What causes "インデックスが範囲外です" in krahets/hello-algo?

Bounds-check before access: `if (index >= 0 && index < list.size()) list.get(index);`

krahets/hello-algo · error · IndexOutOfBoundsException

インデックスが範囲外です

Error message

インデックスが範囲外です

What it means

Thrown by get() in the hand-written dynamic-array class MyList (ja translation of hello-algo). It fires whenever the requested index is negative or >= the current logical element count (size), NOT the backing array length (capacity). The class mimics how java.util.ArrayList enforces bounds: read access is guarded so callers can never reach uninitialized slots in the over-allocated arr[]. The Japanese message 'インデックスが範囲外です' means 'index is out of range'.

Source

Thrown at ja/codes/java/chapter_array_and_linkedlist/my_list.java:37

    public MyList() {
        arr = new int[capacity];
    }

    /* リストの長さを取得（現在の要素数） */
    public int size() {
        return size;
    }

    /* リスト容量を取得する */
    public int capacity() {
        return capacity;
    }

    /* 要素にアクセス */
    public int get(int index) {
        // インデックスが範囲外なら例外を送出する。以下同様
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("インデックスが範囲外です");
        return arr[index];
    }

    /* 要素を更新 */
    public void set(int index, int num) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("インデックスが範囲外です");
        arr[index] = num;
    }

    /* 末尾に要素を追加 */
    public void add(int num) {
        // 要素数が容量を超えると、拡張機構が発動する
        if (size == capacity())
            extendCapacity();
        arr[size] = num;
        // 要素数を更新
        size++;

View on GitHub (pinned to 69932aed18)

Solutions

Bounds-check before access: `if (index >= 0 && index < list.size()) list.get(index);`
Fix the loop bound to use `<` not `<=`, and read size() (not capacity()) each iteration.
If you legitimately need end-position access use list.get(list.size()-1) after confirming size()>0.
Switch to java.util.ArrayList for production code; this class is a pedagogical reimplementation.

Example fix

// before
for (int i = 0; i <= list.capacity(); i++) {
    System.out.println(list.get(i)); // IndexOutOfBoundsException when i >= size
}
// after
for (int i = 0; i < list.size(); i++) {
    System.out.println(list.get(i));
}

Defensive patterns

Strategy: validation

Validate before calling

if (index < 0 || index >= list.size()) {
    throw new IllegalArgumentException("bad index: " + index);
}
int v = list.get(index);

Try / catch

try {
    int v = list.get(index);
} catch (IndexOutOfBoundsException e) {
    // log and recover; index was outside [0, size)
}

Prevention

Always loop with `i < list.size()` and re-read size() if the list mutates.
Never substitute capacity() for size().
Centralize index validation in a helper to avoid scattered bounds bugs.

When it happens

Trigger: Calling myList.get(-1); myList.get(myList.size()); myList.get(myList.size()+k); myList.get(0) on a freshly constructed list whose size()==0; using capacity() (the backing length) instead of size() as a loop bound; using an index value sourced from a different/smaller collection.

Common situations: Off-by-one read loop `for(int i=0;i<=size;i++) get(i)`; confusing capacity() with size() after the list was only partially filled; iterating with an index that becomes stale after a concurrent remove()/clear(); porting code from a 1-based index language.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/a39728ce94c0691a. Report an issue: GitHub.