如何在Java中读取来自文本文件的指定字符

Question

Given I have a text file, I know I can use FileReader to read chars :

in = new FileReader("myFile.txt");
int c;
while ((c = in.read()) != -1)
{ ... }

However, after I do in.read() , would it be possible to backtrack by one character? Is there any way I can change where in.read() is pointing to? Perhaps I could use an iterator?

Answer 1

If you only need to backtrack by one character, consider keeping the previous character in a variable and then referencing that when needed.

If you need to backtrack an unspecified amount, It's probably easier for most files to just save the file contents in memory and process the content there.

The correct answer depends on context.

Answer 2

We could use java.io.PushbackReader.unread

https://docs.oracle.com/javase/7/docs/api/java/io/PushbackReader.html

Please refer the sample here: http://tutorials.jenkov.com/java-io/pushbackreader.html

Answer 3

Assuming you are talking about an input Stream. You can use the int java.io.InputStream.read(byte[] b, int off, int len) method instead, the second parameter "off" (for offset) can be used as the starting point of the inputStream you want to read.

An alternative is to use in.reset() first to re-position the reader to the start of the stream, and then in.skip(long n) to move to the desired position

Answer 4

Depending on what you want to achieve you might have a look at PushbackInputStream or RandomAccessFile .

Find below two snippets to demonstrate the different behaviors. For both the file abc.txt contained a line foobar12345 .

The PushbackInputStream allows you to alter data in a stream to be read later.

try (PushbackInputStream is = new PushbackInputStream(
        new FileInputStream("abc.txt"))) {
    // for demonstration we read only six values from the file
    for (int i = 0; i < 6; i++) {
        // read the next byte value from the stream
        int c = is.read();
        // this is only for visualising the behavior
        System.out.print((char) c);
        // if the current read value equals to character 'b'
        // we push back into the stream a 'B', which
        // would be read in the next iteration
        if (c == 'b') {
            is.unread((byte) 'B');
        }
    }
}

outout

foobBa

The RandomAccessFile allows you to read a value on a specific offset in the stream.

try (RandomAccessFile ra = new RandomAccessFile("abc.txt", "r")) {
    // for demonstration we read only six values from the file
    for (int i = 0; i < 6; i++) {
        // read the next byte value from the stream
        int c = ra.read();
        // this is only for visualising the behavior
        System.out.print((char) c);
        // if the current read value equals to character 'b'
        // we move the file-pointer to offset 6, from which
        // the next character would be read
        if (c == 'b') {
            ra.seek(6);
        }
    }
}

output

foob12