Java教科书帮助中的推回流示例。 到底是怎么回事？

Question

I am reading this example in my java book:

// Demonstrate unread().

import java.io.*;

class PushbackInputStreamDemo {
    public static void main(String args[]) {
        String s = "if (a == 4) a = 0;\n";
        byte buf[] = s.getBytes();
        ByteArrayInputStream in = new ByteArrayInputStream(buf);
        int c;

        try ( PushbackInputStream f = new PushbackInputStream(in) )
        {
            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;
                }
            }
        } catch(IOException e) {
            System.out.println("I/O Error: " + e);
        }
    }
}

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

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

When it reaches the single = sign, it reads it, prints out the <- and then unreads the = and puts it back on the pushbackInputStream right? So when it calls c = f.read() , it receives the = again right? so why aren't we in an infinite loop of <- symbols?

Answer 1

The while loop contains two possible branches: one when an = character is encountered, and one for everything else. For the everything-else scenario the character is simply dumped out to the console, so no infinite loop is created.

For the branch where we encounter an = character, it will then read the next character (after that = character) from the stream. If the next character is a second = character then .eq. is dumped out to the console, and nothing is pushed back onto the PushbackInputStream so the == sequence has been fully consumed and when we go around the while loop again it finds whatever comes after the == . So this scenario causes no infinite loop.

If the next character after the first = is not a second = character then <- is dumped out to the console, and then that second character is pushed back onto the stream. Execution of the while loop then re-reads this character and (seeing as we already know it is not an = character) it will be dumped out to the console immediately after the <- . So this scenario does not cause an infinite loop.