Very basic OOP question

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Java

OOP

Hi,
my task is to simulate a radio with the help of OOP.

I have 3 classes: a GUI class for the radio with a label called "lblFreq", a class for the different channels I want to create "Sender.java" and a control class to do the different tasks.

My channels have 2 atributes: "Freq" for the frequency (double) and "Channel" for the name of the channel (String).

public class Sender {
  
  private double Freq;
  private String Channel;
  
  public Sender(double Freq, String Channel) {
    this.Freq = Freq;
    this.Channel = Channel;
  }

  public double getFreq() {
    return Freq;
  }

  public String getChannel() {
    return Channel;
  }

}

my control class should fill it with a name and a frequency:

public class Steuerung {
  
  private RadioGUI dieGUI;
  private Sender dieSender;
  Sender bigFM = new Sender(89.5, "BigFM");


}

and I want to show the frequency in the label in my GUI when I press the button "btnForward":

 private Steuerung dieST;
(....)
 public void btnForward_ActionPerformed(ActionEvent evt) {
    lblFreq.setText(String.valueOf(dieST.bigFM.getFreq()));
    
  }

I'm getting an error as soon as I press this button, the error log is below.

I think it's because I messed something up at

lblFreq.setText(String.valueOf(dieST.bigFM.getFreq()));

I guess.

Thanks for any help.


Error log:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at RadioGUI.btnForward_ActionPerformed(RadioGUI.java:249)
	at RadioGUI$2.actionPerformed(RadioGUI.java:77)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
	at java.awt.Component.processMouseEvent(Component.java:6505)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
	at java.awt.Component.processEvent(Component.java:6270)
	at java.awt.Container.processEvent(Container.java:2229)
	at java.awt.Component.dispatchEventImpl(Component.java:4861)
	at java.awt.Container.dispatchEventImpl(Container.java:2287)
	at java.awt.Component.dispatchEvent(Component.java:4687)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
	at java.awt.Container.dispatchEventImpl(Container.java:2273)
	at java.awt.Window.dispatchEventImpl(Window.java:2719)
	at java.awt.Component.dispatchEvent(Component.java:4687)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
	at java.awt.EventQueue.access$200(EventQueue.java:103)
	at java.awt.EventQueue$3.run(EventQueue.java:694)
	at java.awt.EventQueue$3.run(EventQueue.java:692)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
	at java.awt.EventQueue$4.run(EventQueue.java:708)
	at java.awt.EventQueue$4.run(EventQueue.java:706)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

What I have tried:

Tried other codes at the lblFreq.setText... but it mostly ended with the same error or it wont even start.