Understanding Double Brace Initialization in Java Collections

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:

First Brace: The first brace {} creates an anonymous inner class that extends the collection class (e.g., ArrayList, HashSet, etc.).
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