Array Declaration and Initialization in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an array in Java?

An array in Java is a container that holds a fixed number of values of the same type, stored in a contiguous block of memory.

beginner

How do you declare an integer array named 'numbers' in Java?

You declare it as: int[] numbers; or int numbers[];. Both are valid.

beginner

How do you initialize an array of size 5 in Java?

You initialize it with: numbers = new int[5]; This creates an array that can hold 5 integers.

intermediate

What is the difference between declaration and initialization of an array?

Declaration creates a reference variable for the array. Initialization allocates memory and sets up the array to hold values.

beginner

How do you declare and initialize an array with values 1, 2, 3 in one line?

Use: int[] numbers = {1, 2, 3}; This creates and fills the array immediately.

Which of the following is a valid array declaration in Java?

A arr int[];

B array int arr;

C int[] arr;

D int arr;

In Java, arrays are declared with the type followed by square brackets, like int[] arr;.

What does new int[4] do in Java?

A Creates an array of 4 integers initialized to 0

B Creates an array of 4 integers with random values

C Declares an array variable

D Deletes an array

It creates an array of size 4 with all elements set to 0 by default.

How do you access the third element of an array named 'data'?

A data(3)

B data[3]

C data{3}

D data[2]

Array indices start at 0, so the third element is at index 2.

Which of these initializes an array with values 5, 10, 15?

A int[] arr = {5, 10, 15};

B int arr = new int[5, 10, 15];

C int[] arr = new int(5, 10, 15);

D int arr[] = new int{5, 10, 15};

The correct syntax uses curly braces with values inside during declaration.

What is the default value of elements in a newly created int array in Java?

A null

B 0

C undefined

D 1

Integer arrays are initialized with 0 by default.

Explain how to declare and initialize an array of strings with 3 names in Java.

Think about using curly braces with string values.

Describe the difference between declaring an array and initializing an array in Java.

Consider what happens in memory during each step.

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