How Do I Efficiently Convert a Java List to an Array?

Converting List to Array in Java

Many Java developers encounter the need to convert a List to an Array. This conversion is often necessary for tasks such as passing the List's elements to legacy APIs or manipulating data in specific data structures.

Method 1: toArray(new Foo[0])

The toArray() method provides an efficient way to convert a List to an array. By specifying an empty array of the desired type as an argument, Java automatically creates an array of the appropriate size and copies the List's elements into it.

Foo[] array = list.toArray(new Foo[0]);

This method is particularly useful when the array type is unknown, as the toArray() method can infer it dynamically.

Method 2: toArray(new Foo[list.size()])

Alternatively, developers can pre-size the array to the size of the List using the toArray() method with a pre-allocated array as an argument. However, this method is not recommended due to potential performance drawbacks and concurrency issues.

Foo[] array = new Foo[list.size()];
list.toArray(array);

Update: Preference for toArray(new Foo[0])

In recent Java versions, using toArray(new Foo[0]) is strongly recommended over toArray(new Foo[list.size()]). This is because Java's reflection-based array creation has been optimized, making the empty array version faster and more reliable. Additionally, the pre-sized array approach can lead to concurrency problems if the List is modified during the conversion.

Conclusion

To convert a List to an Array in Java, developers can use the toArray() method. The toArray(new Foo[0]) approach is recommended for its efficiency and reliability, while the toArray(new Foo[list.size()]) approach should be avoided unless necessary for backward compatibility.

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