Java reverse the order of accessing a list in a for loop

Question

I want to reverse the order of accessing a List inside a for()

This is my actual code

for(int i = 0; i < states.size(); i++) {
    System.out.println(states.size());
    states.get(i).update(true); // restore the first blockstate
    states.remove(i); // remove the first blockstate from the list
}

This code works, but I would like to reverse it. I already tried other ways, like using i-- but it did not work. Can someone provide a suggestion?

You can use the built-in method [`Colletions.reverse`](https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#reverse-java.util.List-). — Tunaki, Sep 29 '15 at 11:55
As another note, doing `states.remove(i);` inside a `for` loop can potentially throw a `ConcurrentModificationException`. If you need to go through a collection and remove things as you go along, you should stick to an iterator. — npinti, Sep 29 '15 at 12:00
Do you want to do the same thing, but starting from the back? — Sergey Kalinichenko, Sep 29 '15 at 12:02

score 6 · Accepted Answer · answered Sep 29 '15 at 12:06

I already tried other ways, like using i-- but it did not work.

Reversing a for loop consists of three steps:

Change the initial value to the last value,
Change the condition to stop after passing the initial value
Reversing the increment/decrement (i.e. ++ becomes --)

In your code this change would look as follows:

for(int i = states.size()-1 ; i >= 0 ; i--) {
    System.out.println(states.size());
    states.get(i).update(true); // restore the current blockstate
    states.remove(i); // remove the current blockstate from the list
}

Note that the last value reached in the original loop is states.size()-1, not states.size(), so i needs to start at states.size()-1 as well.

Since your loop eventually clears out the list anyway, but does it one element at a time, you may get better clarity by deleting the call of remove(i), replacing it with states.clear() after the loop.

for(int i = states.size()-1 ; i >= 0 ; i--) {
    System.out.println(states.size());
    states.get(i).update(true); // restore the current blockstate
}
states.clear();

Yhea thats working but how would u do a interval between each loop? i have a new regenBlock(states).runTaskTimer(this, 40, 17); but now isnt working it does the loop very fast — user3000019, Sep 29 '15 at 12:27

Pankaj Saboo · Answer 2 · 2015-09-29T12:42:54.137

Try this:

List list = Lists.reverse(states); 
for(int i = 0; i < list.size(); i++) {
    System.out.println(list.size());
    list.get(i).update(true); // restore the first blockstate
    //list.remove(i); // remove the first blockstate from the list
 }

if u want to remove element when accessing element then u have to use iterator by using simple for it is not possible... it throws concurrent modification exception

Java reverse the order of accessing a list in a for loop

2 Answers2