Common array operations in Java

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Introduction

Arrays help us store many values in one place. Common operations let us add, find, or change these values easily.

You want to keep a list of student scores and find the highest score.

You need to check if a number exists in a list of phone numbers.

You want to change a specific item in a list, like updating a product price.

You want to add new items to a list and see all items.

You want to count how many times a value appears in a list.

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Syntax

Java

public class ArrayOperations {
    private int[] array;
    private int size;

    public ArrayOperations(int capacity) {
        array = new int[capacity];
        size = 0;
    }

    public void add(int value) {
        if (size < array.length) {
            array[size] = value;
            size++;
        }
    }

    public int find(int value) {
        for (int i = 0; i < size; i++) {
            if (array[i] == value) {
                return i; // return index
            }
        }
        return -1; // not found
    }

    public void update(int index, int newValue) {
        if (index >= 0 && index < size) {
            array[index] = newValue;
        }
    }

    public void printAll() {
        for (int i = 0; i < size; i++) {
            System.out.print(array[i] + " ");
        }
        System.out.println();
    }

    public int countOccurrences(int value) {
        int count = 0;
        for (int i = 0; i < size; i++) {
            if (array[i] == value) {
                count++;
            }
        }
        return count;
    }
}

This class uses a fixed-size array and a size variable to track how many items are added.

Indexing starts at 0, so the first element is at index 0.

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Examples

line_end_arrow_notchAdds three numbers and prints: 10 20 30

Java

ArrayOperations arr = new ArrayOperations(5);
arr.add(10);
arr.add(20);
arr.add(30);
arr.printAll();

line_end_arrow_notchPrints nothing because array is empty (size is 0).

Java

ArrayOperations arr = new ArrayOperations(3);
arr.printAll();

line_end_arrow_notchUpdates the first element from 5 to 15 and prints: 15

Java

ArrayOperations arr = new ArrayOperations(3);
arr.add(5);
arr.update(0, 15);
arr.printAll();

line_end_arrow_notchCounts how many times 7 appears and prints: 2

Java

ArrayOperations arr = new ArrayOperations(3);
arr.add(7);
arr.add(7);
arr.add(2);
int count = arr.countOccurrences(7);
System.out.println(count);

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Sample Program

This program shows how to add numbers to the array, find an element's index, update a value, print all elements, and count occurrences of a value.

Java

public class Main {
    public static void main(String[] args) {
        ArrayOperations numbers = new ArrayOperations(5);

        System.out.println("Before adding elements:");
        numbers.printAll();

        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        numbers.add(20);

        System.out.println("After adding elements:");
        numbers.printAll();

        int index = numbers.find(20);
        System.out.println("Index of 20: " + index);

        numbers.update(1, 25); // change 20 at index 1 to 25
        System.out.println("After updating index 1 to 25:");
        numbers.printAll();

        int count20 = numbers.countOccurrences(20);
        System.out.println("Count of 20: " + count20);
    }
}

class ArrayOperations {
    private int[] array;
    private int size;

    public ArrayOperations(int capacity) {
        array = new int[capacity];
        size = 0;
    }

    public void add(int value) {
        if (size < array.length) {
            array[size] = value;
            size++;
        }
    }

    public int find(int value) {
        for (int i = 0; i < size; i++) {
            if (array[i] == value) {
                return i;
            }
        }
        return -1;
    }

    public void update(int index, int newValue) {
        if (index >= 0 && index < size) {
            array[index] = newValue;
        }
    }

    public void printAll() {
        for (int i = 0; i < size; i++) {
            System.out.print(array[i] + " ");
        }
        System.out.println();
    }

    public int countOccurrences(int value) {
        int count = 0;
        for (int i = 0; i < size; i++) {
            if (array[i] == value) {
                count++;
            }
        }
        return count;
    }
}

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Important Notes

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Adding an element takes O(1) time if space is available.

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Finding an element takes O(n) time because it may check all elements.

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Updating an element is O(1) because you access by index directly.

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Counting occurrences is O(n) because it checks all elements.

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Common mistake: Trying to add more elements than the array capacity causes no addition.

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Use arrays when you know the maximum size and want fast access by index.

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Summary

Arrays store multiple values in order and allow easy access by position.

Common operations include adding, finding, updating, printing, and counting values.

Understanding these helps manage lists of data simply and efficiently.