Array Declaration and Initialization in DSA Java

Mental Model

An array is a fixed-size collection of items stored next to each other in memory, where each item can be accessed by its position number.

Analogy: Think of an array like a row of mailboxes, each with a number. You can put a letter in any mailbox or take it out by knowing its number.

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [ ]   [ ]   [ ]   [ ]   [ ]

Dry Run Walkthrough

Input: Declare an integer array of size 5 and initialize it with values 10, 20, 30, 40, 50

Goal: Create an array that holds these 5 numbers in order and can be accessed by their positions

Step 1: Declare an integer array variable named arr with size 5

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [ ]   [ ]   [ ]   [ ]   [ ]

Why: We need to reserve space for 5 integers before storing values

Step 2: Assign value 10 to arr[0]

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [10]  [ ]   [ ]   [ ]   [ ]

Why: Store the first number at the first position

Step 3: Assign value 20 to arr[1]

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [10]  [20]  [ ]   [ ]   [ ]

Why: Store the second number at the second position

Step 4: Assign value 30 to arr[2]

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [10]  [20]  [30]  [ ]   [ ]

Why: Store the third number at the third position

Step 5: Assign value 40 to arr[3]

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [10]  [20]  [30]  [40]  [ ]

Why: Store the fourth number at the fourth position

Step 6: Assign value 50 to arr[4]

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [10]  [20]  [30]  [40]  [50]

Why: Store the fifth number at the fifth position

Result:

Index:  [0] -> [1] -> [2] -> [3] -> [4]
Value:  [10]  [20]  [30]  [40]  [50]

Annotated Code

DSA Java

public class ArrayExample {
    public static void main(String[] args) {
        // Step 1: Declare an integer array of size 5
        int[] arr = new int[5];
        
        // Step 2-6: Initialize array elements
        arr[0] = 10; // assign 10 to first element
        arr[1] = 20; // assign 20 to second element
        arr[2] = 30; // assign 30 to third element
        arr[3] = 40; // assign 40 to fourth element
        arr[4] = 50; // assign 50 to fifth element
        
        // Print the array elements
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i]);
            if (i < arr.length - 1) {
                System.out.print(" -> ");
            }
        }
        System.out.print(" -> null\n");
    }
}

int[] arr = new int[5];

Reserve space for 5 integers in memory

arr[0] = 10;

Store 10 at first position

arr[1] = 20;

Store 20 at second position

arr[2] = 30;

Store 30 at third position

arr[3] = 40;

Store 40 at fourth position

arr[4] = 50;

Store 50 at fifth position

for (int i = 0; i < arr.length; i++) {

Traverse array from start to end to print values

System.out.print(arr[i]);

Print current element value

Output

10 -> 20 -> 30 -> 40 -> 50 -> null

Complexity Analysis

Time: O(n) because we initialize and print each of the n elements once

Space: O(n) because the array stores n elements in contiguous memory

vs Alternative: Compared to using individual variables, arrays use less code and allow easy access by index

Edge Cases

Declare array with size zero

Array is created with no elements; accessing any index causes error

DSA Java

int[] arr = new int[0];

Access index outside array bounds

Throws ArrayIndexOutOfBoundsException at runtime

DSA Java

for (int i = 0; i < arr.length; i++) {

When to Use This Pattern

When you need to store multiple items of the same type in order and access them by position, use arrays because they provide fixed-size indexed storage.

Common Mistakes

Mistake: Trying to access or assign values outside the declared array size
Fix: Always use indices from 0 up to array length minus one to avoid errors

Mistake: Forgetting to initialize array elements before use
Fix: Assign values to each element before reading them to avoid default zero or unexpected values

Summary

An array holds multiple values of the same type in a fixed-size sequence accessible by index.

Use arrays when you want to group related data items and access them quickly by position.

Remember arrays reserve space upfront and use zero-based indexing to find each element.

Practice

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

easy

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

B. int arr = new int[5];

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

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

Solution

1. Step 1: Understand Java array declaration syntax

   In Java, arrays are declared with the type followed by square brackets and initialized with the new keyword and size inside square brackets.

