从缓冲读取器流全局读取

Question

so Im trying to write this class which is going to parse a file and read commands from it.

I want the ctor to just open the stream and do nothing else. while I parse the file in other class methods. But I'm getting a nullpointerexception when I try to read the file in the methods I made.

help will be apprecated :D

public class Parser {
private BufferedReader _input;
private String _command;

public Parser(String filename) throws FileNotFoundException {
    FileInputStream fstream = new FileInputStream(filename);

    BufferedReader _input = new BufferedReader(new InputStreamReader(fstream));
}

public boolean hasMoreCommands() throws IOException {

    String line;
    if ( (line = _input.readLine()) != null) {
        return true;
    } else {
        _input.close();
        return false;
    }
}

public void advance() throws IOException {
    String line;
    do {
        line = _input.readLine().trim();
    } while (line.equals("") || line.substring(0,2).equals(COMMENT_SIGN)); 

    String[] splittedLine = line.split(COMMENT_SIGN);
    _command = splittedLine[0];
    _command = _command.replace(" ", ""); 
}

my main for testing it + the exception trace

public static void main(String[] args) throws IOException {
    Parser input = null;
    input = new Parser("D:\\test.asm");
    System.out.println( input.hasMoreCommands());

}

Exception in thread "main" java.lang.NullPointerException
at nand6.Parser.hasMoreCommands(Parser.java:40)
at nand6.Parser.main(Parser.java:116)

Answer 1

Have a look at this snippet of code

public Parser(String filename) throws FileNotFoundException {
FileInputStream fstream = new FileInputStream(_filename);

BufferedReader _input = new BufferedReader(new InputStreamReader(fstream));
}

Change

BufferedReader _input = new BufferedReader(new InputStreamReader(fstream));

to

_input = new BufferedReader(new InputStreamReader(fstream));

Your mistake : You are creating another local variable of type *BufferedReader _input* hence your class level variable is still null resulting in a NullPointerException

Answer 2

You are basically defining a new BufferedReader object called "_input" inside your constructor. You think that after calling the constructor you instantiate "_input" outside the constructor. But you are not, it refers to null. That's why you are getting NullPointerException. Just remove "BufferedReader" in front of "_input" inside your constructor, in order to refer to correct object.