What is Double Brace initialization in Java?

double-brace-initialize, initialization, java

Solution

Double brace initialisation creates an anonymous class derived from the specified class (the outer braces), and provides an initialiser block within that class (the inner braces). e.g.

new ArrayList<Integer>() {{
   add(1);
   add(2);
}};

Note that an effect of using this double brace initialisation is that you're creating anonymous inner classes. The created class has an implicit `this` pointer to the surrounding outer class. Whilst not normally a problem, it can cause grief in some circumstances e.g. when serialising or garbage collecting, and it's worth being aware of this.

Problem

Original source

Related problems

• Meaning of new Class(...){{...}} initialization idiom
• What is an initialization block?
• Efficiency of Java "Double Brace Initialization"?