When the Internet age first arrived , those using Windows systems that were still restricted to 8.3 filename formats had to create web pages with names ending in . HTM , while those using Macintosh or UNIX computers could use the recommended . html filename extension . This also became a problem for programmers experimenting with the Java programming language , since it requires source code files to have the four - letter suffix . java and compiles object code output files with the five - letter . class suffix .

Question: what is the filename extension used for all java source files
Answer: . java