Array declaration and initialization in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the correct way to declare an array of integers in Java?

You declare an array of integers by specifying the type followed by square brackets and the array name, like this: int[] numbers;

beginner

How do you initialize an array with specific values at the time of declaration?

You can initialize an array with values using curly braces, like this: int[] numbers = {1, 2, 3, 4};

beginner

What happens if you declare an array but do not initialize it?

The array variable will point to null and trying to use it will cause a NullPointerException. You must initialize it before use.

beginner

How do you create an empty array of size 5 for integers?

You create it by specifying the size with new: int[] numbers = new int[5];. This creates an array with 5 elements, all initialized to 0.

beginner

Can you change the size of an array after it is created in Java?

No, arrays in Java have a fixed size once created. To have a resizable collection, use classes like ArrayList.

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

A int[] arr = {10, 20, 30};

B int arr = new int[3];

C int arr[] = new int;

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

Option A correctly declares and initializes an integer array with 3 elements.

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

A 1

B null

C undefined

D 0

Integer arrays are initialized with 0 by default for each element.

How do you declare an array variable without creating the array object yet?

A int[] arr;

B int[] arr = new int[];

C int arr = new int[5];

D int arr[] = {1, 2, 3};

Option A declares the array variable but does not create the array object.

What happens if you try to access an element outside the array bounds?

A Returns null

B Throws ArrayIndexOutOfBoundsException

C Compiles but ignores the access

D Returns 0

Accessing outside the array size throws an ArrayIndexOutOfBoundsException at runtime.

Which syntax correctly creates an array of 4 strings?

A String[] names = {4};

B String names = new String[4];

C String[] names = new String[4];

D String names[] = new String;

Option C correctly creates a string array with 4 elements.

Explain how to declare and initialize an integer array with 5 elements in Java.

Think about the type, square brackets, and using new int[5].

Describe what happens if you try to use an array variable that has been declared but not initialized.

Consider what the variable points to before creating the array.

Practice

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

easy

A. int[] numbers;

B. array int numbers;

C. int numbers[] = new array;

D. int numbers = new int[];

Solution

1. Step 1: Understand Java array declaration syntax

   In Java, arrays are declared by specifying the type followed by square brackets and the variable name.

2. Step 2: Check each option for correct syntax

   int[] numbers; uses int[] numbers; which is the correct syntax. Other options misuse keywords or syntax.

3. Final Answer:

   int[] numbers; -> Option A

4. Quick Check:

   Correct array declaration = int[] numbers; [OK]

Hint: Remember: type[] name; declares an array in Java [OK]

Common Mistakes:

• Using 'array' keyword which doesn't exist in Java
• Assigning new array without specifying size or values
• Placing brackets incorrectly

2. Which of the following lines correctly initializes an array of 5 integers in Java?

easy

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

B. int arr = new int[5];

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

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

Solution

1. Step 1: Understand array initialization with size

   To create an array of fixed size, use new type[size] syntax.

2. Step 2: Validate each option

   int[] arr = new int[5]; correctly declares and initializes an integer array of size 5. Others have syntax errors or missing size.

3. Final Answer:

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

4. Quick Check:

   Array initialization with size = new int[5] [OK]

Hint: Use new type[size] to initialize arrays in Java [OK]

Common Mistakes:

• Missing square brackets in declaration
• Not specifying size when using new
• Using parentheses instead of brackets

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

int[] nums = {1, 2, 3, 4};
System.out.println(nums[2]);

medium

A. 1

B. 2

C. 3

D. 4

Solution

1. Step 1: Understand array indexing in Java

   Arrays start at index 0, so nums[0]=1, nums[1]=2, nums[2]=3, nums[3]=4.

2. Step 2: Identify the printed element

   The code prints nums[2], which is the third element, value 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Index 2 value = 3 [OK]

Hint: Remember array index starts at 0 in Java [OK]

Common Mistakes:

• Confusing index 2 with the second element
• Off-by-one errors in indexing
• Misreading array initialization

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

int arr[5];

medium

A. Array size cannot be specified in declaration without initialization

B. Missing semicolon at the end

C. Array must be declared as int arr = new int[5];

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 inside square brackets during declaration without initialization.

2. Step 2: Analyze the given code

   int arr[5]; tries to declare an array with size 5 but does not initialize it, which is invalid syntax.

3. Final Answer:

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

4. Quick Check:

   Size only with new keyword = correct [OK]

Hint: Specify size only when using new keyword [OK]

Common Mistakes:

• Trying to declare size in brackets without new
• Confusing Java with C/C++ array syntax
• Forgetting to initialize array after declaration

5. How can you declare and initialize a String array with the values "apple", "banana", and "cherry" in one line in Java?

hard

A. String[] fruits = new String{"apple", "banana", "cherry"};

B. String[] fruits = new String[]{"apple", "banana", "cherry"};

C. String fruits[] = new String[3]{"apple", "banana", "cherry"};

D. String fruits = {"apple", "banana", "cherry"};

Solution

1. Step 1: Understand array initialization with values

   Java allows two ways to initialize arrays with values: shorthand without new keyword only at declaration, or with new keyword and braces.

2. Step 2: Check each option

   String[] fruits = new String[]{"apple", "banana", "cherry"}; is correct. Other options misuse the new keyword by missing brackets after the type, specifying size with initializer list (invalid syntax), or missing array declaration brackets.

3. Final Answer:

   String[] fruits = new String[]{"apple", "banana", "cherry"}; -> Option B

4. Quick Check:

   Use new Type[]{values} for inline initialization [OK]

Hint: Use new Type[]{values} or shorthand only at declaration [OK]

Common Mistakes:

• Mixing shorthand and new keyword incorrectly
• Missing square brackets in declaration
• Assigning array values without brackets