Array length property in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code using array length?

Consider the following Java code snippet. What will be printed when it runs?

Java

public class Main {
    public static void main(String[] args) {
        int[] numbers = {5, 10, 15, 20, 25};
        System.out.println(numbers.length);
    }
}

A 6

B 5

C Compilation error

D 4

💡 Hint

Remember that the length property of an array gives the number of elements it contains.

The array 'numbers' has 5 elements, so numbers.length returns 5.

❓ Predict Output

intermediate

What is the output when accessing length of a multidimensional array?

Look at this Java code. What will it print?

Java

public class Main {
    public static void main(String[] args) {
        int[][] matrix = new int[3][4];
        System.out.println(matrix.length);
    }
}

A 3

B 4

C 7

D Runtime error

💡 Hint

The length property on a 2D array returns the number of rows.

The 'matrix' has 3 rows, so matrix.length is 3.

❓ Predict Output

advanced

What happens if you try to assign to the length property of an array?

What will happen when this Java code runs?

Java

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

A Runtime exception

B 5

C 3

D Compilation error

💡 Hint

The length property of an array is final and cannot be changed.

In Java, the length property of an array is final and cannot be assigned a new value, causing a compilation error.

❓ Predict Output

advanced

What is the output of this code accessing length of a null array?

What will this Java program print or do?

Java

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

A 0

B Compilation error

C NullPointerException at runtime

D Prints 'null'

💡 Hint

Accessing a property on a null reference causes an error at runtime.

Since arr is null, accessing arr.length causes a NullPointerException at runtime.

🧠 Conceptual

expert

How many elements are in the array after this code runs?

What is the number of elements in the array 'data' after executing this code?

Java

public class Main {
    public static void main(String[] args) {
        int[] data = new int[10];
        data = new int[5];
        System.out.println(data.length);
    }
}

A 5

B 10

C 15

D Compilation error

💡 Hint

The variable 'data' is reassigned to a new array before printing length.

The original array of length 10 is replaced by a new array of length 5, so data.length is 5.

Practice

1. What does the length property of an array in Java represent?

easy

A. The number of elements stored in the array

B. The last index of the array

C. The total memory size of the array

D. The sum of all elements in the array

Solution

1. Step 1: Understand the meaning of length in arrays

   The length property gives the count of elements stored in the array, not the last index or memory size.

2. Step 2: Differentiate from other array properties

   The last index is always length - 1, so length itself is the total number of elements.

3. Final Answer:

   The number of elements stored in the array -> Option A

4. Quick Check:

   Array length = number of elements [OK]

Hint: Remember: length counts elements, not indexes [OK]

Common Mistakes:

• Confusing length with last index
• Thinking length is a method needing parentheses
• Assuming length gives memory size

2. Which of the following is the correct way to get the length of an array named arr in Java?

easy

A. arr.size()

B. arr.length()

C. arr.length

D. arr.getLength()

Solution

1. Step 1: Recall syntax for array length in Java

   In Java, arrays have a property called length accessed without parentheses.

2. Step 2: Identify incorrect options

   arr.length() and arr.getLength() are invalid because length is not a method. arr.size() is used for collections, not arrays.

3. Final Answer:

   arr.length -> Option C

4. Quick Check:

   Array length is a property, no parentheses [OK]

Hint: Use arr.length, not parentheses [OK]

Common Mistakes:

• Adding parentheses after length
• Using size() which is for lists
• Trying to call getLength() method

3. What will be the output of the following Java code?

int[] numbers = {5, 10, 15, 20};
System.out.println(numbers.length);

medium

A. 4

B. 3

C. 5

D. Compilation error

Solution

1. Step 1: Count the elements in the array

   The array numbers has 4 elements: 5, 10, 15, and 20.

2. Step 2: Use the length property

   numbers.length returns the number of elements, which is 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Array length = 4 elements [OK]

Hint: Count elements, length equals that number [OK]

Common Mistakes:

• Counting indexes instead of elements
• Expecting length() method call
• Thinking length includes extra space

4. Identify the error in this Java code snippet:

int[] data = new int[5];
System.out.println(data.length());

medium

A. Array length property does not exist in Java

B. Array size must be declared with curly braces, not square brackets

C. The array must be initialized with values before accessing length

D. Array length is accessed with parentheses, causing a compile error

Solution

1. Step 1: Check how length is accessed

   The code uses data.length() with parentheses, but length is a property, not a method.

2. Step 2: Confirm correct syntax

   The correct way is data.length without parentheses. Using parentheses causes a compile-time error.

3. Final Answer:

   Array length is accessed with parentheses, causing a compile error -> Option D

4. Quick Check:

   length is a property, no parentheses [OK]

Hint: Remember: length is property, no () [OK]

Common Mistakes:

• Using parentheses after length
• Confusing array initialization syntax
• Thinking length needs array values first

5. Given the array int[] values = {2, 4, 6, 8, 10};, which code snippet correctly prints all elements using the array's length property?

hard

A. for (int i = 0; i <= values.length; i++) { System.out.println(values[i]); }

B. for (int i = 0; i < values.length; i++) { System.out.println(values[i]); }

C. for (int i = 1; i < values.length; i++) { System.out.println(values[i]); }

D. for (int i = 1; i <= values.length; i++) { System.out.println(values[i]); }

Solution

1. Step 1: Understand array indexing and length

   Array indexes start at 0 and go up to length - 1. So valid indexes are 0 to 4 for this array.

2. Step 2: Analyze each loop condition

   for (int i = 0; i <= values.length; i++) { System.out.println(values[i]); } uses i <= values.length, which tries to access index 5 (out of bounds). Options B and C start at 1, skipping the first element. for (int i = 0; i < values.length; i++) { System.out.println(values[i]); } correctly loops from 0 to 4.

3. Final Answer:

   for (int i = 0; i < values.length; i++) { System.out.println(values[i]); } -> Option B

4. Quick Check:

   Loop from 0 to length-1 to access all elements [OK]

Hint: Loop: i from 0 to length-1, use < i not <= [OK]

Common Mistakes:

• Using <= length causing index out of bounds
• Starting loop at 1 skipping first element
• Confusing length with last index