problemfile.java

// s is type Sheet
// this is 	"at musicNotesDatabase.PanelSheets.fillInstrumentList(PanelSheets.java:107)"
SheetInstrument si = (SheetInstrument) s;

Sheet.java

package musicNotesDatabase;

import java.util.UUID;

class Sheet {
	private int instrument;
	private UUID uuid;

	public Sheet(int instrument, UUID uuid) {
		this.instrument = instrument;
		this.uuid = uuid;
	}

	public int getInstrumentId() {
		return instrument;
	}

	public UUID getPDFUUID() {
		return uuid;
	}
	
	public String toString() {
		return null;
	}

}

SheetInstrument.java

package musicNotesDatabase;

import java.sql.SQLException;
import java.util.UUID;

public class SheetInstrument extends Sheet {
	private String InstrumentName;

	public SheetInstrument(int instrument, UUID uuid) {
		super(instrument, uuid);
	}

	public void setInstrumentName(Database db, String lang) throws SQLException {
		InstrumentName = db.getInstrumentName(getInstrumentId(), lang);
	}

	public String getInstrumentName() {
		return InstrumentName;
	}

}

traceback

Exception in thread "AWT-EventQueue-1" java.lang.ClassCastException: musicNotesDatabase.Sheet cannot be cast to musicNotesDatabase.SheetInstrument
	at musicNotesDatabase.PanelSheets.fillInstrumentList(PanelSheets.java:107)
	at musicNotesDatabase.PanelSheets.onCompositionSelect(PanelSheets.java:93)
	at musicNotesDatabase.PanelSheets.lambda$1(PanelSheets.java:59)
	at javax.swing.JList.fireSelectionValueChanged(JList.java:1796)
	at javax.swing.JList$ListSelectionHandler.valueChanged(JList.java:1810)
	at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:184)
	at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:154)
	at javax.swing.DefaultListSelectionModel.setValueIsAdjusting(DefaultListSelectionModel.java:685)
	at javax.swing.JList.setValueIsAdjusting(JList.java:2140)
	at javax.swing.plaf.basic.BasicListUI$Handler.mouseReleased(BasicListUI.java:2796)
	at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290)
	at java.awt.Component.processMouseEvent(Component.java:6533)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
	at java.awt.Component.processEvent(Component.java:6298)
	at java.awt.Container.processEvent(Container.java:2236)
	at java.awt.Component.dispatchEventImpl(Component.java:4889)
	at java.awt.Container.dispatchEventImpl(Container.java:2294)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
	at java.awt.Container.dispatchEventImpl(Container.java:2280)
	at java.awt.Window.dispatchEventImpl(Window.java:2746)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
	at java.awt.EventQueue.access$500(EventQueue.java:97)
	at java.awt.EventQueue$3.run(EventQueue.java:709)
	at java.awt.EventQueue$3.run(EventQueue.java:703)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
	at java.awt.EventQueue$4.run(EventQueue.java:731)
	at java.awt.EventQueue$4.run(EventQueue.java:729)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
	at org.GNOME.Accessibility.AtkWrapper$5.dispatchEvent(AtkWrapper.java:700)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)