How to Fix MyIterable Constructor Error?

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Question

“The constructor MyIterable should accept parameter of type List<String>.”

I am unable to complete the “Get Hands-On with an Iterable Variable in For Loops” Trailhead module because I keep getting the following error message:

Here is the code I have for my MyIterable class, where its constructor is supposed to accept a List<String> parameter:

public class MyIterable implements Iterable<String> {
    
    private List<String> strings;
    
    public MyIterable(List<String> strList) {
        strings = strList;
        strings.iterator();
    }
    
    public Iterator<String> iterator() {
        return strings.iterator();
    }
}

I am following the instructions, but I am still facing the error. How can I fix this?

Solution

The error arises because the constructor is not being recognized properly due to an unnecessary call to strings.iterator() within the constructor. The issue can be resolved by ensuring that the constructor only accepts the List<String> and stores it without calling the iterator method inside the constructor.

Here is the corrected version of the MyIterable class with the right constructor:

public class MyIterable implements Iterable<String> {
    private List<String> strings;

    public MyIterable(List<String> strings) {
        this.strings = strings;
    }

    public Iterator<String> iterator() {
        return strings.iterator();
    }
}

In this version, the constructor correctly accepts a List<String> and assigns it to the strings field. The iterator() method then returns the iterator for that list when needed. This ensures that the class conforms to the Iterable<String> interface without unnecessary calls inside the constructor.

Test Class:

To validate the functionality of your iterable class, you can use the following test class:

@IsTest
public class MyIterableTest {
    @IsTest
    static void testIterableForLoop() {
        List<String> strings = new List<String>{'Hello', 'World'};

        MyIterable iterable = new MyIterable(strings);

        for (String str : iterable) {
            System.debug(str);
        }
    }
}

Explanation of the Code:

In the first code snippet, the MyIterable class implements the Iterable<String> interface, meaning it must define an iterator() method that returns an Iterator<String>. The constructor accepts a List<String> and assigns it to the strings field.
The iterator() method simply returns the iterator for the strings list, enabling it to be iterated using a for-each loop. The constructor doesn’t need to call strings.iterator() inside itself, as it’s handled by the iterator method.
The second code snippet is a test class MyIterableTest, marked with @IsTest to make it a test class. It creates a List<String>, passes it to the MyIterable constructor, and iterates over the list in a for-each loop, verifying that the iterator() method works as expected.
The test class checks if the MyIterable class functions correctly by printing the values of the list ('Hello' and 'World') to the debug log, ensuring the class is correctly handling the iteration and implementing the Iterable interface as required.

Summing Up

The error occurs because the constructor in the MyIterable class is incorrectly calling the strings.iterator() method inside itself. To resolve this, the constructor should simply accept a List<String> and assign it to the strings field without invoking the iterator method.

The MyIterable class implements the Iterable<String> interface, which requires an iterator() method that returns an iterator for the strings list. This allows the class to be used in a for-each loop to iterate over the list of strings.

A test class can be used to validate that the MyIterable class functions as expected. It creates a list, passes it to the class, and checks if the iteration works by printing the values to the debug log. This ensures that the MyIterable class can be used correctly for iteration.

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