Java 花括号初始化

1. Overview

In this quick tutorial, we’ll show how double braces can be used for creating and initializing objects in a single Java expression.

We’ll also look at why this technique can be considered an anti-pattern.

2. Standard Approach

Normally we initialize and populate a set of countries as follows:

@Test
public void whenInitializeSetWithoutDoubleBraces_containsElements() {
    Set<String> countries = new HashSet<String>();                
    countries.add("India");
    countries.add("USSR");
    countries.add("USA");
 
    assertTrue(countries.contains("India"));
}

As can be seen from above example, we are doing the following:

1. Create an instance of HashSet
2. Add countries to the HashSet
3. Finally, we assert whether the country is present in the HashSet

3. Using Double Brace

However, we can actually combine the creation and initialization in a single statement; this is where we make use of double braces:

@Test
public void whenInitializeSetWithDoubleBraces_containsElements() {
    Set<String> countries = new HashSet<String>() {
        {
           add("India");
           add("USSR");
           add("USA");
        }
    };
 
    assertTrue(countries.contains("India"));
}

As can be seen from above example, we are:

1. Creating an anonymous inner class which extends HashSet
2. Providing an instance initialization block which invokes the add method and adds the country name to the HashSet
3. Finally, we can assert whether the country is present in the HashSet

4. Advantages of Using Double Braces

There are some simple advantages of using double braces:

• Fewer lines of code compared to the native way of creation and initialisation
• The code is more readable
• Creation initialization is done in the same expression

5. Disadvantages of Using Double Braces

Disadvantages of using double braces are:

• Obscure, not widely known way to do the initialization
• It creates an extra class every time we use it
• Doesn’t support the use of the “diamond operator” – a feature introduced in Java 7
• Doesn’t work if the class we are trying to extend is marked final
• Holds a hidden reference to the enclosing instance, which may cause memory leaks

It’s due to these disadvantages that double brace initialization is considered as an anti-pattern.

6. Alternatives

6.1. Stream Factory Methods

Instead, we can make good use of the new Java 8 Stream API to initialize our Set:

@Test
public void whenInitializeUnmodifiableSetWithDoubleBrace_containsElements() {
    Set<String> countries = Stream.of("India", "USSR", "USA")
      .collect(collectingAndThen(toSet(), Collections::unmodifiableSet));
 
    assertTrue(countries.contains("India"));
}

6.2. Java 9 Collections Factory Methods

Also, Java 9 will bring a set of useful factory methods that will make the following possible:

List<String> list = List.of("India", "USSR", "USA");
Set<String> set = Set.of("India", "USSR", "USA");

You can read more about this in this article.

7. Conclusion

In this concise tutorial, we discussed the usage of double braces along with its advantages and disadvantages.

The implementation of these examples can be found in the GitHub project – this is a Maven-based project, so it should be easy to import and run as-is.