I'm going to explain TCP:
The basic concept is that you have to run a "Server" on a machine. That server accepts clients waiting for a connection. Each connection goes over a port (you know, I hope...).
Always use ports above 1024 because ports lower than 1025 are most of the time reserved for standard protocols (like HTTP (80), FTP (21), Telnet, ...)
However, creating a Server in Java is done this way:
ServerSocket server = new ServerSocket(8888); // 8888 is the port the server will listen on.
"Socket" is the word you are probably looking for if you want to do research.
And to connect your client to a server you have to write this:
Socket connectionToTheServer = new Socket("localhost", 8888); // First param: server-address, Second: the port
But now, there isn't still a connection. The server has to accept the waiting client (as I noticed here above):
Socket connectionToTheClient = server.accept();
Done! Your connection is established! Communicating is just like File-IO. The only thing you have to keep in mind is that you have to decide when you want to flush the buffer and really send the data through the socket.
Using a PrintStream for text-writing is very handy:
OutputStream out = yourSocketHere.getOutputStream();
PrintStream ps = new PrintStream(out, true); // Second param: auto-flush on write = true
ps.println("Hello, Other side of the connection!");
// Now, you don't have to flush it, because of the auto-flush flag we turned on.
A BufferedReader for text-reading is the good (best*) option:
InputStream in = yourSocketHere.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String line = br.readLine();
System.out.println(line); // Prints "Hello, Other side of the connection!", in this example (if this would be the other side of the connection.
Hopefully you can start with networking with this information!
PS: Of course, all networking code have to be try-catched for IOExceptions.
EDIT: I forgot to write why it isn't always the best option. A BufferedReader uses a buffer and read as much as it can into the buffer. But sometimes you don't want that the BufferedReader steals the bytes after the newline and put them into his own buffer.
Short example:
InputStream in = socket.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(in));
// The other side says hello:
String text = br.readLine();
// For whatever reason, you want to read one single byte from the stream,
// That single byte, just after the newline:
byte b = (byte) in.read();
But the BufferedReader has already that byte, you want to read, in his buffer. So calling in.read() will return the byte following on the last byte in the buffer of the reader.
So, in this situation the best solution is to use DataInputStream and manage it your own way to know how long the string will be and read only that number of bytes and convert them into a string. Or: You use
DataInputStream.readLine()
This method doesn't use a buffer and reads byte by byte and checks for a newline. So this method doesn't steal the bytes from the underlying InputStream.
EDIT: You can develop your own protocol in which you can request a method call using Java Reflexion. For example:
String className = ...;
String methodName = ...;
Class[] methodParamTypes = ...;
Object[] methodParams = ...;
Class cl = Class.forName(className);
Method me = cl.getDelcaredMethod(methodName, methodParamTypes);
Object returnValue = me.invoke(this, methodParams);
Once you have your object you can send it to the other side of the connection with Serialization: ObjectOuputStreams and ObjectInputStreams. With these two classes you can write and read objects through a stream.
Object obj = ...; // Your object you want to write through the stream. (Needs to implement java.io.Serializable)
ObjectOutputStream oos = new ObjectOuptputStream(socket.getOutputStream());
oos.writeObject(oos);
oos.reset(); // (***)
// Don't close it! Otherwise the connection will be closed as well.
And on the other side of the connection:
ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
Object object = ois.readObject(); // Read the object
// Don't close it!
// Then cast it to whatever you want.
(***): Check out my question for further information about reset() and when to use it.
