Array length property in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the length of the array.

Java

int[] numbers = {1, 2, 3, 4, 5};
System.out.println(numbers[1]);

Drag options to blanks, or click blank then click option'

A.length()

B.count

C.size

D.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using .length() like a method causes a compile error.

Using .size or .count are not valid for arrays.

In Java, arrays have a length property (without parentheses) to get their size.

2fill in blank

medium

Complete the code to loop through the array using its length property.

Java

int[] values = {10, 20, 30};
for (int i = 0; i < values[1]; i++) {
    System.out.println(values[i]);
}

Drag options to blanks, or click blank then click option'

A.count

B.size()

C.length

D.length()

Attempts:

3 left

💡 Hint

Common Mistakes

Using .length() causes a compile error.

Using .size() or .count are invalid for arrays.

The loop uses values.length to know how many times to run.

3fill in blank

hard

Fix the error in accessing the array length property.

Java

String[] names = {"Anna", "Bob", "Cara"};
int size = names[1];
System.out.println(size);

Drag options to blanks, or click blank then click option'

A.length()

B.length

C.size

D.count

Attempts:

3 left

💡 Hint

Common Mistakes

Using .length() instead of .length.

Trying to use .size or .count which are not valid for arrays.

The correct way to get the length of an array is using .length without parentheses.

4fill in blank

hard

Fill both blanks to create a map of array elements to their lengths.

Java

Map<String, Integer> map = new HashMap<>();
String[] words = {"cat", "dog", "bird"};
for (String word : words) {
    map.put(word, word[1]);
}
System.out.println(map.size() == words[2]);

Drag options to blanks, or click blank then click option'

A.length()

B.length

C.size()

D.count

Attempts:

3 left

💡 Hint

Common Mistakes

Using .length for strings causes errors.

Using .length() for arrays causes errors.

Strings use .length() method to get length, arrays use .length property.

5fill in blank

hard

Fill all three blanks to create a dictionary of word lengths for words longer than 3 characters.

Java

Map<String, Integer> wordLengths = new HashMap<>();
String[] words = {"apple", "bat", "carrot", "dog"};
for (String [1] : words) {
    if ([1].[2]() > 3) {
        wordLengths.put([3], [3].length());
    }
}
System.out.println(wordLengths);

Drag options to blanks, or click blank then click option'

Aword

Blength

Clength()

Dwords

Attempts:

3 left

💡 Hint

Common Mistakes

Using array name instead of loop variable.

Using .length property instead of .length() method for strings.

Use word as the loop variable, length() to get string length, and word to put in the map.

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