memory - Why Initializing object to null is not dereferenced the object in java? -

i working application have implemented observer pattern set slider value on change in model. have button called clear clear window , set observer null before removing listener .so after did memory mapping profiler .but scared find object still there. in 2nd exp before making observer null first remove listener make null , found profiler object collected garbage collector. below provide snipnet of code .any light on appreciated .

  controls = new createcontrols();         scene scene = new scene(new group());          stage.settitle("table view sample");         stage.setwidth(300);         stage.setheight(500);          final vbox vbox = new vbox();         vbox.setspacing(5);         vbox.setpadding(new insets(10, 0, 0, 10));         controlbox = controls.creatcontrols();         vbox.getchildren().add(controlbox);         final button clear = new button("clear");         vbox.getchildren().add(clear);         eventhandler deletehnadler=     new eventhandler<actionevent>(){              @override             public void handle(actionevent arg0) {                 // todo auto-generated method stub                 vbox.getchildren().clear();                 // controls.dispose();                 controls=null;                                }                 }; 

createcontrol class:

package com.nxp.designpattern.observer;  import javafx.event.actionevent; import javafx.event.eventhandler; import javafx.geometry.insets; import javafx.scene.control.button; import javafx.scene.control.slider; import javafx.scene.layout.vbox;  public class createcontrols implements observerinterface {      private slider slider;     private static int counter;     private subject m_subject;     private button next_button;     private button previous_buttbutton;     private eventhandler<actionevent> nexteventhandler;     private eventhandler<actionevent> prevhandler;      public createcontrols() {         m_subject = new subject();         observerimpl.getobserverimpl().addlistener(this);      }      public vbox creatcontrols() {         final vbox vbox = new vbox();         vbox.setspacing(5);         vbox.setpadding(new insets(10, 0, 0, 10));         next_button = new button("next");         previous_buttbutton = new button("previous");          slider = new slider();         vbox.getchildren().add(next_button);         vbox.getchildren().add(previous_buttbutton);         vbox.getchildren().add(slider);         nexteventhandler = new eventhandler<actionevent>() {              @override             public void handle(actionevent arg0) {                 // todo auto-generated method stub                 counter += 10;                 m_subject.setvalue(counter);             }         };         next_button.addeventhandler(actionevent.action, nexteventhandler);         prevhandler = new eventhandler<actionevent>() {              @override             public void handle(actionevent arg0) {                 // todo auto-generated method stub                 counter -= 10;                 m_subject.setvalue(counter);             }         };         previous_buttbutton.addeventhandler(actionevent.action, prevhandler);         return vbox;     }      @override     public void changeinvalue(int value) {         slider.setvalue(value);      }      void dispose() {         next_button.removeeventhandler(actionevent.action, nexteventhandler);         // nexteventhandler=null;         previous_buttbutton.removeeventhandler(actionevent.action, prevhandler);         // prevhandler=null;         observerimpl.getobserverimpl().removelistener(this);     } } 

"in 2nd exp before making observer null first remove listener make null"
problem/solution. variable pointer, setting null doesn't actual object , listener still has reference it'll never garbage collected. have remove references.