试图理解Android中的XML Pull Parser - Java

Question

I was following a tutorial for reading RSS feeds using XML. Please know that I am a very visual learner and the person teaching tried to explain it in words but sadly I didn't understand :(. So I'm hoping someone can explain.

 StringBuilder tempBuffer = new StringBuilder();

        try{
            URL xmlUrl = new URL(URLPath);
            HttpURLConnection httpURLConnection = (HttpURLConnection) xmlUrl.openConnection();
            int response = httpURLConnection.getResponseCode();
            Log.d("Downloaded Data", " Response Code:" + response);

            InputStream is = httpURLConnection.getInputStream();
            InputStreamReader isr = new InputStreamReader(is);

            int charRead;
            char[] buffer = new char[500];

            while(true){

                charRead = isr.read(buffer);

                if(charRead <= 0){
                    break;
                }


                tempBuffer.append(String.copyValueOf(buffer, 0, charRead));

            }
            return tempBuffer.toString();

Now the thing I want to try to understand is the char[] buffer = new char[500]. What does it mean when we do charRead = isr.read(buffer), in regard to reading the stream?

Answer 1

It reads up to 500 bytes (the length of the buffer) from the stream and puts it in buffer. charRead is the number of characters it actually read- somewhere from 0 to 500, depending on how much data was waiting. charRead can also be less than 0 in case of an error.

Answer 2

I'll try to explain it as a visual example.

Imagine an InputStream as a conveyor belt coming out of a hole in the wall (the network). There is a series of char s on the conveyor belt, but you need to bring them to you before you can take them off the conveyor belt and process them into your bucket ( buffer , which can only hold 500 char ). When you call isr.read(buffer) , the conveyor belt operator is told to start the conveyor belt. The conveyor belt will start to roll, dumping the char objects into the bucket ( buffer ). This will continue until either

A. The conveyor belt dumps 500 chars into the bucket ( buffer ), or

B. The conveyor belt runs out of chars coming out of the hole in the wall (network).

The line of code:

if(charRead <= 0){
    break;
}

Tells the conveyor belt that if NOTHING came out of the hole in the wall when the button was pressed to stop pressing the button and exit the loop.

The line of code:

tempBuffer.append(String.copyValueOf(buffer, 0, charRead));

tells the conveyor belt operator to stop what he's doing (after the conveyor belt stopped moving because either 500 chars got dumped into the bucket or the conveyor belt ran out of chars ) and take everything that's in the bucket ( buffer ), and take out charRead amount of chars . Basically, this is everything that's in the bucket, because we know that charRead is how many chars came off the conveyor belt and went into the bucket, so if we take charRead chars from the bucket, we know that we got all of them. These are appended onto the StringBuilder , and the loop starts over again!