Answer

Varargs is a special syntax that allows a method to accept a variable (any) number of arguments of one type. In the method declaration, such a parameter is denoted as Type... name (for example, int... nums). Inside the method, the arguments are treated as a regular array.

Example:

public void printNumbers(int... nums) {
    for (int n : nums) {
        System.out.print(n + " ");
    }
}

You can call it like this:

printNumbers(1, 2, 3, 4);
// or
printNumbers();

Internal implementation: When a method with varargs is called, the compiler automatically creates an array of the corresponding type.

Limitations and nuances:

• There can be only one varargs parameter, and it must always be the last in the list of parameters.
• The varargs expansion (Type...) is essentially an array (Type[]).
• Passing null: printNumbers(null) will cause an NPE when attempting to iterate.
• Overloading methods with varargs can lead to ambiguity in calls.

Trick question

Question: "Can you declare a method with multiple varargs parameters?"

Common mistake: Many believe that you can declare public void example(int... a, String... b), but this will cause a compilation error.

Correct answer: No, there can be only one varargs parameter, and it must be the last.

Examples of real errors due to ignorance of the topic nuances

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Story

In a corporate logging library, overloaded versions of methods with varargs and arrays were implemented. When passing an array instead of a set of elements, the wrong version of the method was applied, resulting in the array being logged as an object rather than its contents. This caused confusion when analyzing logs.

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Story

In an integration service, a method call with varargs accepted null without checking for NPE. The system unexpectedly "crashed" when trying to iterate elements, as at least an empty array was expected.

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Story

In one e-commerce project, there was confusion with method overloads: there was a method send(String subject, String... emails) and another — send(String subject, String[] emails). When passing an array, the compiler implicitly chose the wrong method. The result: emails were not sent to clients.