使用printwriter与System.out.print打印整数

Question

I'm trying to read the bytes I have in an array to the console and if I use a PrintWriter object, nothing is printed to the screen, however, using System.out.println() works fine. Why?

Here is what my code looks like:

private static void readByteArray(byte[] bytes) {
   ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
   PrintWriter pw = new PrintWriter(System.out);

   int c;
   while((c = bais.read()) != -1) {
   pw.print(c);
}

That code doesn't work, but if I do this:

private static void readByteArray(byte[] bytes) {
   ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
   PrintWriter pw = new PrintWriter(System.out);

   int c;
   while((c = bais.read()) != -1) {
   System.out.println(c);
}

It prints.

What is the difference? According to this http://docs.oracle.com/javase/7/docs/api/java/io/PrintWriter.html the PrintWriter method print(int i) prints an integer so I'm confused.

Answer 1

The System.out variable is referencing an object of type PrintStream which wraps a BufferedOutputStream (at least in Oracle JDK 7). When you call one of the printX() or write() methods on PrintStream , it internally flushes the buffer of the underlying BufferedOutputStream .

That doesn't happen with PrintWriter . You have to do it yourself. Alternatively, you can create a PrintWriter with an autoFlush property set to true which will flush on each write.

PrintWriter pw = new PrintWriter(System.out, true);

If you read this constructor's javadoc , it states

• autoFlush A boolean; if true, the println, printf, or format methods will flush the output buffer