What causes "Index: {}, array1 Length: 0" in Blankj/AndroidUtilCode?

When array1 is empty, pass index 0 (the only valid position) or simply use array2 directly.

Blankj/AndroidUtilCode · error · IndexOutOfBoundsException

Index: {}, array1 Length: 0

Error message

Index: {}, array1 Length: 0

What it means

ArrayUtils.realAddArr throws IndexOutOfBoundsException("Index: N, array1 Length: 0") when the first array is empty (length 0) and the requested insertion index is not 0. With an empty target array, only index 0 is valid; any other index has no position to insert at.

Source

Thrown at lib/utilcode/src/main/java/com/blankj/utilcode/util/ArrayUtils.java:763

        if (result == null) return null;
        return (float[]) result;
    }

    @Nullable
    public static double[] add(@Nullable double[] array1, int index, @Nullable double[] array2) {
        Object result = realAddArr(array1, index, array2, Double.TYPE);
        if (result == null) return null;
        return (double[]) result;
    }

    @Nullable
    private static Object realAddArr(@Nullable Object array1, int index, @Nullable Object array2, Class clss) {
        if (array1 == null && array2 == null) return null;
        int len1 = getLength(array1);
        int len2 = getLength(array2);
        if (len1 == 0) {
            if (index != 0) {
                throw new IndexOutOfBoundsException("Index: " + index + ", array1 Length: 0");
            }
            return realCopy(array2);
        }
        if (len2 == 0) {
            return realCopy(array1);
        }
        if (index > len1 || index < 0) {
            throw new IndexOutOfBoundsException("Index: " + index + ", array1 Length: " + len1);
        }
        Object joinedArray = Array.newInstance(array1.getClass().getComponentType(), len1 + len2);
        if (index == len1) {
            System.arraycopy(array1, 0, joinedArray, 0, len1);
            System.arraycopy(array2, 0, joinedArray, len1, len2);
        } else if (index == 0) {
            System.arraycopy(array2, 0, joinedArray, 0, len2);
            System.arraycopy(array1, 0, joinedArray, len2, len1);
        } else {
            System.arraycopy(array1, 0, joinedArray, 0, index);

View on GitHub (pinned to 7b4caf9e54)

Solutions

When array1 is empty, pass index 0 (the only valid position) or simply use array2 directly.
Clamp/validate the index to [0, len1] before calling add: if len1==0 force index=0.
Prefer the single-arg ArrayUtils.add(array1, array2) (append) when you only need concatenation, to avoid manual indexing.

Example fix

// before
int[] empty = new int[0];
int[] out = ArrayUtils.add(empty, 2, new int[]{7}); // throws: Index: 2, array1 Length: 0

// after
int[] out = ArrayUtils.add(empty, 0, new int[]{7}); // valid: only index 0 on empty array

Defensive patterns

Strategy: validation

Validate before calling

private static int[] safeAdd(int[] a1, int index, int[] a2) {
    int len1 = a1 == null ? 0 : a1.length;
    if (len1 == 0) index = 0; // only valid position for an empty array
    return ArrayUtils.add(a1, index, a2);
}

Type guard

private static boolean isValidAddIndex(Object array1, int index) {
    int len = java.lang.reflect.Array.getLength(array1);
    return index >= 0 && index <= len && (len != 0 || index == 0);
}

Prevention

When array1 is empty, pass index 0 only.
Prefer the append overload ArrayUtils.add(array1, array2) for plain concatenation.
Clamp the insertion index to [0, len1] before calling the indexed add.

When it happens

Trigger: Calling any typed ArrayUtils.add(emptyArray, index, array2) overload (Object[], int[], byte[], etc.) where array1.length==0 and index != 0. The private realAddArr is shared by every typed add overload.

Common situations: Passing an empty array plus a non-zero index from a variable that defaulted to -1 or a stale position; merging into an array that was filtered to empty without resetting the insertion index; off-by-one where callers assume len1-based index is allowed.

Related errors

AI-assisted analysis of Blankj/AndroidUtilCode@7b4caf9e54 (2026-08-14). Data as JSON: /api/errors/ff300d4569960323. Report an issue: GitHub.