One-dimensional arrays in Java

Introduction

One-dimensional arrays help you store many values of the same type in a single place. This makes it easy to organize and use related data together.

When you want to keep a list of student scores in a test.

When you need to store daily temperatures for a week.

When you want to hold a sequence of names or words.

When you want to process a collection of numbers for calculations.

When you want to keep track of inventory counts for products.

Syntax

Java

public class OneDimensionalArray {
    public static void main(String[] args) {
        // Declare and create an array of integers
        int[] numbers = new int[5];

        // Assign values to the array
        numbers[0] = 10;
        numbers[1] = 20;
        numbers[2] = 30;
        numbers[3] = 40;
        numbers[4] = 50;

        // Access and print values
        for (int i = 0; i < numbers.length; i++) {
            System.out.println("Element at index " + i + ": " + numbers[i]);
        }
    }
}

The array size is fixed once created and cannot be changed.

Array indexes start at 0, so the first element is at index 0.

Examples

This creates an empty array with no elements.

Java

int[] emptyArray = new int[0];
// An array with zero elements

This array holds only one value at index 0.

Java

int[] singleElementArray = new int[1];
singleElementArray[0] = 100;
// Array with one element at index 0

You can create and fill an array in one step using curly braces.

Java

int[] numbers = {5, 10, 15, 20};
// Array initialized with values directly

Use length - 1 to get the last index because indexes start at 0.

Java

int lastElement = numbers[numbers.length - 1];
// Access the last element of the array

Sample Program

This program creates an array of 5 ages, prints them, changes one age, and prints the array again to show the update.

Java

public class OneDimensionalArrayExample {
    public static void main(String[] args) {
        // Create an array to store 5 ages
        int[] ages = new int[5];

        // Assign ages
        ages[0] = 21;
        ages[1] = 25;
        ages[2] = 30;
        ages[3] = 22;
        ages[4] = 28;

        // Print all ages before change
        System.out.println("Ages before update:");
        for (int i = 0; i < ages.length; i++) {
            System.out.println("Index " + i + ": " + ages[i]);
        }

        // Update the age at index 2
        ages[2] = 35;

        // Print all ages after change
        System.out.println("\nAges after update:");
        for (int i = 0; i < ages.length; i++) {
            System.out.println("Index " + i + ": " + ages[i]);
        }
    }
}

Important Notes

Accessing an index outside the array size causes an ArrayIndexOutOfBoundsException.

Arrays have a fixed size; to change size, create a new array and copy values.

Use loops to process all elements easily.

Summary

One-dimensional arrays store multiple values of the same type in order.

Indexes start at 0 and go up to length - 1.

Arrays have fixed size and are useful for grouping related data.

Practice

1.

What is the starting index of a one-dimensional array in Java?

easy

A. 1

B. 0

C. -1

D. Depends on the array size

Solution

1. Step 1: Understand Java array indexing

   Java arrays always start indexing from 0, meaning the first element is at index 0.

2. Step 2: Confirm the starting index

   Since the first element is at index 0, the starting index is 0 regardless of array size.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   Array index starts at 0 [OK]

Hint: Remember, Java arrays always start at zero index [OK]

Common Mistakes:

• Thinking arrays start at 1
• Assuming negative indexes
• Confusing with other languages

2.

Which of the following is the correct way to declare and create an integer array of size 5 in Java?

?

easy

A. int[] arr = int[5];

B. int arr = new int[5];

C. int arr[] = new int();

D. int[] arr = new int[5];

Solution

1. Step 1: Check array declaration syntax

   In Java, arrays are declared with type[], for example int[]. The new keyword creates the array with size.

2. Step 2: Validate each option

   int[] arr = new int[5]; uses correct syntax: int[] arr = new int[5]; Options A, B, and C have syntax errors or missing parts.

3. Final Answer:

   int[] arr = new int[5]; -> Option D

4. Quick Check:

   Correct array declaration uses new int[size] [OK]

Hint: Use 'type[] name = new type[size];' for arrays [OK]

Common Mistakes:

• Missing new keyword
• Wrong brackets placement
• Using parentheses instead of brackets

3.

What will be the output of the following Java code?

int[] numbers = {10, 20, 30, 40};
System.out.println(numbers[2]);

medium

A. 40

B. 20

C. 30

D. ArrayIndexOutOfBoundsException

Solution

1. Step 1: Identify the array elements and indexes

   The array 'numbers' has elements: index 0=10, 1=20, 2=30, 3=40.

2. Step 2: Access the element at index 2

   numbers[2] refers to the third element, which is 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Index 2 value is 30 [OK]

Hint: Remember array index starts at 0, so 2 means third element [OK]

Common Mistakes:

• Confusing index 2 with 1 or 3
• Expecting 40 instead of 30
• Thinking index starts at 1

4.

Find the error in the following code snippet:

int[] arr = new int[3];
arr[3] = 10;
System.out.println(arr[3]);

medium

A. Index 3 is out of bounds for array of size 3

B. Missing initialization of array elements

C. Array size should be 4

D. Syntax error in array declaration

Solution

1. Step 1: Check array size and valid indexes

   The array size is 3, so valid indexes are 0, 1, and 2.

2. Step 2: Identify the invalid index usage

   Accessing arr[3] is invalid and causes ArrayIndexOutOfBoundsException.

3. Final Answer:

   Index 3 is out of bounds for array of size 3 -> Option A

4. Quick Check:

   Array indexes go from 0 to size-1 [OK]

Hint: Array indexes must be less than size [OK]

Common Mistakes:

• Using index equal to size
• Assuming array auto-expands
• Ignoring runtime exceptions

5.

Given an integer array arr of size 5 initialized as {2, 4, 6, 8, 10}, which code snippet correctly doubles each element's value?

?

hard

A. for(int i = 0; i < arr.length; i++) { arr[i] *= 2; }

B. for(int i = 0; i <= arr.length; i++) { arr[i] = arr[i] * 2; }

C. for(int i = 1; i < arr.length; i++) { arr[i] = arr[i] * 2; }

D. for(int i = 0; i < arr.length - 1; i++) { arr[i] = arr[i] * 2; }

Solution

1. Step 1: Understand array length and loop bounds

   The array length is 5, so valid indexes are 0 to 4. Loop must run from 0 to less than length.

2. Step 2: Evaluate each option's loop condition

   for(int i = 0; i <= arr.length; i++) { arr[i] = arr[i] * 2; } uses i <= arr.length which causes out of bounds error at i=5. for(int i = 0; i < arr.length; i++) { arr[i] *= 2; } correctly uses i < arr.length. for(int i = 1; i < arr.length; i++) { arr[i] = arr[i] * 2; } starts at 1, missing index 0. for(int i = 0; i < arr.length - 1; i++) { arr[i] = arr[i] * 2; } stops at length-1, missing last element.

3. Final Answer:

   for(int i = 0; i < arr.length; i++) { arr[i] *= 2; } -> Option A

4. Quick Check:

   Loop from 0 to length-1 doubles all elements [OK]

Hint: Use i < arr.length to avoid index errors [OK]

Common Mistakes:

• Using <= arr.length causing errors
• Starting loop at 1 missing first element
• Stopping loop early missing last element