Arrays Utility API

copyOf() & fill()

Arrays.copyOf(arr, newLen) copies and resizes (pads with 0/null/false). Arrays.copyOfRange(arr, from, to) extracts a subarray. Arrays.fill(arr, val) initializes every element; Arrays.fill(arr, from, to, val) fills a subrange.

O(n)

How It Works

Arrays.copyOf(arr, newLen) allocates a fresh array of the requested length and copies elements over, padding with zeros, false, or null when the new length exceeds the original — the idiomatic way to 'resize' a fixed-length Java array. Arrays.copyOfRange(arr, from, to) extracts a subarray with an inclusive from and exclusive to, and the to index may legally exceed the source length, padding the tail.

Arrays.fill(arr, val) overwrites every slot with one value, with a from/to overload for subranges — the standard way to initialize DP tables or reset visited markers. All of these are O(n) in the elements touched, and copies are shallow: for object arrays the references are duplicated, not the objects they point to.

Step-by-Step Visualization

Arrays.copyOf(arr, 3) — takes first 3 elements

Result[1, 2, 3]

1/4

Code

Java

int[] arr = {1, 2, 3, 4, 5};

// ─── copyOf ───────────────────────────────────────────────────
Arrays.copyOf(arr, 3);          // [1, 2, 3]           — truncate
Arrays.copyOf(arr, 8);          // [1,2,3,4,5,0,0,0]  — pad with 0s

// ─── copyOfRange (from inclusive, to exclusive) ───────────────
Arrays.copyOfRange(arr, 1, 4);  // [2, 3, 4]
Arrays.copyOfRange(arr, 3, 10); // [4,5,0,0,0,0,0]    — pads if to > length

// ─── fill ─────────────────────────────────────────────────────
int[] dp = new int[6];
Arrays.fill(dp, Integer.MAX_VALUE);   // all cells = MAX (DP sentinel)
Arrays.fill(dp, 1, 4, 0);            // fill subrange [1, 4) with 0

int[][] grid = new int[3][4];
for (int[] row : grid) Arrays.fill(row, -1);  // 2D: fill row by row

// ─── System.arraycopy (faster bulk copy) ─────────────────────
System.arraycopy(arr, 0, dest, 0, 5); // (src, srcPos, dst, dstPos, len)

Tips & Gotchas

1copyOf with a larger length pads with 0 / null / false

2copyOfRange(arr, from, to): from is inclusive, to is exclusive

3fill() is the fastest way to initialize an array to a non-zero value

4For 2D arrays, fill each row individually in a loop

Practice Problems

1Merge Sorted Array
2Rotate Array
3Coin Change
4Range Addition

About the Arrays Utility API Pattern

java.util.Arrays methods and array ↔ collection conversion patterns. These utilities handle sorting, searching, copying, filling, and bridging between primitive arrays and Java collections.

Key insight

When you see 'subarray', 'contiguous', or 'in-place', think arrays. The key is reducing brute-force O(n²) to O(n) using sliding window, two pointers, or prefix sums.

Common Array Interview Problems

Two Sum
Best Time to Buy & Sell Stock
Maximum Subarray
Merge Intervals
Product of Array Except Self
Container With Most Water

Frequently Asked Questions

What is the difference between copyOf and clone for arrays?

arr.clone() always duplicates the array at its exact current length, while Arrays.copyOf lets you grow or truncate in the same call. Both are shallow copies, so for a 2D array you must copy each row individually or the clones will share row references.

How do I fill a 2D array with a value?

Arrays.fill only works on one dimension, so loop over the rows and call Arrays.fill(row, val) on each. Beware of initializing rows via reference assignment — every row would alias the same array, and writing one cell would appear to write a whole column.