Java Interface Implement

Question

I am studying Java, Spring and POI. I see https://poi.apache.org/spreadsheet/quick-guide.html and follow it.
At "New WorkBook", next code exists.

Workbook wb = new XSSFWorkbook();
FileOutputStream fileOut = new FileOutputStream("workbook.xlsx");
wb.write(fileOut);
fileOut.close();

Running above code, it works well.
I found out that "Workbook" is NOT class BUT interface. I learned that interface HAVE TO be implemented. But I CANNOT found the implementation. Where is it implemented? What must I study to comprehend it?

Answer 1

Oficial Oracle tutorial about interfaces: https://docs.oracle.com/javase/tutorial/java/concepts/interface.html