Array declaration and initialization in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array initialization and access

What is the output of the following Java code?

Java

public class Main {
    public static void main(String[] args) {
        int[] numbers = new int[3];
        numbers[0] = 5;
        numbers[1] = 10;
        numbers[2] = 15;
        System.out.println(numbers[1]);
    }
}

A 0

B 5

C 10

D 15

💡 Hint

Remember array indices start at 0.

The array 'numbers' has elements 5, 10, and 15 at indices 0, 1, and 2 respectively. Accessing numbers[1] prints 10.

❓ Predict Output

intermediate

Output of array initialization with values

What will be printed by this Java program?

Java

public class Main {
    public static void main(String[] args) {
        String[] fruits = {"Apple", "Banana", "Cherry"};
        System.out.println(fruits.length);
    }
}

A 3

B 2

C 0

D Compilation error

💡 Hint

The length property tells how many items are in the array.

The array 'fruits' has three elements, so fruits.length is 3.

❓ Predict Output

advanced

Output of multi-dimensional array initialization

What is the output of this Java code?

Java

public class Main {
    public static void main(String[] args) {
        int[][] matrix = new int[2][3];
        matrix[0][1] = 7;
        matrix[1][2] = 9;
        System.out.println(matrix[1][2] + matrix[0][1]);
    }
}

A ArrayIndexOutOfBoundsException

B 79

C 0

D 16

💡 Hint

Add the two assigned values from the matrix.

matrix[0][1] is 7 and matrix[1][2] is 9, their sum is 16.

❓ Predict Output

advanced

Output of array default values

What will this Java program print?

Java

public class Main {
    public static void main(String[] args) {
        boolean[] flags = new boolean[4];
        System.out.println(flags[2]);
    }
}

A true

B false

C null

D Compilation error

💡 Hint

Arrays of booleans are initialized with false by default.

Boolean arrays in Java default to false for all elements.

🧠 Conceptual

expert

Number of elements in a jagged array

Consider the following Java code snippet. How many elements does the jagged array contain in total?

Java

public class Main {
    public static void main(String[] args) {
        int[][] jagged = new int[3][];
        jagged[0] = new int[2];
        jagged[1] = new int[4];
        jagged[2] = new int[3];
    }
}

A 9

B 7

C 3

D 12

💡 Hint

Add the lengths of each inner array.

The jagged array has inner arrays of lengths 2, 4, and 3. Total elements = 2 + 4 + 3 = 9.

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