Programming Tutorials

PushbackInputStream example program in Java

By: Sam Chen in Java Tutorials on 2007-09-15  

One of the novel uses of buffering is the implementation of pushback. Pushback is used on an input stream to allow a byte to be read and then returned (that is, "pushed back") to the stream. The PushbackInputStream class implements this idea. It provides a mechanism to "peek" at what is coming from an input stream without disrupting it. PushbackInputStream has the following constructors:

PushbackInputStream(InputStream inputStream)
PushbackInputStream(InputStream inputStream, int numBytes)

The first form creates a stream object that allows one byte to be returned to the input stream. The second form creates a stream that has a pushback buffer that is numBytes long. This allows multiple bytes to be returned to the input stream.

Beyond the familiar methods of InputStream, PushbackInputStream provides unread(), shown here:

void unread(int ch)
void unread(byte buffer[ ])
void unread(byte buffer, int offset, int numChars)

The first form pushes back the low-order byte of ch. This will be the next byte returned by a subsequent call to read(). The second form returns the bytes in buffer. The third form pushes back numChars bytes beginning at offset from buffer. An IOException will be thrown if there is an attempt to return a byte when the pushback buffer is full. Java 2 makes a small change to PushbackInputStream: it now implements the skip() method.

Here is an example that shows how a programming language parser might use a PushbackInputStream and unread() to deal with the difference between the = = operator for comparison and the = operator for assignment:

// Demonstrate unread().
import java.io.*;

class PushbackInputStreamDemo {
    public static void main(String args[]) throws IOException {
        String s = "if (a == 4) a = 0;\\n";
        byte buf[] = s.getBytes();
        ByteArrayInputStream in = new ByteArrayInputStream(buf);
        PushbackInputStream f = new PushbackInputStream(in);
        int c;
        while ((c = f.read()) != -1) {
            switch (c) {
                case '=':
                    if ((c = f.read()) == '=')
                        System.out.print(".eq.");
                    else {
                        System.out.print("<-");
                        f.unread(c);
                    }
                    break;
                default:
                    System.out.print((char) c);
                    break;
            }
        }
    }
}

Here is the output for this example. Notice that = = was replaced by ".eq." and = was replaced by "<-".

if (a .eq. 4) a <- 0;

Caution: PushbackInputStream has the side effect of invalidating the mark() or reset() methods of the InputStream used to create it. Use markSupported() to check any stream on which you are going to use mark()/reset().

This tutorial is an extract from the "The Complete Reference Part 2 by Herbert Schildt".