Package com.puppycrawl.tools.checkstyle.checks.coding

Class AvoidDoubleBraceInitializationCheck

  • All Implemented Interfaces:
    Configurable, Contextualizable
    public class AvoidDoubleBraceInitializationCheck
extends AbstractCheck

    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.

    To configure the check: 

     <module name="AvoidDoubleBraceInitialization"/>

    Which results in the following violations: 

     class MyClass {
     List<Integer> list1 = new ArrayList<>() { // violation
         {
             add(1);
         }
     };
     List<String> list2 = new ArrayList<>() { // violation
         ;
         // comments and semicolons are ignored
         {
             add("foo");
         }
     };
 }

    Check only looks for double brace initialization and it ignores cases where the anonymous class has fields or methods. Though these might create the same memory issues as double brace, the extra class members can produce side effects if changed incorrectly. 

     class MyClass {
     List<Object> list = new ArrayList<>() { // OK, not pure double brace pattern
         private int field;
         {
             add(new Object());
         }
     };
 }

    Parent is com.puppycrawl.tools.checkstyle.TreeWalker

    Violation Message Keys:

    • avoid.double.brace.init
    Since:
    8.30