将从文件读取的数据捕获到字符串流Java中

Question

I'm coming from a C++ background, so be kind on my n00bish queries...

I'd like to read data from an input file and store it in a stringstream. I can accomplish this in an easy way in C++ using stringstreams. I'm a bit lost trying to do the same in Java.

Following is a crude code/way I've developed where I'm storing the data read line-by-line in a string array. I need to use a string stream to capture my data into (rather than use a string array).. Any help?

    char dataCharArray[] = new char[2];
    int marker=0;
    String inputLine;
    String temp_to_write_data[] = new String[100];

    // Now, read from output_x into stringstream

    FileInputStream fstream = new FileInputStream("output_" + dataCharArray[0]);

    // Convert our input stream to a BufferedReader
    BufferedReader in = new BufferedReader (new InputStreamReader(fstream));

    // Continue to read lines while there are still some left to read
    while ((inputLine = in.readLine()) != null )
    {
        // Print file line to screen
        // System.out.println (inputLine);
        temp_to_write_data[marker] = inputLine;
        marker++;
    }

EDIT:

I think what I really wanted was a StringBuffer. I need to read data from a file (into a StringBuffer, probably) and write/transfer all the data back to another file.

Answer 1

In Java, first preference should always be given to buying code from the library houses:

http://commons.apache.org/io/api-1.4/org/apache/commons/io/IOUtils.html http://commons.apache.org/io/api-1.4/org/apache/commons/io/FileUtils.html

In short, what you need is this:

FileUtils.readFileToString(File file)

Answer 2

I also come from C++, and I was looking for a class similar to the C++ 'StringStreamReader', but I couldn't find it. In my case (which I think was very simple), I was trying to read a file line by line and then read a String and an Integer from each of these lines. My final solution was to use two objects of the class java.util.Scanner, so that I could use one of them to read the lines of the file directly to a String and use the second one to re-read the content of each line (now in the String) to the variables (a new String and a positive 'int'). Here's my code:

try {
    //"path" is a String containing the path of the file we want to read
    Scanner sc = new Scanner(new BufferedReader(new FileReader(new File(path))));
    while (sc.hasNextLine()) { //while the file isn't over
        Scanner scLine = new Scanner(sc.nextLine());
        //sc.nextLine() returns the next line of the file into a String
        //scLine will now proceed to scan (i.e. analyze) the content of the string
        //and identify the string and the positive 'int' (what in C++ would be an 'unsigned int')
        String s = scLine.next(); //this returns the string wanted
        int x;
        if (!scLine.hasNextInt() || (x = scLine.nextInt()) < 0) return false;
        //scLine.hasNextInt() analyzes if the following pattern can be interpreted as an int
        //scLine.nextInt() reads the int, and then we check if it is positive or not
        //AT THIS POINT, WE ALREADY HAVE THE VARIABLES WANTED AND WE CAN DO
        //WHATEVER WE WANT WITH THEM
        //in my case, I put them into a HashMap called 'hm'
        hm.put(s, x);
    }
    sc.close();
    //we finally close the scanner to point out that we won't need it again 'till the next time
} catch (Exception e) {
    return false;
}
return true;

Hope that helped.

Answer 3

StringBuffer is one answer, but if you're just writing it to another file, then you can just open an OutputStream and write it directly out to the other file. Holding a whole file in memory is probably not a good idea.

Answer 4

In you simply want to read a file and write another one:

BufferedInputStream in = new BufferedInputStream( new FileInputStream( "in.txt" ) );
BufferedOutputStream out = new BufferedOutputStream( new FileOutputStream( "out.txt" ) );
int b;
while ( (b = in.read()) != -1 ) {
    out.write( b );
}

If you want to read a file into a string:

StringWriter out = new StringWriter();
BufferedReader in = new BufferedReader( new FileReader( "in.txt" ) );
int c;
while ( (c = in.read()) != -1 ) {
    out.write( c );
}
StringBuffer buf = out.getBuffer();

This can be made more efficient if you read using byte arrays. But I recommend that you use the excellent apache common-io. IOUtils ( http://commons.apache.org/io/api-1.4/org/apache/commons/io/IOUtils.html ) will do the loop for you.

Also, you should remember to close the streams.