RMI, and Java GUIs

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RMI, and Java GUIs

• 4-3-2002

2
Comments

• Everyone should be filling out and turning in
  group evaluations. These are largely for your
  protection. It can be used to point out where
  there are problems in group mechanics.
• I had planned on talking about sequence diagrams
  today, but Im going to push that back a week and
  discuss some Java topics instead.

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Remote Method Invocation

• Java makes virtually all network communication
  easier than it is in C/C. This is particularly
  true when using RMI.
• With RMI you can treat objects on other computers
  as if they were local on your own computer and
  call methods on them accordingly. Of course,
  there is a lag when you do this, but you dont
  have to explicitly worry about the socketing and
  whatnot.

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Remote Interfaces

• The entity you get for an object remotely is
  actually not the object itself, but an interface
  that object implements. The interface must
  extend java.rmi.Remote.
• The implementation class should also extend
  java.rmi.server.UnicastRemoteObject.
• Note that this implies that the implementation
  can have more functionality than the remote
  interface does.
• All remote methods can throw java.rmi.RemoteExcept
  ion.

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Passing

• Different types of objects are passed differently
• Remote objects - any object that extends
  java.rmi.Remote is passed as a remote object.
  You get a skeleton that does network
  communication.
• Serializable objects - If an object is not Remote
  it must be serializable and then it is passed by
  value.
• Primitives - No pass by reference of primitives

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Registering and Lookup

• Once you have a remote object, it can pass you
  others, but getting the first one takes a
  different approach.
• An object can register itself with the rmi
  registry using the rebind method of
  java.rmi.Naming. (You have to start a local
  registry with rmiregistry first.)
• Objects can get a remote reference to a
  registered object using the lookup method of
  java.rmi.Naming.

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Compiling

• Once you have written your code you first compile
  it with a normal Java compiler. After that you
  have to do another step to create stub and
  skeleton classes that do most of the work behind
  RMI. You to this is the RMI compiler, rmic.
  Just run rmic specifying the name of the
  implementation class.

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Java GUIs

• One of the greatest things about Java, from an
  application building standpoint, is how easy it
  is to make a GUI in it.
• Since 5 out of 8 of the groups HAVE to do this to
  implement anything we will take a few minutes to
  discuss it.
• Java has two graphics libraries, AWT and Swing.
  They work very similarly.

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AWT

• The AWT library uses inheritance extensively.
  Components of the GUI are represented by the
  component class. Container is a subclass of
  Component that can have other components added to
  it. Each of these has other subclasses
  representing buttons, windows, text fields,
  labels, etc.

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Layouts

• To make Java GUIs portable, they use layout
  managers to position and size various components.
  You can place things at specific locations on
  the screen, but that isnt very portable.
• Every container has a layout manager it uses to
  place the components in it. By nesting
  containers and layout managers you can get great
  flexibility.

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Some Layout Managers

• Here are some of the layout managers in AWT.
• FlowLayout - places one component after another
  like text on a page.
• GridLayout - places components on a regular grid.
• BorderLayout - Objects can be placed a north,
  south, east, west, or center.
• GridBagLayout - A powerful layout manager for
  placing objects in an irregular grid.

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Events

• No GUI is complete unless it can react to the
  user. User interactions with a GUI are called
  events and we want to be able to create code to
  handle events.
• In Java (since version 1.1) we do this be
  attaching event listeners to GUI components.
  When an event happens for an object all the
  appropriate listeners for that object are called.

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Events and Listeners

• There are a fair number of event types and
  listeners.
• The listeners include ActionListener,
  MouseListener, KeyListener, and FocusListener.
  Each listener type has certain methods that it
  must define.
• Action events are some of the most common and
  happen when you do things like click buttons.

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Swing

• Swing is actually based on AWT, but provides a
  look and feel that is machine independent. It
  has pretty much all the same objects but the
  names typically start is J (i.e. JButton and
  JFrame).
• Swing also has other classes AWT didnt. These
  include JTable, JTree, JColorChooser, and
  JToolBar.
• AWT or Swing for project?