Chapter 21: One-Dimensional Array in Java

This chapter goes deep on one-dimensional arrays only: patterns you will write in homework, interviews, and small utilities. 2D arrays are Chapter 22.

1. Topic title

One-dimensional arrays: scanning, aggregating, searching, copying, and safe indexing

2. What it means

A 1D array is a single row of slots: type[] name. You already know creation from Chapter 20. Here we focus on what you do with that row: read every cell, update cells, find answers that depend on all cells, and copy data without sharing unwanted references.

3. Why it is used

Most list-shaped data starts life as a 1D array or later moves to ArrayList. Understanding loops + indices makes both easier: summing a bill, finding the max test score, reversing a sequence, shifting elements.

4. Real-world example

A teacher stores 30 quiz scores in int[]. Functions of that array: average, highest, count of scores below passing. All are single-pass or two-pass loops over one dimension.

5. Java code example

import java.util.Arrays;
 
public class OneDimensionalArrayDemo {
    public static void main(String[] args) {
        int[] scores = {55, 80, 72, 90, 65};
 
        int sum = 0;
        for (int s : scores) {
            sum += s;
        }
        double average = (double) sum / scores.length;
        System.out.println("Sum = " + sum + ", average = " + average);
 
        int max = scores[0];
        for (int i = 1; i < scores.length; i++) {
            if (scores[i] > max) {
                max = scores[i];
            }
        }
        System.out.println("Max score = " + max);
 
        int[] original = {1, 2, 3};
        int[] grow = Arrays.copyOf(original, 5);
        System.out.println("grown: " + Arrays.toString(grow));
 
        printTotal(new int[] {10, 20, 30});
    }
 
    static void printTotal(int[] values) {
        int t = 0;
        for (int v : values) {
            t += v;
        }
        System.out.println("anonymous array total = " + t);
    }
}

Arrays.toString is great for debugging—it prints [a, b, c].

6. Explanation of code

• Sum / average: enhanced for accumulates sum; cast to double before divide so you do not lose decimals.
• Max: start with first element; scan rest with index loop when you need i.
• Arrays.copyOf(original, 5) — new array length 5, first elements copied, extra slots default (0 for int).
• new int[] {10, 20, 30} — anonymous array passed directly without storing in a variable first.

7. Common mistakes

Integer division on averages — forgot (double) sum.

Starting max at 0 when all scores could be negative—use first element or Integer.MIN_VALUE with care.

Off-by-one in loops—i <= scores.length is wrong for indexing.

Copy with assignment int[] b = a — aliases same object; changing b[0] changes a[0]. Use copyOf or manual copy when you need independence.

8. Best practices

Use Arrays.copyOf / System.arraycopy instead of hand-written copy loops unless you are learning.

Extract tiny methods like average(int[] data) once patterns repeat.

Validate empty arrays before dividing by length—length == 0 guard.

9. Small practice task

Given int[] data = {3, -1, 7, 7, 2}:

1. Count how many times the maximum value appears.
2. Build a new array that is the reverse of data (do not reverse in place first if you want extra practice—either way is fine).

Print results with Arrays.toString where helpful.

Beginner tip

When stuck, print Arrays.toString(arr) after every “clever” loop iteration during debugging. Clarity beats pride.