On Java 17 and older non-static inner classes contain an implicit reference to their enclosing instance. Static nested classes do not have such a reference. Therefore, making an inner class static prevents a common cause of memory leaks and uses slightly less memory per instance of the class.

The implicit reference was removed for non-Serializable inner classes in Java 18.

Example:

  public class Outer {
    class Inner { // not static
      public void foo() {
        bar("x");
      }

      private void bar(String string) {}
    }
  }

After the quick-fix is applied:

  public class Outer {
    static class Inner {
      public void foo() {
        bar("x");
      }

      private void bar(String string) {}
    }
  }