You can use expand to replace tabs with spaces.
For example, if your editor has tab set as 4 spaces then use:

expand -4 MyFile.java > temp

VERIFY WITH AN EDITOR THAT IT DID THE RIGHT THING IN temp THEN

mv temp MyFile.java

The default is 8 spaces so

expand MyFile.java > temp

will replace tabs with 8 spaces.

DO NOT use "expand MyFile.java > MyFile.java".
This will result in an EMPTY FILE. You must use an intermediate temp
file. This occurs because the file redirection creates a new empty file
the overwrites the original before the expand reads the original.