Java Intermediate

Arrays Class in Java

Q: Why does Arrays.asList() not support add() or remove()?

Arrays.asList() returns a fixed-size list backed by the array. The array has a fixed size, so the list cannot grow or shrink. set() works because it replaces elements in place. Wrap with new ArrayList<>(Arrays.asList(arr)) to get a fully mutable list.

📅 March 17, 2026 ⏱ 3 min read 🎯 Intermediate

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📍 Part of: Arrays → Topic 5 of 8

Java Arrays class utility methods — Arrays.

⚙️ Intermediate — basic Java knowledge assumed

In this tutorial, you'll learn

Java Arrays class utility methods — Arrays.

Arrays.sort() uses dual-pivot Quicksort for primitives (O(n log n)) and Timsort for objects.
binarySearch() only works correctly on a sorted array — always sort first.
copyOf() creates a new array; changes to the copy do not affect the original.

✦ Plain-English analogy ✦ Real code with output ✦ Interview questions

⚡Quick Answer

java.util.Arrays provides static utility methods for arrays: sort() for O(n log n) sorting, binarySearch() for O(log n) search on sorted arrays, copyOf() for copying, fill() for initialisation, stream() for functional operations, and asList() to get a fixed-size List view.

Sorting Arrays

Example · JAVA

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package io.thecodeforge.java.arrays;

import java.util.Arrays;

public class ArraysSortDemo {
    public static void main(String[] args) {
        int[] numbers = {5, 2, 8, 1, 9, 3};
        Arrays.sort(numbers);
        System.out.println(Arrays.toString(numbers)); // [1, 2, 3, 5, 8, 9]

        // Sort a range (from index 1 to 4, exclusive end)
        int[] partial = {5, 2, 8, 1, 9, 3};
        Arrays.sort(partial, 1, 4);  // sorts indices 1,2,3 only
        System.out.println(Arrays.toString(partial)); // [5, 1, 2, 8, 9, 3]

        // Sort objects — natural order (Comparable)
        String[] words = {"banana", "apple", "cherry", "date"};
        Arrays.sort(words);
        System.out.println(Arrays.toString(words));   // [apple, banana, cherry, date]

        // Sort objects — custom Comparator (by length)
        Arrays.sort(words, (a, b) -> a.length() - b.length());
        System.out.println(Arrays.toString(words));   // [date, apple, banana, cherry]
    }
}

▶ Output

[1, 2, 3, 5, 8, 9]
[5, 1, 2, 8, 9, 3]
[apple, banana, cherry, date]
[date, apple, banana, cherry]

Searching, Copying, and Filling

Example · JAVA

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package io.thecodeforge.java.arrays;

import java.util.Arrays;

public class ArraysUtilities {
    public static void main(String[] args) {
        int[] sorted = {1, 3, 5, 7, 9, 11};

        // binarySearch — MUST be sorted first
        System.out.println(Arrays.binarySearch(sorted, 7));  // 3 (index)
        System.out.println(Arrays.binarySearch(sorted, 4));  // negative = not found

        // copyOf — new array, truncates or pads with zeros
        int[] copy = Arrays.copyOf(sorted, 4);
        System.out.println(Arrays.toString(copy));   // [1, 3, 5, 7]

        int[] extended = Arrays.copyOf(sorted, 8);
        System.out.println(Arrays.toString(extended)); // [1, 3, 5, 7, 9, 11, 0, 0]

        // copyOfRange — copy a slice
        int[] slice = Arrays.copyOfRange(sorted, 2, 5);
        System.out.println(Arrays.toString(slice));  // [5, 7, 9]

        // fill — initialise all elements
        int[] grid = new int[5];
        Arrays.fill(grid, -1);
        System.out.println(Arrays.toString(grid));   // [-1, -1, -1, -1, -1]

        // equals and deepEquals
        int[] a = {1, 2, 3};
        int[] b = {1, 2, 3};
        System.out.println(a == b);           // false — different references
        System.out.println(Arrays.equals(a, b)); // true  — element comparison
    }
}

▶ Output

3
-3
[1, 3, 5, 7]
[1, 3, 5, 7, 9, 11, 0, 0]
[5, 7, 9]
[-1, -1, -1, -1, -1]
false
true

Arrays.stream and Arrays.asList

Example · JAVA

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package io.thecodeforge.java.arrays;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class ArraysStreamDemo {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

        // Arrays.stream — IntStream for primitives
        int sum = Arrays.stream(numbers).sum();
        System.out.println("Sum: " + sum);  // 55

        double avg = Arrays.stream(numbers).average().orElse(0);
        System.out.println("Avg: " + avg);  // 5.5

        int[] evens = Arrays.stream(numbers)
                           .filter(n -> n % 2 == 0)
                           .toArray();
        System.out.println(Arrays.toString(evens));  // [2, 4, 6, 8, 10]

        // Arrays.asList — fixed-size List view (add/remove throw UnsupportedOperationException)
        String[] arr = {"a", "b", "c"};
        List<String> list = Arrays.asList(arr);
        list.set(0, "x");  // set works — modifies underlying array
        System.out.println(Arrays.toString(arr));  // [x, b, c]
        // list.add("d");  // throws UnsupportedOperationException

        // For a mutable list, wrap: new ArrayList<>(Arrays.asList(arr))
    }
}

▶ Output

Sum: 55
Avg: 5.5
[2, 4, 6, 8, 10]
[x, b, c]

🎯 Key Takeaways

Arrays.sort() uses dual-pivot Quicksort for primitives (O(n log n)) and Timsort for objects.
binarySearch() only works correctly on a sorted array — always sort first.
copyOf() creates a new array; changes to the copy do not affect the original.
Arrays.asList() returns a fixed-size view — you cannot add or remove elements.
Arrays.equals() compares element by element; == compares array references.

Interview Questions on This Topic

QWhat is the difference between Arrays.equals() and == for arrays?
QWhy does Arrays.asList() not support the add() method?
QWhat does Arrays.binarySearch() return when the element is not in the array?

Frequently Asked Questions

Why does Arrays.asList() not support add() or remove()?

Arrays.asList() returns a fixed-size list backed by the array. The array has a fixed size, so the list cannot grow or shrink. set() works because it replaces elements in place. Wrap with new ArrayList<>(Arrays.asList(arr)) to get a fully mutable list.

What does binarySearch() return when the element is not found?

It returns -(insertion_point) - 1, where insertion_point is the index where the element would be inserted to keep the array sorted. For example, searching for 4 in [1, 3, 5, 7] returns -3 (insertion point is 2, so -(2)-1 = -3). This lets you derive the insertion point from the negative result.

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