Here is an example of how to use the DemuxInputStream class from the Apache Commons IO library in Java:

import java.io.IOException;
import org.apache.commons.io.input.DemuxInputStream;

class Example {
    public static void main(String[] args) {
        DemuxInputStream dis = new DemuxInputStream();
        try {
            // add a stream consumer
            dis.addStream(new StreamConsumer() {
                public void stream(byte[] b, int off, int len) {
                    // process the stream data
                }
            });
            // write data to the DemuxInputStream
            dis.write(new byte[]{1, 2, 3});
            dis.flush();
        } finally {
            dis.close();
        }
    }
}

This code creates a new DemuxInputStream and add a StreamConsumer to it. The DemuxInputStream is a multiplexed input stream that allows multiple InputStreamConsumer instances to read data from a single input stream.

The addStream method is used to add a StreamConsumer to the DemuxInputStream. This StreamConsumer will be called when data is written to the DemuxInputStream using the write method.

In this example, the write method is used to write an array of bytes to the DemuxInputStream and the flush method is used to ensure that the data is processed by the StreamConsumer.

It is important to call the close method when you are done with the DemuxInputStream to release any resources it is holding.

Note that DemuxInputStream is an abstract class, it needs to be subclassed to be usable.