ArrayListアルゴリズムの実装

語彙

English	意味
shift/ʃɪft/	シフト
delete/dɪˈliːt/	削除
insert/ˈɪnsɜːt/	挿入

English

Adjacent removals reveal a skipped element

Starting with [0, 0, 5], remove index 0 and then increment i. The second zero has shifted into index 0, so that forward loop skips it and leaves [0, 5].
For the forward while-loop taught here, stay at the same index after removal and advance only when keeping an element. It then removes both zeros and finishes with [5].

Adjacent removals reveal the skipped-element bug

English

Keep the current forward index after removal

After remove(i), every later element shifts one place left. If an element now occupies i, it has not yet been checked; inspect it on the next iteration.
The loop below accepts a non-null list of non-null Integers and removes every zero in place. It uses current size each iteration and performs no get call on an empty list.

Example

import java.util.ArrayList;

public class RemoveZeros {
    public static void removeZeros(ArrayList<Integer> list) {
        int i = 0;
        while (i < list.size()) {
            if (list.get(i) == 0) {
                list.remove(i);
            } else {
                i++;
            }
        }
    }
}

English

Explain why this loop terminates

Each iteration either removes one element and reduces size, or keeps one and increases i. Thus size() - i, the number of positions still to check, decreases by one each iteration.
Starting with [0, 0, 5], the states are [0, 5] at i=0, [5] at i=0, then [5] at i=1. The final condition is false, so no out-of-bounds access occurs.

English

A backward loop follows a different rule

A reverse loop starts at the final existing index: for (int i = list.size() - 1; i >= 0; i--). Remove matching elements with remove(i); the next i is always one lower.
Deleting at i shifts only later indices, which have already been checked. Earlier unchecked elements retain their indices, so decrementing after a removal is correct here; “never advance after removal” is not a universal rule.

Watch out

Choose an index update that matches the direction of traversal. In the forward while-loop, stay after removal; in the backward for-loop, continue decrementing. Direct structural changes during an enhanced loop are a separate unsafe pattern, even when no fail-fast exception appears.

実践

ここで示す前方whileループにおいて、i を進めるのは...場合のみです。