util.Arrays.asList(getEnemies()) );

#24

That is what I’m currently using… But what is the best way to make ‘getEnemies’?

#25

It’s not what you’re currently using, because it throws a ConcurrentModificationException.

What do you mean how to make getEnemies()? Why are you using an array in the first place?

#26

I find that the simple technique like this:


for(int i = 0; i < array.size(); i++) {
     System.out.println(i);
}

Works best. I have tried to iterate through array lists using an advanced for loop and I mean it works, but it seems to not like Slick2D and other libraries i use so I have gotten errors many a time.

If you wanted to get as close as you can to an advance for loop and iterate though objects in the for loop you could do this:


ArrayList<Player> list = new ArrayList<Player>();

Player tempPlayer;

for(int i = 0; i < list.size(); i++) {
     tempPlayer = list.get(i);

     //This line is just to show a use. Not necessary.
     System.out.println("X: " + tempPlayer.x, "Y: " + tempPlayer.y);
}

Good luck with your games, man!

#27

I like Streams too, so if you use Java 8 you can use them!


// One Operation
array.stream().forEach(val -> val.doSomething());

// Multiple Operations:
array.stream().forEach(val -> {
    val.doSomething();
    val.doSomeOtherThings();
});

// You could even make crazy things like this:
array.stream().filter(val -> val.isAThing())
              .forEach(thing -> thing.doSomething());

Just wanted to show some examples for this :wink:
For some more methods you can look into the Java Doc.

#28

If you think that’s crazy…

#29

Didn’t want to make it too complicated :smiley:

#30

@Jacob_Pickens: That method will cause silent problems if you remove entities during iteration.

#31

In my CS class we wrote a collection that implements set operations (union, difference, etc.), and to prove a point to one of my classmates I wrote mine so that each method had only one statement, so here’s the intersection of sets:


return list.stream().distinct()
    .flatMap(t -> generate(() -> t).limit(min(getFrequencyOf(t), other.getFrequencyOf(t))))
    .collect(() -> new ArrayListBagExtension<>(type), (c, e) -> c.add(e), (c1, c2) -> c1.list.addAll(c2.list));

getFrequencyOf is a bit more sane: [icode]return (int) list.stream().filter(e -> anEntry.equals(e)).count();[/icode]

Don’t try this at home kids!

#32

It’s for my game engine… I’m using usual arrays from get methods for simplicity and security…

#33

Ask yourself if using arrays is really simpler and more secure and why.

I wouldn’t convert back and forth between arrays and lists all the time, but just go with List everywhere in the API (except for primitive types) .

If you insist on converting lists to arrays, use [icode]list.toArray(new Type[list.size()])[/icode] to avoid unnecessary allocations (see toArray javadoc)

#34

Compare this:


for (int i = 0; i < things.size(); i++) {
    Thing thing = things.get(i);

    // Do Something.
}

To this:


for (Thing thing : things) {
    // Do Something.
}

Huge difference, much faster to do IMO.

  • Jev
#35

Looks faster only because it’s “smaller”… but it does a lot of things under the hood that you might not need or want it to do and stops you doing a bunch of things that you might want it to do. Know thy requirements! The question for the whole topic title is loaded.

Cas :slight_smile:

#36

I’m not switching between the two all the time… I’m just converting from ArrayList to arrays when the programmer wants it…

Before I was using ‘array.toArray(new [0]);’ but I just found out by looking at the code that it will create a new array if I do that…

#37

Best way to iterate through ArrayList?

Once.

#38

You should only convert an ArrayList to an array when you know that you aren’t going to be adding anything more to it and the libraries/frameworks you’re working with require it for many of their functions IMO.

  • Jev
#39

Also I forgot to say…
I use array.toArray(new [array.size()]); because it creates a copy of the array so if the array changes during the loop (which happens alot) then nothing bad will happen…

#40

Have you already tested List’s backwards loop remove() at least once: ???

for ( int len = projectiles.size(), i = len; i-- != 0; )
  if (projectiles.get(i).removeCriteria()) {
    projectiles.set(i, projectiles.get(--len));
    projectiles.remove(len);
  }

No need to have any extra array copy or whatever! :stuck_out_tongue: