One-dimensional arrays in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array element modification

What is the output of this Java code snippet?

Java

public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        arr[2] = arr[0] + arr[4];
        System.out.println(arr[2]);
    }
}

A 8

B 3

C 5

D 6

💡 Hint

Remember array indices start at 0. arr[2] is changed to arr[0] + arr[4].

arr[0] is 1 and arr[4] is 5, so arr[2] becomes 1 + 5 = 6.

❓ Predict Output

intermediate

Sum of array elements

What is the output of this Java program?

Java

public class Main {
    public static void main(String[] args) {
        int[] numbers = {10, 20, 30, 40};
        int sum = 0;
        for (int i = 0; i < numbers.length; i++) {
            sum += numbers[i];
        }
        System.out.println(sum);
    }
}

A 100

B 10

C 40

D 90

💡 Hint

Add all elements in the array.

Sum is 10 + 20 + 30 + 40 = 100.

🔧 Debug

advanced

Identify the error in array access

What error does this Java code produce when run?

Java

public class Main {
    public static void main(String[] args) {
        int[] arr = new int[3];
        arr[3] = 10;
        System.out.println(arr[3]);
    }
}

A SyntaxError

B NullPointerException

C ArrayIndexOutOfBoundsException

D No error, prints 10

💡 Hint

Check the valid indices for an array of size 3.

Array indices go from 0 to length-1. arr[3] is out of bounds.

❓ Predict Output

advanced

Output after array element swap

What is the output of this Java program?

Java

public class Main {
    public static void main(String[] args) {
        int[] arr = {7, 8, 9};
        int temp = arr[0];
        arr[0] = arr[2];
        arr[2] = temp;
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }
}

A 9 8 7

B 7 8 9

C 8 7 9

D 9 7 8

💡 Hint

The first and last elements are swapped.

arr[0] and arr[2] are swapped, so array becomes {9, 8, 7}.

🧠 Conceptual

expert

Number of elements after array initialization

How many elements does the array contain after this declaration?

int[] arr = new int[5];

A 0

B 5

C 6

D 4

💡 Hint

The number inside the brackets defines the size.

The array is created with 5 elements indexed 0 to 4.

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