将InputStream转换为固定长度的字符串的Stream <String>

Question

Like in Convert InputStream into Stream<String> given a Charset I want to convert an InputStream is into a Stream<String> stream . But this time instead of splitting the InputStream at the new line characters, I want to split it into parts of equal length. So all strings of the stream would have the same length (with a possible exception on the last element of the stream, that may be shorter).

Answer 1

I don't think this is possible using class library methods only, so you'll have to write your own logic that follows the same pattern as BufferedReader.lines :

1. InputStreamReader - Start by creating an InputStreamReader
2. Iterator<String> - Implement a custom Iterator subclass that splits the stream into pieces however you want. It sounds like you want to implement hasNext() and next() to call a readPart() that reads at most N characters.
3. Spliterators.spliteratorUnknownSize - Pass the iterator to this method to create a Spliterator.
4. StreamSupport.stream - Pass the Spliterator to this method to create a stream.

Ultimately, the class library just doesn't have builtins for reading from an input stream and converting into fixed-size strings, so you have to write those for #1/#2. After that, converting to a stream in #3/#4 isn't too bad since there are class library methods to help.

Answer 2

There is no direct support for this. You can create a straight-forward factory method:

static Stream<String> strings(InputStream is, Charset cs, int size) {
    Reader r=new InputStreamReader(is, cs);
    CharBuffer cb=CharBuffer.allocate(size);
    return StreamSupport.stream(new Spliterators.AbstractSpliterator<String>(
        Long.MAX_VALUE, Spliterator.ORDERED|Spliterator.NONNULL) {
            public boolean tryAdvance(Consumer<? super String> action) {
                try { while(cb.hasRemaining() && r.read(cb)>0); }
                catch(IOException ex) { throw new UncheckedIOException(ex); }
                cb.flip();
                if(!cb.hasRemaining()) return false;
                action.accept(cb.toString());
                cb.clear();
                return true;
            }
        }, false).onClose(()->{
            try { r.close(); }catch(IOException ex) { throw new UncheckedIOException(ex); }
        });
}

It can be used like:

try(Stream<String> chunks=strings(is, StandardCharsets.UTF_8, 100)) {
    // perform operation with chunks
}