1. Create class 'SimpleClass' and insert this code:

  //---------------------------
  public class SimpleClass {

      public final String SimpleConstant = "const";
      private String SimpleString = "";
    
      public SimpleClass(){
          SimpleString = "text";
      }
  }
  //---------------------------

2. Select "text"

3. Go to Refactor->Introduce Fields...

4. Set Name: 'SimpleConstant', Access: 'public', check 'Declare Final', Initialize in: 'Field', than press Ok

>> Constant SimpleConstant has been added, although is is already defined, so a bug is occured.

Product Version: NetBeans IDE 7.3.1 (Build 201306052037)
Java: 1.7.0_25; Java HotSpot(TM) 64-Bit Server VM 23.25-b01
Runtime: Java(TM) SE Runtime Environment 1.7.0_25-b30
System: Windows 7 version 6.1 running on amd64; Cp1252; en_US (nb)