checkstyle – AvoidDoubleBraceInitialization

AvoidDoubleBraceInitialization

Since Checkstyle 8.30

Description

Detects double brace initialization.

Rationale: Double brace initialization (set of Instance Initializers in class body) may look cool, but it is considered as anti-pattern and should be avoided. This is also can lead to a hard-to-detect memory leak, if the anonymous class instance is returned outside and other object(s) hold reference to it. Created anonymous class is not static, it holds an implicit reference to the outer class instance. See this blog post and article for more details. Check ignores any comments and semicolons in class body.

Examples

To configure the check:

<module name="Checker">
  <module name="TreeWalker">
    <module name="AvoidDoubleBraceInitialization"/>
  </module>
</module>

Which results in the following violations:

class Example1 {
  // violation below 'Avoid double brace initialization.'
  List<Integer> list1 = new ArrayList<>() {
    {
      add(1);
    }
  };
  // violation below 'Avoid double brace initialization.'
  List<String> list2 = new ArrayList<>() {
        ;
    // comments and semicolons are ignored
    {
      add("foo");
    }
  };
  List<Object> list = new ArrayList<>() {
    // OK, as it is not double brace pattern
    private int field;
    {
      add(new Object());
    }
  };
}