Common array operations in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array sum calculation

What is the output of this Java code that sums all elements in an integer array?

Java

int[] numbers = {2, 4, 6, 8};
int sum = 0;
for (int num : numbers) {
    sum += num;
}
System.out.println(sum);

A 30

B 24

C SyntaxError

D 20

💡 Hint

Add each element of the array to the sum variable inside the loop.

The code adds 2 + 4 + 6 + 8 which equals 20, so the output is 20.

❓ Predict Output

intermediate

Output of array reverse print

What does this Java code print when it prints the array elements in reverse order?

Java

int[] arr = {1, 3, 5, 7};
for (int i = arr.length - 1; i >= 0; i--) {
    System.out.print(arr[i] + " ");
}

A 7 5 3 1

B 1 3 5 7

C 7 5 3

D RuntimeError

💡 Hint

The loop starts from the last index and goes down to zero.

The loop prints elements starting from index 3 to 0, so output is '7 5 3 1 '.

❓ Predict Output

advanced

Output of array element replacement

What is the output after replacing all negative numbers with zero in this array?

Java

int[] data = {4, -1, 7, -3, 0};
for (int i = 0; i < data.length; i++) {
    if (data[i] < 0) {
        data[i] = 0;
    }
}
for (int num : data) {
    System.out.print(num + " ");
}

A SyntaxError

B 4 0 7 0 0

C 4 0 7 -3 0

D 4 -1 7 -3 0

💡 Hint

Check each element and replace if it is less than zero.

Negative values -1 and -3 are replaced with 0, so output is '4 0 7 0 0 '.

❓ Predict Output

advanced

Output of array copy with System.arraycopy

What is the output of this code that copies part of one array into another?

Java

int[] source = {10, 20, 30, 40, 50};
int[] dest = {1, 2, 3, 4, 5};
System.arraycopy(source, 1, dest, 2, 2);
for (int i : dest) {
    System.out.print(i + " ");
}

A 1 2 20 30 5

B 10 20 30 40 50

C 1 2 10 20 5

D ArrayIndexOutOfBoundsException

💡 Hint

System.arraycopy copies 2 elements starting at index 1 from source to index 2 in dest.

Elements 20 and 30 from source replace dest[2] and dest[3], so output is '1 2 20 30 5 '.

🧠 Conceptual

expert

Error type when accessing invalid array index

What error does this Java code produce when trying to access an invalid index in an array?

Java

int[] arr = {5, 10, 15};
System.out.println(arr[5]);

A SyntaxError

B NullPointerException

C ArrayIndexOutOfBoundsException

D ClassCastException

💡 Hint

Accessing an index outside the array length causes a runtime exception.

Index 5 is outside the array bounds (length 3), so Java throws ArrayIndexOutOfBoundsException.

Practice

1. Which of the following correctly declares an array of integers with 5 elements in Java?

easy

A. int[] numbers = int[5];

B. int numbers = new int[5];

C. int numbers[] = new int;

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

Solution

1. Step 1: Understand Java array declaration syntax

   In Java, arrays are declared with the type followed by square brackets, then the variable name, and initialized with 'new' and size.

2. Step 2: Check each option for correct syntax

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

3. Final Answer:

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

4. Quick Check:

   Correct array declaration = int[] numbers = new int[5]; [OK]

Hint: Remember: type[] name = new type[size]; for arrays [OK]

Common Mistakes:

• Missing square brackets after type
• Not using 'new' keyword
• Incorrect variable initialization

2. Which of the following is the correct way to access the third element of an array named arr in Java?

easy

A. arr(3);

B. arr[2];

C. arr[3];

D. arr.get(2);

Solution

1. Step 1: Recall Java array indexing

   Java arrays use zero-based indexing, so the first element is at index 0, second at 1, third at 2.

2. Step 2: Identify correct syntax for accessing elements

   Elements are accessed with square brackets []. So arr[2] accesses the third element. Options A and D use incorrect syntax, and C accesses the fourth element.

3. Final Answer:

   arr[2]; -> Option B

4. Quick Check:

   Third element index = 2 [OK]

Hint: Arrays start at 0, so third element is arr[2] [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing
• Trying to use methods like get() on arrays

3. What is the output of the following Java code?

int[] nums = {1, 2, 3, 4, 5};
int sum = 0;
for (int i = 0; i < nums.length; i++) {
    sum += nums[i];
}
System.out.println(sum);

medium

A. 15

B. 10

C. 5

D. 0

Solution

1. Step 1: Understand the array and loop

   The array nums contains 1, 2, 3, 4, 5. The loop runs from i=0 to i=4 (nums.length is 5).

2. Step 2: Calculate the sum of all elements

   sum = 0 + 1 + 2 + 3 + 4 + 5 = 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   Sum of 1 to 5 = 15 [OK]

Hint: Sum array by adding all elements in a loop [OK]

Common Mistakes:

• Off-by-one error in loop
• Starting sum at wrong value
• Using wrong loop condition

4. The following code is intended to print all elements of the array data, but it causes a runtime error. What is the error?

int[] data = {10, 20, 30};
for (int i = 1; i <= data.length; i++) {
    System.out.println(data[i]);
}

medium

A. ArrayIndexOutOfBoundsException because loop goes beyond last index

B. Syntax error due to wrong for loop syntax

C. Prints all elements correctly

D. NullPointerException because array is not initialized

Solution

1. Step 1: Analyze loop indices and array length

   The array has length 3, valid indices are 0, 1, 2. The loop runs i from 1 to 3 inclusive.

2. Step 2: Identify index causing error

   When i = 3, data[3] is accessed, which is out of bounds and causes ArrayIndexOutOfBoundsException.

3. Final Answer:

   ArrayIndexOutOfBoundsException because loop goes beyond last index -> Option A

4. Quick Check:

   Loop index must be less than length [OK]

Hint: Loop indices must be 0 to length-1 for arrays [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Starting loop at 1 instead of 0
• Ignoring zero-based indexing

5. Given an integer array arr, which code snippet correctly counts how many elements are greater than 10?

hard

A. int count = 0;
for (int i = 0; i < arr.length; i++) {
if (arr[i] >= 10) {
count++;
}
}
System.out.println(count);

B. int count = 0;
for (int i = 1; i <= arr.length; i++) {
if (arr[i] > 10) {
count++;
}
}
System.out.println(count);

C. int count = 0;
for (int i = 0; i < arr.length; i++) {
if (arr[i] > 10) {
count++;
}
}
System.out.println(count);

D. int count = 0;
for (int i = 0; i < arr.length; i++) {
if (arr[i] < 10) {
count++;
}
}
System.out.println(count);

Solution

1. Step 1: Understand the counting condition

   We want to count elements strictly greater than 10, so condition is arr[i] > 10.

2. Step 2: Check loop indices and condition correctness

   int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] > 10) { count++; } } System.out.println(count); uses correct zero-based indexing and correct condition. int count = 0; for (int i = 1; i <= arr.length; i++) { if (arr[i] > 10) { count++; } } System.out.println(count); has out-of-bounds error. int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] >= 10) { count++; } } System.out.println(count); counts elements >= 10 (includes 10). int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] < 10) { count++; } } System.out.println(count); counts elements less than 10, which is wrong.

3. Final Answer:

   int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] > 10) { count++; } } System.out.println(count); -> Option C

4. Quick Check:

   Count elements with arr[i] > 10 using correct loop [OK]

Hint: Use i < arr.length and condition arr[i] > 10 to count [OK]

Common Mistakes:

• Using <= instead of < in loop
• Using wrong comparison operator
• Starting loop at 1 causing index errors