2. Step 2: Check each option for correct syntax

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

3. Final Answer:

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

4. Quick Check:

   Correct Java array declaration = int[] arr = new int[5]; [OK]

Hint: Use 'new' keyword and specify size in brackets [OK]

Common Mistakes:

• Forgetting 'new' keyword
• Using wrong brackets or missing type
• Declaring array without brackets

2. Which of the following is a valid way to declare and initialize an array with values in Java?

easy

A. int[] arr = new int[] {1, 2, 3, 4};

B. int arr[] = new int{1, 2, 3, 4};

C. int[] arr = new int{1, 2, 3, 4};

D. int arr = {1, 2, 3, 4};

Solution

1. Step 1: Understand array initialization with values

   In Java, to initialize an array with values using 'new', syntax is: int[] arr = new int[] {values};

2. Step 2: Check each option

   int[] arr = new int[] {1, 2, 3, 4};: correct syntax. int arr[] = new int{1, 2, 3, 4};: incorrect declaration style and missing [] after new int. int[] arr = new int{1, 2, 3, 4};: missing [] after new int. int arr = {1, 2, 3, 4};: arr not declared as array.

3. Final Answer:

   int[] arr = new int[] {1, 2, 3, 4}; -> Option A

4. Quick Check:

   Array init with 'new int[]' = int[] arr = new int[] {1, 2, 3, 4}; [OK]

Hint: Use 'new int[]' when initializing separately [OK]

Common Mistakes:

• Omitting 'new' when required
• Using wrong brackets
• Assigning values without array type

3. What will be the output of the following Java code?

int[] arr = new int[3];
arr[0] = 5;
arr[2] = 10;
System.out.println(arr[1]);

medium

A. 5

B. 0

C. 10

D. Compilation error

Solution

1. Step 1: Understand default values in integer arrays

   When an integer array is created with 'new int[3]', all elements are initialized to 0 by default.

2. Step 2: Check assigned values and output

   arr[0] = 5 and arr[2] = 10 are assigned. arr[1] is not assigned, so it remains 0. The print statement outputs arr[1], which is 0.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   Unassigned int array element = 0 [OK]

Hint: Unassigned int array elements default to 0 [OK]

Common Mistakes:

• Assuming unassigned elements have garbage values
• Confusing index positions
• Expecting runtime error for unassigned index

4. Identify the error in the following Java array declaration:

int arr[5];

medium

A. Missing semicolon at the end

B. Array must be declared with curly braces

C. Array size cannot be specified in declaration without initialization

D. Array size must be a variable, not a number

Solution

1. Step 1: Understand Java array declaration rules

   In Java, you cannot specify the size of the array in the declaration alone. Size must be specified during initialization with 'new'.

2. Step 2: Analyze the given code

   int arr[5]; tries to declare an array with size 5 without initialization, which is invalid syntax in Java.

3. Final Answer:

   Array size cannot be specified in declaration without initialization -> Option C

4. Quick Check:

   Size only with 'new' keyword [OK]

Hint: Specify size only with 'new' keyword during initialization [OK]

Common Mistakes:

• Using C-style array declaration
• Trying to declare size without 'new'
• Confusing declaration and initialization

5. Given the following code, what will be the output?

int[] arr = {1, 2, 3};
arr = new int[] {4, 5};
System.out.print(arr.length);

Choose the correct output and explain why.

hard

A. 3

B. Runtime error

C. Compilation error

D. 2

Solution

1. Step 1: Understand array reassignment

   Initially, arr points to an array of length 3 with values {1, 2, 3}. Then arr is reassigned to a new array with values {4, 5} of length 2.

2. Step 2: Check the length property after reassignment

   After reassignment, arr.length reflects the new array length, which is 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Array length after reassignment = 2 [OK]

Hint: Array variable can point to new array; length updates accordingly [OK]

Common Mistakes:

• Assuming original array length remains
• Expecting error on reassignment
• Confusing array contents with length