How to merge the data into file

there is one file with name "file1.txt" it consists of these data inside the file aaa
bbb
ccc

"file2.txt" contains the data inside the file 666
777
888



my output should be in this format in file3.txt aaa
666
bbb
777
ccc
888 in this format

This is a basic concept of java I/O
see this example : File read write