codice:
if(s.equals(libri.getLibro(c).getTitolo())) b=true;
ma il problema rimane lo stesso, ovvero non esegue l'else ...l'errore è questo qui
codice:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at lavoro3_biblioteca.Biblioteca.jButton3MouseClicked(Biblioteca.java:306)
	at lavoro3_biblioteca.Biblioteca.access$200(Biblioteca.java:11)
	at lavoro3_biblioteca.Biblioteca$3.mouseClicked(Biblioteca.java:109)
	at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:253)
	at java.awt.Component.processMouseEvent(Component.java:6376)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
	at java.awt.Component.processEvent(Component.java:6138)
	at java.awt.Container.processEvent(Container.java:2085)
	at java.awt.Component.dispatchEventImpl(Component.java:4735)
	at java.awt.Container.dispatchEventImpl(Container.java:2143)
	at java.awt.Component.dispatchEvent(Component.java:4565)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4621)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4291)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4212)
	at java.awt.Container.dispatchEventImpl(Container.java:2129)
	at java.awt.Window.dispatchEventImpl(Window.java:2478)
	at java.awt.Component.dispatchEvent(Component.java:4565)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:679)
	at java.awt.EventQueue.access$000(EventQueue.java:85)
	at java.awt.EventQueue$1.run(EventQueue.java:638)
	at java.awt.EventQueue$1.run(EventQueue.java:636)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
	at java.awt.EventQueue$2.run(EventQueue.java:652)
	at java.awt.EventQueue$2.run(EventQueue.java:650)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:649)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Ciao,

vedendo cosi a occhio il codice credo che ti dia quell'errore tipo di errore perchè l'oggetto JLabel non istanziato.

Spero di esserti stato d'aiuto.