Why does Throwable.printStackTrace takes a PrintWriter, instead of a Writer?

Question

Why does public void printStackTrace(PrintStream s) in the Throwable class take a PrintWriter, instead of a Writer?

I think Writer should be the norm (code to the interface, not a specific implementation), but it could be something that I'm missing, hence this question.

Answer 1

Writer is an abstract class which PrintWriter extends. Throwable.printStackTrace takes a PrintWriter because it calls the println method on it which is not available in the Writer class.println is called because it needs to print the stack trace, one stack frame per line.

Answer 2

There could be a couple of reason

• The original implementation was to use PrintStream and the improved version of that is PrintWriter.
• The method prints lines of text and while it could create a PrintWriter on demand this is not ideal.

What they could have done is accept a Appendable which is the parent of Writer like this.

public void printStackTrace(Appendable appendable) {
    PrintWriter pw = (appendable instanceof PrintWriter) ? (PrintWriter) appendable :
        (appendable instanceof Writer) ? new PrintWriter((Writer) appendable) :
        new printWriter(new AppendableWriter(appendable));