Text files.
Notepad creates, reads, and writes text files. Text editor programs and program editor programs use text files. Compilers (such as javac) expect the source program to be a text file. Many application programs are written to process input data from a text file and to write their output to a text file.
You can write a Java program to deal with any kind of file (since fundamentally all files are collections of bytes and Java can certainly read bytes.) But it is particularly easy to write programs that do input and output with text files.
Have your programs so far been doing input and output of text?