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?

If you are using an IDE, such as Eclipse or IntelliJ, you can place your cursor on `XSSFWorkbook` and hit F3 or right click on it to jump to the definition of the class. If you are using just a simple editor and the command line, you can run a search in your file system for `XSSFWorkbook.java` and/or `XSSFWorkbook.class` to find the implementation. — Raul Santelices, Jan 13 '16 at 02:24

score 0 · Accepted Answer · answered Jan 13 '16 at 02:30

0

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

answered Jan 13 '16 at 02:30

Alejandro Goñi

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score 0 · Answer 2 · answered Jan 13 '16 at 02:38

That is interface instance.
Details: Using an Interface as a Type

In short, you can create instance of interface.

Case 1: for anonymous class.

Workbook wb = new Workbook() {
// Some implementation
}

Case 2: Use it as type. (Yes! your case!)

Workbook wb = new XSSFWorkbook();

see also: Strategy Design Pattern in Java. It is very useful idea.

Java Interface Implement

2 Answers2