concatenation : Java Glossary

// simple concatenation
final String greeting = "Happy" + " " + "birthday.";

Javac does concatenation of String literals, such as in the example above, at compile time, so they have no run-time overhead.

Javac implements concatenation with calls to StringBuffer in JDK (Java Development Kit) 1.4- and StringBuilder in 1.5+. These are not as efficient as using StringBuilder directly since there is no way to specify the size of the buffer.

Alternate Concatenation Techniques

• StringBuffer
• StringBuilder
• StringWriter
• CharArrayWriter
• FastCat
• LazyString. Don’t concatenate appended Strings, just save individual appended Strings up in an ArrayList, and procrastinate the concatenation to the last minute, when you know the exact size of the buffer you need to compose the String.
• Use a StringWriter implemented as an ArrayList<char[]> of 8096 char buffers. Allocate an additional buffer if needed to handle each append. Appended Strings may sometimes have to span buffers. Buffers are completely filled before starting another. To convert to a String at the end, allocate a char[] buffer the exact size and copy over each of the 8096-char buffers. Ideally convert to a String without yet another copy.

Concatenating Files Programmatically

• As a legacy from the pre-DOS CPM (Cost Per thousand/Mille impressions) days, some text files have an EOL (End Of Line) character as the last character. These will have to be removed when you concatenate.
• If your text file don’t end with a \r\n or \n etc, you will have to insert one before the next chunk.
• If your target file is the same as one of your component files, you must copy the output to a temporary file, delete the target, then rename the temporary to the target, or use logic that produces the same effect.
• If you concatenation merely tacks files onto the end of the target, you can avoid copying the target itself. The problem with this sort of logic is if the process fails part way through, the original target file is corrupted. The advantage is that it is quicker — which might be important for a very long file.
• For any sort of copy where the target file already exists, it is wise to copy to a temporary file then do a delete and rename at the last second. This way you don’t corrupt the target file if the copy fails part way through.

Concatenating Files at the Command Prompt

Rem glue a.txt, b.txt and c.txt into a new file d.txt
Rem leaving a.txt, b.txt and c.txt as they were.
copy a.txt + b.txt + c.txt  d.txt

Take Command does not support these options. It presumes text files do not use obsolete EOL characters.

It is safe to append onto your target like this:

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