Common array operations in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the syntax to declare and initialize an array of integers with values 1, 2, 3 in Java?

You can declare and initialize an integer array like this:

int[] numbers = {1, 2, 3};

beginner

How do you find the length of an array named arr in Java?

Use arr.length to get the number of elements in the array.
Example:

int size = arr.length;

beginner

How can you loop through all elements of an array arr using a for-each loop?

Use the for-each loop syntax:

for (int element : arr) {
    // use element
}

This goes through each item in arr one by one.

intermediate

How do you copy an array source to a new array copy in Java?

You can use Arrays.copyOf:

int[] copy = Arrays.copyOf(source, source.length);

This creates a new array with the same elements.

beginner

How do you sort an array arr of integers in Java?

Use Arrays.sort(arr); to sort the array in ascending order.
Example:

Arrays.sort(arr);

Which of the following is the correct way to declare an array of strings named names?

A String[] names = new String;

B String[] names;

C String names = new String[];

D String names[] = ();

Option B correctly declares an array of strings. Options A, B, and C are invalid.

How do you access the third element of an array arr?

A arr.get(3)

B arr[3]

C arr[2]

D arr(2)

Arrays in Java start at index 0, so the third element is at index 2.

What does arr.length return?

A The number of elements in the array

B The last element of the array

C The size of the array in bytes

D The index of the last element

arr.length gives the total number of elements stored in the array.

Which method sorts an array arr in Java?

A Arrays.sort(arr);

B arr.sort();

C Collections.sort(arr);

D arr.sorted();

The static method Arrays.sort() sorts arrays. Collections.sort() is for lists.

How do you loop through all elements of an array arr using a for-each loop?

A for each (int x in arr) { }

B for (int i = 0; i < arr.length; i++) { }

C for (int i = 1; i <= arr.length; i++) { }

D for (int x : arr) { }

Option D is the correct for-each loop syntax in Java.

Explain how to declare, initialize, and access elements in a Java array.

Think about how you create a list of items and get each item by its position.

Describe how to loop through all elements of an array and how to sort an array in Java.

Consider how you would read each item one by one and then arrange them in order.

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