One-dimensional arrays in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a one-dimensional array in Java?

A one-dimensional array in Java is a collection of elements of the same type stored in a single row. It allows you to store multiple values in a single variable, accessed by an index starting at 0.

beginner

How do you declare a one-dimensional array of integers named 'numbers' with 5 elements in Java?

You declare it like this:
int[] numbers = new int[5];
This creates an array that can hold 5 integers, all initially set to 0.

beginner

How do you access the third element in a one-dimensional array named 'data'?

You access it using its index:
data[2]
Remember, array indexes start at 0, so the third element is at index 2.

intermediate

What happens if you try to access an array index that is out of bounds in Java?

Java throws an ArrayIndexOutOfBoundsException. This means you tried to access an index less than 0 or greater than or equal to the array length.

beginner

How can you initialize a one-dimensional array with values at the time of declaration?

You can write:
int[] numbers = {10, 20, 30, 40, 50};
This creates an array with 5 elements already set to those values.

What is the index of the first element in a Java array?

A 1

B 0

C -1

D Depends on the array size

Java arrays start indexing at 0, so the first element is at index 0.

Which of the following is the correct way to declare a one-dimensional array of doubles named 'prices' with 10 elements?

A double prices = new double[10];

B double prices[] = new double;

C double prices[] = new double(10);

D double[] prices = new double[10];

The correct syntax is double[] prices = new double[10]; to declare an array of 10 doubles.

What will happen if you try to access array element at index 5 in an array declared as int[] arr = new int[5];?

A Returns the last element

B Returns 0

C Throws ArrayIndexOutOfBoundsException

D Creates a new element at index 5

Since valid indexes are 0 to 4, accessing index 5 causes an ArrayIndexOutOfBoundsException.

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

A items.length

B items.size()

C items.length()

D length(items)

In Java, arrays have a public final field called length accessed as items.length.

Which of these initializes an array with values 1, 2, 3 in one line?

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

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

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

D int[] arr = (1, 2, 3);

You can initialize an array with values using int arr[] = {1, 2, 3};.

Explain what a one-dimensional array is and how you declare one in Java.

Think about a list of items stored in a single row.

Describe how to access and modify elements in a one-dimensional array in Java.

Imagine picking or changing an item in a numbered list.

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