http://xmlgraphics.apache.org/fop/: Apache FOP (Formatting Objects Processor) is the world's first print formatter driven by XSL formatting objects (XSL-FO) and the world's first output independent formatter. It is a Java application that reads a formatting object (FO) tree and renders the resulting pages to a specified output. Output formats currently supported include PDF, PCL, PS, AFP, TIFF, PNG, SVG, XML (area tree representation), Print, AWT and TXT. The primary output target is PDF. (Apache Software Foundation) 
public final class org.apache.fop.layoutmgr.ElementListObserver
  minor version: 0
  major version: 59
  flags: flags: (0x0031) ACC_PUBLIC, ACC_FINAL, ACC_SUPER
  this_class: org.apache.fop.layoutmgr.ElementListObserver
  super_class: java.lang.Object
{
  private static java.util.List activeObservers;
    descriptor: Ljava/util/List;
    flags: (0x000a) ACC_PRIVATE, ACC_STATIC

  private void <init>();
    descriptor: ()V
    flags: (0x0002) ACC_PRIVATE
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // org.apache.fop.layoutmgr.ElementListObserver this
         0: .line 31
            aload 0 /* this */
            invokespecial java.lang.Object.<init>:()V
         1: .line 32
            return
        end local 0 // org.apache.fop.layoutmgr.ElementListObserver this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    2     0  this  Lorg/apache/fop/layoutmgr/ElementListObserver;

  public static void addObserver(org.apache.fop.layoutmgr.ElementListObserver$Observer);
    descriptor: (Lorg/apache/fop/layoutmgr/ElementListObserver$Observer;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
        start local 0 // org.apache.fop.layoutmgr.ElementListObserver$Observer observer
         0: .line 41
            invokestatic org.apache.fop.layoutmgr.ElementListObserver.isObservationActive:()Z
            ifne 2
         1: .line 42
            new java.util.ArrayList
            dup
            invokespecial java.util.ArrayList.<init>:()V
            putstatic org.apache.fop.layoutmgr.ElementListObserver.activeObservers:Ljava/util/List;
         2: .line 44
      StackMap locals:
      StackMap stack:
            getstatic org.apache.fop.layoutmgr.ElementListObserver.activeObservers:Ljava/util/List;
            aload 0 /* observer */
            invokeinterface java.util.List.add:(Ljava/lang/Object;)Z
            pop
         3: .line 45
            return
        end local 0 // org.apache.fop.layoutmgr.ElementListObserver$Observer observer
      LocalVariableTable:
        Start  End  Slot      Name  Signature
            0    4     0  observer  Lorg/apache/fop/layoutmgr/ElementListObserver$Observer;
    MethodParameters:
          Name  Flags
      observer  

  public static void removeObserver(org.apache.fop.layoutmgr.ElementListObserver$Observer);
    descriptor: (Lorg/apache/fop/layoutmgr/ElementListObserver$Observer;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
        start local 0 // org.apache.fop.layoutmgr.ElementListObserver$Observer observer
         0: .line 53
            invokestatic org.apache.fop.layoutmgr.ElementListObserver.isObservationActive:()Z
            ifeq 2
         1: .line 54
            getstatic org.apache.fop.layoutmgr.ElementListObserver.activeObservers:Ljava/util/List;
            aload 0 /* observer */
            invokeinterface java.util.List.remove:(Ljava/lang/Object;)Z
            pop
         2: .line 56
      StackMap locals:
      StackMap stack:
            return
        end local 0 // org.apache.fop.layoutmgr.ElementListObserver$Observer observer
      LocalVariableTable:
        Start  End  Slot      Name  Signature
            0    3     0  observer  Lorg/apache/fop/layoutmgr/ElementListObserver$Observer;
    MethodParameters:
          Name  Flags
      observer  

  public static void observe(java.util.List, java.lang.String, java.lang.String);
    descriptor: (Ljava/util/List;Ljava/lang/String;Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=5, args_size=3
        start local 0 // java.util.List elementList
        start local 1 // java.lang.String category
        start local 2 // java.lang.String id
         0: .line 65
            invokestatic org.apache.fop.layoutmgr.ElementListObserver.isObservationActive:()Z
            ifeq 7
         1: .line 66
            aload 1 /* category */
            ifnonnull 3
         2: .line 67
            new java.lang.NullPointerException
            dup
            ldc "category must not be null"
            invokespecial java.lang.NullPointerException.<init>:(Ljava/lang/String;)V
            athrow
         3: .line 69
      StackMap locals:
      StackMap stack:
            getstatic org.apache.fop.layoutmgr.ElementListObserver.activeObservers:Ljava/util/List;
            invokeinterface java.util.List.iterator:()Ljava/util/Iterator;
            astore 4
            goto 6
      StackMap locals: java.util.List java.lang.String java.lang.String top java.util.Iterator
      StackMap stack:
         4: aload 4
            invokeinterface java.util.Iterator.next:()Ljava/lang/Object;
            astore 3 /* activeObserver */
        start local 3 // java.lang.Object activeObserver
         5: .line 70
            aload 3 /* activeObserver */
            checkcast org.apache.fop.layoutmgr.ElementListObserver$Observer
            aload 0 /* elementList */
            aload 1 /* category */
            aload 2 /* id */
            invokeinterface org.apache.fop.layoutmgr.ElementListObserver$Observer.observe:(Ljava/util/List;Ljava/lang/String;Ljava/lang/String;)V
        end local 3 // java.lang.Object activeObserver
         6: .line 69
      StackMap locals:
      StackMap stack:
            aload 4
            invokeinterface java.util.Iterator.hasNext:()Z
            ifne 4
         7: .line 73
      StackMap locals: java.util.List java.lang.String java.lang.String
      StackMap stack:
            return
        end local 2 // java.lang.String id
        end local 1 // java.lang.String category
        end local 0 // java.util.List elementList
      LocalVariableTable:
        Start  End  Slot            Name  Signature
            0    8     0     elementList  Ljava/util/List;
            0    8     1        category  Ljava/lang/String;
            0    8     2              id  Ljava/lang/String;
            5    6     3  activeObserver  Ljava/lang/Object;
    MethodParameters:
             Name  Flags
      elementList  
      category     
      id           

  public static boolean isObservationActive();
    descriptor: ()Z
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: .line 77
            getstatic org.apache.fop.layoutmgr.ElementListObserver.activeObservers:Ljava/util/List;
            ifnull 1
            iconst_1
            ireturn
      StackMap locals:
      StackMap stack:
         1: iconst_0
            ireturn
      LocalVariableTable:
        Start  End  Slot  Name  Signature
}
SourceFile: "ElementListObserver.java"
NestMembers:
  org.apache.fop.layoutmgr.ElementListObserver$Observer
InnerClasses:
  public abstract Observer = org.apache.fop.layoutmgr.ElementListObserver$Observer of org.apache.fop.layoutmgr.ElementListObserver