End of file NullPointerException

问题:

What I wanted is to reach EOF by typing Ctrl + z from command line with BufferedReader reading from console. The following code does so. But the problem is, it issues a NullPointerException after reaching EOF. Is there a way to skip this exception? Or more precisely, what is the proper way of reaching EOF with BufferedReader reading from console?

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

class EOF {

    public static void main(String args[]) {
        String s = "";
        String EOF = "^z";
        BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
        try {
            while (!s.equals(EOF)) {
                s = read.readLine();   
            }
        } catch (IOException e) {} 
    }

}

回答1:

Or more precisely, what is the proper way of reaching EOF with bufferedReader reading from console?

Currently you're actually detecting the characters '^' and 'z' it's not like '^' is really a control character.

The exception you're getting is actually a hint as to how you should be handling this. From the docs for BufferedReader.readLine:

Returns:
A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

So basically you should loop until readLine returns null.

String line;
while((line = read.readLine()) != null)
{
    // Do something with line
}

回答2:

See how much a debugger can help:

After I press ctrl + z, s has null value, hence you're getting this exception, since it's like writing !null.equals(EOF).

Why?

Because BufferedReader#readLine returns "null if the end of the stream has been reached".

回答3:

Just use null as EOF signal.

    while((s=read.readLine())!= null)
    {
        .....
    }

回答4:

BufferedReader input = new BufferedReader(new InputStreamReader(System.in));

String str;
while((str=input.readLine()) != null ) {
    // 
}