doublebrace

Doublebrace, often referred to as double‑brace initialization, is a technique used primarily in Java to create and initialize an object, typically a collection, in a single expression. The syntax involves an anonymous inner class and an instance initializer block, such as:

new ArrayList<String>(){{ add("alpha"); add("beta"); }}.

The outer pair of braces instantiates an anonymous class extending the designated type, while the inner pair executes an instance initializer that runs immediately after construction. This allows developers to populate lists, sets, or maps inline without having to write separate code blocks or loops. While it can make code more concise, it also introduces several subtleties.

The anonymous subclass carries an implicit reference to its enclosing instance if used in a non‑static context. This retains memory until the subclass is garbage‑collected, potentially causing memory leaks in long‑running applications. Moreover, because the subclass extends the original type, it is not exactly the same as the original class; for example, serializing an ArrayList created this way will produce a class name that includes the anonymous inner class, which can break compatibility. The extra class also inflates the class hierarchy, which may affect performance and clarity of debugging output.

Despite these caveats, double‑brace initialization remains popular in code snippets and quick prototypes. Many modern development styles discourage its use in production code, preferring explicit builders, static factory methods, or utility methods such as Arrays.asList or Collections.unmodifiableList to achieve the same concise construction without the aforementioned risks.