How to pass an ArrayList to a varargs method parameter?

java

array

varargs

java-best-practices

byAlex Kataev·Nov 6, 2024

The toArray(T[] a) method of ArrayList allows the conversion of an ArrayList to a varargs. Below is an example:

public void varargsMethod(String... args) {
    // Implementation
}

ArrayList<String> list = new ArrayList<>(Arrays.asList("element1", "element2", "element3"));
varargsMethod(list.toArray(new String[0])); // Converting List to Array for varargs

new String[0] acts as a type indicator for toArray() to create a String array. Always ensure that the array type matches the varargs type, else you'll bump into a ClassCastException.

Matching array and vararg types

When passing an ArrayList to a varargs method, the array type has to be precisely defined. By feeding new String[list.size()], you provide an appropriately-sized array, sidestepping the toArray() method having to allocate a new one.

ArrayList<WorldLocation> locations = // ... initialization

public void travel(WorldLocation... destinations) {
    // Travel the world! If only it was that easy in real life!
}

travel(locations.toArray(new WorldLocation[locations.size()]));

Supressing compiler warnings

If you encounter unchecked warnings when converting collections to arrays in generic context, suppress them only when you're certain of your collection's type stability.

@SuppressWarnings("unchecked")
public void methodWithVarargs(T... params) {
    // ...
}

List<T> myList = // ...
methodWithVarargs(myList.toArray((T[]) new Object[0]));  // Enough typecasting to put a Broadway show to shame!

The usage of the @SuppressWarnings annotation should be restricted to avoiding masked type mismatch issues.

Looping through varargs

To handle each element of your varargs individually, simply iterate using a for loop:

for (String element : args) {
    // Now processing: element
}

This method functions incredibly well when you have an array of elements derived from an ArrayList, needing custom processing in the method.

Avoiding potential errors

Incorrect array size

While a zero-length array (new String[0]) is fine due to Java optimisations, allocating an oversized array uses unnecessary memory and can introduce null elements.