Java不兼容类型场景

Question

I'm learning Java and I've come across a scenario that has me confused.

Here is some code:

     long frameLength = stream.getFrameLength(); 
     int frameSize = stream.getFormat().getFrameSize(); 


    //total number of bytes = bytes per frame * total number of frames

    byte[] totalBytes = new byte[frameLength * frameSize];

The int type cannot take the value of stream.getFrameLength() as it's too big. That's why we use long .

But we can't use long because we need to pass in the value of (frameLength * frameSize) into a byte[] holder which has to be an int because java needs int sized arrays.

So, the above code will not work.

After some research I discovered this:

int frameLength = (int) stream.getFrameLength(); 

That seems to work, I'm confused about the extra "(int)" part added, what is this and what does it mean?

How is the above different to:

 int frameLength = stream.getFrameLength(); or
 long frameLength = stream.getFrameLength(); 

Here's the working code:

AudioInputStream stream = AudioSystem.getAudioInputStream(f);
    int frameLength = (int) stream.getFrameLength(); //This is the number of bytes per frame in the audio stream
    int frameSize = (int) stream.getFormat().getFrameSize(); //Number of frames in the audio stream.

       //total number of bytes = bytes per frame * total number of frames

    byte[] totalBytes = new byte[frameLength * frameSize];

I'm confused, I'd appreciate some help!

Answer 1

That seems to work, I'm confused about the extra "(int)" part added, what is this and what does it mean?

It casts whatever comes out of stream.getFrameLength() to an integer. Note that if it was a long that is larger than Integer.MAX_VALUE you will get an integer overflow .

If you really need that int initialization of the byte array I would do the multiplication first:

long frameLength = stream.getFrameLength();
int frameSize = stream.getFormat().getFrameSize();

Then see if you won't get integer overflow:

long length = frameSize * frameLength // its a long because that won't overflow as fast as int.
if (length > Integer.MAX_VALUE)
    throw new IllegalStateException("Frame too large. Integer overflow!")
else 
   byte[] totalBytes = new byte[frameLength * frameSize];

This assumes the overflow won't happen (since we do not handle the error). If you do encounter this, you should find another solution in stead of a byte array.