Exception.getCause() returning null when trying to find the source of an exception

why does System.out.println(e.getCause()); gives null? And can store whole HashSet collection like this?

private void saving() throws IOException, ClassNotFoundException {
    ObjectOutputStream out = null;
    try {
        out = new ObjectOutputStream(new BufferedOutputStream(new FileOutputStream(dataFile)));

        out.writeObject(c);

    } catch (java.io.NotSerializableException e) {
        System.out.println(e.getCause());
    } finally {
        out.close();
    }
}

Used printStackTrace() instead of getCause()

java.io.NotSerializableException: Data$1
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1180)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1528)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1493)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1416)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
    at java.util.HashSet.writeObject(HashSet.java:284)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:616)
    at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:962)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1480)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1416)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
    at Prozor.saving(Prozor.java:81)
    at Prozor.actionPerformed(Prozor.java:57)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2012)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2335)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:404)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:253)
    at java.awt.Component.processMouseEvent(Component.java:6268)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6033)
    at java.awt.Container.processEvent(Container.java:2045)
    at java.awt.Component.dispatchEventImpl(Component.java:4629)
    at java.awt.Container.dispatchEventImpl(Container.java:2103)
   开发者_JAVA技巧 at java.awt.Component.dispatchEvent(Component.java:4455)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4633)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4297)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4227)
    at java.awt.Container.dispatchEventImpl(Container.java:2089)
    at java.awt.Window.dispatchEventImpl(Window.java:2517)
    at java.awt.Component.dispatchEvent(Component.java:4455)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:649)
    at java.awt.EventQueue.access$000(EventQueue.java:96)
    at java.awt.EventQueue$1.run(EventQueue.java:608)
    at java.awt.EventQueue$1.run(EventQueue.java:606)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:116)
    at java.awt.EventQueue$2.run(EventQueue.java:622)
    at java.awt.EventQueue$2.run(EventQueue.java:620)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:619)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:275)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:200)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:185)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:177)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:138)

Still do not see the problem, except that it snaped on first object of collection, and collection is implementing Serializable, all objects also implement Serializable and threads have transient where declared

When an exception is chained, the getCause method is used to get the original cause. In this case, the exception was not chained from any other layer, hence getCause returns null. You should use e.printStackTrace() instead, to get the reason for the exception. Most likely, this would be because one or more keys/values in your HashSet are of a type that is not implementing java.io.Serializable.

Use printStackTrace() instead of getCause().

And yes, you can save a whole collection (as long as it holds objects that implement Serilizable).

Below code gave me exact cause of the exception

try{
 // code
}
catch(Exception ex){
    Exception exe = new Exception();
    String causeString="";
    exe.initCause(ex);
    if(exe.getCause()!=null && exe.getCause().getCause()!=null)
     causeString=  exe.getCause().getCause().getLocalizedMessage(); 
}  

You can use this method to get cause.

private void saving() throws IOException, ClassNotFoundException {
    ObjectOutputStream out = null;
    try {
        out = new ObjectOutputStream(new BufferedOutputStream(new FileOutputStream(dataFile)));

        out.writeObject(c);

    } catch (java.io.NotSerializableException e) {
        System.out.println(e.fillInStackTrace());
    } finally {
        out.close();
    }
}