Writer or OutputStream?

Question

I'm designing a library where a class should have an ability to be able to convert itself internals into text. Which class shall I use: OutputStream or Writer? And what is the key difference between them (in my case)?

public interface Memento {
  void save(OutputStream stream);
  void save(Writer writer);
}

Which one?

Bina

Writers/Readers deal with character data and Streams deal with binary data. – Chromaticness

@Chromaticness Cleared up my doubts in one simple sentence. Thank you haha – Hypo

Answer 1

An OutputStream is a byte-oriented stream. Any text you write has to be encoded as bytes using some encoding (most commonly ISO-8859-1 or UTF-8). A Writer is a character-oriented stream that may or may not internally encode characters as bytes, depending on what it is writing to.

EDIT If you are designing a library, then if you provide an OutputStream-oriented interface to which text is to be written, you really should provide client classes the ability to control the encoding to be used.

Darr

Answer 2

Text? Writer. It is intended for handling characters, honors encoding.

Stream/array of bytes? OutputStream. Works on raw bytes, has no notion of characters, encodings, strings, etc.

Callahan

and what's about OutputStreamWriter? is it used to convert the byte to charset? – Underfur

@UnKnown - An OutputStreamWriter is a Writer that happens to use an OutputStream as the destination. When writing text, it will convert characters to bytes using whatever encoding was specified when constructing the OutputStreamWriter (or a default encoding that depends on the platform). For converting bytes to characters, you would use a Reader. – Darr

Answer 3

1) In many cases preferable can be overriding toString() or providing a similar method to convert ... internals into text. The advantage for user of such method is flexibility. For instance, if the consumer of such method can:

• Save it as a part of some POJO to database
• Incorporate it as a field into some JSON object
• Save to a stream
• Save it to a writer

It can be disadvantage in some cases, e.g. when the text representation is relatively big (like 100 MB) and there are many requests simultaneously that produce such objects. This can require too much resources (CPU, RAM). In such case direct writing to a stream or to a writer can be preferable.

2) If you expect that you object can be used in many different contexts, then it makes sense to provide both, saving to a stream and saving to a writer. For instance, HttpServletResponse provides both, getWriter() and getOutputStream(), so that everyone can decide what is better in the his particular case. Or Jackson's JsonFactory provides methods * createGenerator()* for File, OutputStream and Writer, giving the consumer much freedom.

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