Question:
I am new to Java and am currently trying toassociate idioms and techniques from otherlanguages/environments with like capabilities inJava. Specifically, I am trying to determine howto perform inter-thread and inter-processcommunication. How does one pass data, in theform of an Object or Object reference, betweentwo threads?

Answer:
Assume that the Object(data) beingcommunicated is private to the sending class/packageand not within the scope of the receiving class/package,although the definition of the Object being passedis known to the receiving class/package.

Can this type of communication be performedusing Piped Streams?

One advantage of threads over processes isthat because threads share an address space, sharedmemory communication is fairly easy to set up, providedyou synchronize access to shared data.

Java also provides the classes PipedInputStream andPipedOutputStream specifically for message passingstyle communication between threads. In the examplebelow, I create a pipeline of four threads:

generator ->filter -> amplifier -> accumulator 

Each arrow (->) represents a PipedOutputStream tothe thread on the left, and a PipedInputStream tothe thread on the right. Generator fills itsoutput pipe with integers from 1 to 100. Filterreads from its input stream, and places only eveninputs into its output stream. Amplifier readsnumbers from its input stream, and places theirsquares in its output stream. Finally, accumulatoraccumulates the sum of all inputs from its inputstream.

*/import java.io.*;// Generators fill output pipe with ints from first to lastclass Generator extends Thread {   private DataOutputStream dout;   private int first, last;   public Generator(PipedOutputStream out, int start, int end) {      // filter pipes into data streams      dout = new DataOutputStream(out);      first = start;      last = end;   }   public void run() {      for(int i = first; i <= last; i++) {         try {            dout.writeInt(i);            sleep(5); // don't be selfish         }         catch(IOException e) {}         catch(InterruptedException e){}      }   }} // Generator// Accumulators sum all ints in input pipeclass Accumulator extends Thread {   private DataInputStream din;   private int accum = 0;   public Accumulator(PipedInputStream in) {      // filter pipes into data streams      din = new DataInputStream(in);   }   public void run() {      boolean done = false;      while(!done) {         try {            accum += din.readInt();            sleep(5);         }         catch(IOException e) { done = true; }         catch(InterruptedException e) {}      }      System.out.println("Result = " + accum); // for now   }} // Accumulator// Filters put even ints from input stream into// output stream (i.e. they filter out odds)class Filter extends Thread {   private DataInputStream din;   private DataOutputStream dout;   private int next;   public Filter(PipedInputStream in, PipedOutputStream out) {      // filter pipes into data streams      din = new DataInputStream(in);      dout = new DataOutputStream(out);   }   public void run() {      boolean done = false;      while(!done) {         try {            next = din.readInt();            if (next % 2 == 0)               dout.writeInt(next);            sleep(5);         }         catch(IOException e) { done = true; }         catch(InterruptedException e) {}      }   }} // Filter// Amplifiers put squares of ints from input stream// into output streamclass Amplifier extends Thread {   private DataInputStream din;   private DataOutputStream dout;   private int next;   public Amplifier(PipedInputStream in, PipedOutputStream out) {      din = new DataInputStream(in);      dout = new DataOutputStream(out);   }   public void run() {      boolean done = false;      while(!done) {         try {            next = din.readInt();            dout.writeInt(next * next);            sleep(5);         }         catch(IOException e) { done = true; }         catch(InterruptedException e) {}      }   }} // Amplifier// computes sum of even squares of ints from 1 to 100// = 2^2 + 4^2 + ... + 98^2 + 100^2public class SumOfEvenSquares {   public static void main(String args[]) {      try {         // make some pipes         PipedOutputStream pipe1 = new PipedOutputStream();         PipedInputStream pipe2 = new PipedInputStream(pipe1);         //try { pipe1.connect(pipe2); } catch(IOException e) {}         PipedOutputStream pipe3 = new PipedOutputStream();         PipedInputStream pipe4 = new PipedInputStream(pipe3);         //try { pipe3.connect(pipe4); } catch(IOException e) {}         PipedOutputStream pipe5 = new PipedOutputStream();         PipedInputStream pipe6 = new PipedInputStream(pipe5);         //try { pipe5.connect(pipe6); } catch(IOException e) {}         // make some threads         Generator g = new Generator(pipe1, 1, 10);         Filter f = new Filter(pipe2, pipe3);         Amplifier a = new Amplifier(pipe4, pipe5);         Accumulator ac = new Accumulator(pipe6);         // start threads         g.start();         f.start();         a.start();         ac.start();         // close pipes         pipe1.close();         pipe2.close();         pipe3.close();         pipe4.close();         pipe5.close();         pipe6.close();     }     catch(IOException e) {}   }} // SumOfEvenSquares