Understanding Double Brace Initialization in Java Collections

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질문:

Explain the term “Double Brace Initialisation” in Java?

답변:

Double Brace Initialization is a technique in Java that allows you to create and initialize collections (like List, Set, or Map) in a more concise and readable way. It involves using an anonymous inner class to create an instance of a collection and then immediately populating it with elements.

Here's how it works:

  1. First Brace: The first brace {} creates an anonymous inner class that extends the collection class (e.g., ArrayList, HashSet, etc.).
  2. Second Brace: The second brace {} is used to define an instance initializer block, where you can add elements to the collection.

Example

Here’s an example of using Double Brace Initialization with a List:

import java.util.ArrayList;
import java.util.List;

public class DoubleBraceInitializationExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<String>() {{
            add("Apple");
            add("Banana");
            add("Cherry");
        }};
        
        System.out.println(list);
    }
}

Explanation of the Example

  • new ArrayList<String>() creates an instance of ArrayList.
  • The {{ ... }} block is an instance initializer that allows you to add elements to the list right after its creation.
  • The add method is called within the initializer block to populate the list with "Apple", "Banana", and "Cherry".

Advantages

  • Conciseness: It reduces boilerplate code for initializing collections.
  • Readability: It makes the code more readable by keeping the initialization close to the declaration.

Disadvantages

  • Anonymous Class Overhead: Each use of Double Brace Initialization creates an anonymous inner class, which can lead to increased memory usage and potential memory leaks if the inner class holds a reference to the outer class.
  • Serialization Issues: If the collection is serialized, the anonymous class may cause issues since it is not a named class.
  • Less Familiar: Some developers may find this syntax less familiar or harder to understand, especially if they are not accustomed to inner classes.

Conclusion

Double Brace Initialization is a useful technique for quickly initializing collections in Java, but it should be used judiciously, considering the potential downsides related to performance and readability.