Java扫描器从我的文本文件读取null吗？

Question

I'm writing some code to read an input file of book titles, and putting the read lines into an array and trying to print out the array. But when I try to print out the array, it just returns 'null' for each read line. I'm not sure what I'm doing wrong or what my code is doing. Any suggestions? Thanks!

Code:

    import java.io.*;
import java.util.*;
public class LibraryInputandOutputs {
    public static void main(String args[]) throws IOException{
        int lineCount = 0;
        File inputFile = new File("bookTitles.inp.txt");
        Scanner reader = new Scanner(inputFile);
        while(reader.hasNextLine()) {
            reader.nextLine();
            lineCount++;
        }

        String[] bookArray = new String[lineCount];
        while (reader.hasNextLine()) {
            for (int i = 0; i < lineCount; i++) {
                bookArray[i] = reader.next();
             }
        }
        for (int k = 0; k < lineCount; k++) {
            System.out.println(bookArray[k]);
        }
        reader.close();
        inputFile.close();

    }
}

My text file I'm reading from is 20 book titles, all on different lines. My output on the terminal is 20 lines of null.

Answer 1

Lets break this down:

1. This reads every line of the input file, counts each one, and then discards them:

    while(reader.hasNextLine()) { reader.nextLine(); lineCount++; } 

   You are now at the end of file.

2. Allocate a string array that is large enough.

    String[] bookArray = new String[lineCount]; 

3. Attempt to read more lines. The loop will terminate immediately because reader.hasNextLine() will return false . You are already at the end of file.

   So you the statement assigning to bookArray[i] won't be executed.

    while (reader.hasNextLine()) { for (int i = 0; i < lineCount; i++) { bookArray[i] = reader.next(); } } 

4. Since bookArray[i] = ... was never executed above, all of the array elements will still be null .

    for (int k = 0; k < lineCount; k++) { System.out.println(bookArray[k]); } 

---

One solution is to open and read the file twice.

Another solution is to "reset" the file back to the beginning. (A bit complicated.)

Another solution would be to use a List rather than an array so that you don't need to read the file twice.

Another solution is to search the javadocs for a method that will read all lines of a file / stream as an array of strings.

(Some of these may be precluded by the requirements of your exercise. You work it out ... )

---

The nested loop in step 3 is also wrong. You don't need a for loop inside a while loop. You need a single loop that "iterates" the over the lines and also increments the array index ( i ). They don't both need to be done by the loop statement itself. You could do one or the other (or both) in the loop body.

Answer 2

Stephen C has already pointed out the main problems with your logic. You're trying to loop twice through the file but you've already reached the end of the file the first time. Don't loop twice. "Merge" both the while loops into one, remove that for loop inside the while loop and collect all the book titles. You can then use the size of the list to print them later on. My Java might be rusty but here it goes -

import java.io.*;
import java.util.*;
public class LibraryInputandOutputs {
    public static void main(String args[]) throws IOException {

        // int lineCount = 0; - You don't need this.

        File inputFile = new File("bookTitles.inp.txt");
        Scanner reader = new Scanner(inputFile);

        // Use an array list to collect book titles.
        List<String> bookArray = new ArrayList<>(); 

        // Loop through the file and add titles to the array list.
        while(reader.hasNextLine()) {
            bookArray.add(reader.nextLine());
            // lineCount++; - not needed
        }

        // Not needed -
        // while (reader.hasNextLine()) {
        //     for (int i = 0; i < lineCount; i++) {
        //         bookArray[i] = reader.next();
        //      }
        // }

        // Use the size method of the array list class to get the length of the list 
        // and use it for looping.
        for (int k = 0; k < bookArray.size(); k++) {  
            System.out.println(bookArray[k]);
        }

        reader.close();
        inputFile.close();

    }
}

Answer 3

I agree with Stephen C. In particular, using a List is usually better than an array because it's more flexible. If you need an array, you can always use toArray() after the List is filled.

Are your book titles on separate lines? If so you might not need a Scanner class, and could use something like a BufferedReader or LineNumberReader.