Java RMI

1) Compute.java

package compute;

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface Compute extends Remote {
   <T> T executeTask(Task<T> t) throws RemoteException;
}

2) Task.java

package compute;

public interface Task<T> {
   T execute();
}

cd c:\home\waldo\src
javac compute\Compute.java compute\Task.java
jar cvf compute.jar compute\*.class

3) ComputeEngine.java

package engine;

import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.UnicastRemoteObject;
import compute.Compute;
import compute.Task;

public class ComputeEngine implements Compute{
   public ComputeEngine(){
      super();
   }
  
   public<T> T executeTask(Task<T> t){
      return t.execute();
   }

   public static void main(String[] args){
      if(System.getSecurityManager() == null){
         System.setSecurityManager(new SecurityManager());
      }
      try{
         String name = "Compute";
         Compute engine = new ComputeEngine();
         Compute stub = (Compute) UnicastRemoteObject.exportObject(engine, 0);
         Registry registry = LocateRegistry.getRegistry();
         registry.rebind(name, stub);
      } catch(Exception e){
         System.err.println("ComputeEngine exception: ");
         e.printStackTrace();
      }
   }
}

cd c:\home\ann\src
javac -cp c:\home\ann\public_html\classes\compute.jar
    engine\ComputeEngine.java

Pi.java

package client;
import compute.Task;
import java.io.Serializable;
import java.math.BigDecimal;

public class Pi implements Task<BigDecimal>, Serializable{
   private final int digits;
   public Pi(int digits){
      this.digits = digits;
   }
   public BigDecimal execute(){
      return computePi(digits);
   }
   public static BigDecimal computePi(int digits){
      return result;
   }
}

ComputPi.java

package client;

import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.math.BigDecimal;
import compute.Compute;

public class ComputePi{
   public static void main(String args[]){
      if(System.getSecurityManager() == null){
         System.setSecurityManager(new SecurityManager());
      }
      try{
         String name = "Compute";
         Registry registry = LocateRegistry.getRegistry(args[0]);
         Compute comp = (Compute) registry.lookup(name);
         Pi task = new Pi(Integer.parseInt(args[1]);
         BigDecimal pi = comp.executeTask(task);
         System.out.println(pi);
      } catch(Exception e){
         System.err.println("ComputerPi exception:");
         e.printStackTrace();
      }
   }
}

cd c:\home\jones\src
javac -cp c:\home\jones\public_html\classes\compute.jar
   client\ComputePi.java client\Pi.java
mkdir c:\home\jones\public_html\classes\client
cp client\Pi.class
   c:\home\jones\public_html\classes\client

server.policy

grant codeBase "file:/home/ann/src/" {
   permission java.security.AllPermission;
};

client.policy

grant codeBase "file:/home/jones/src/" {
   permission java.security.AllPermission;
};

start rmiregistry (or javaw if start is not available)

java -cp c:\home\ann\src;c:\home\ann\public_html\classes\compute.jar
       -Djava.rmi.server.codebase=file:/c:/home/ann/public_html/classes/compute.jar
       -Djava.rmi.server.hostname=mycomputer.example.com
       -Djava.security.policy=server.policy
          engine.ComputeEngine

java -cp c:\home\jones\src;c:\home\jones\public_html\classes\compute.jar
       -Djava.rmi.server.codebase=file:/c:/home/jones/public_html/classes/
       -Djava.security.policy=client.policy
          client.ComputePi mycomputer.example.com 45

Objective-C Header and Class Implementation

Person.h

#import <Foundation/Foundation.h>

@interface Person:NSObject{

   NSString *name;

   int age;

}

- (NSString *)name;

- (void) setName: (NSString *) newName;

- (int)age;

- (void) setAge: (int) newAge;

//actions

- (BOOL) isAllowedToVote;

- (void castBallot;

@end

Person.m

#import "Person.h"

@implementation Person

- (int) age{

   return age;

}

- (void) setAge: (int) value{

   age = value;

}

//... and other methods

@end

Using a class

id anObject = [[SomeClass alloc] init];

- (id) init

{

   self = [super init];

   if(self){

     //initializations

   }

   return self;

}

- (Person *) createPersonWithName: (NSString *) aName{

   Person *aPerson = [[Person alloc] init];

   [aPerson setName:aName];

   [aPerson setAge:21];

   return aPerson;

}

Class Inheritance

@interface Student : Person {

   NSString *major;

}

@end

@implementation Student

// Constructor for a student

- (id) init

{

   self = [super init];

   if(self != nil){

      //set attributes

      [self setMajor:@"Undefined"];

   }

   return self;

}

@end

Objective-C Selector

id object;
SEL anAction = @selector(action:);
if([object respondsToSelector:anAction]){
   [object performSelector:anAction withObject:self];
}

Objective-C Object Messaging

[receiver message]
[receiver message:argument]
[receiver message:arg1 argument2:arg2]
int status = [receiver message];
[receiver makeGroup:group, memberOne, memberTwo, memberThree];

[rectangle draw]
[rectangle setColor:blue]
[rectangle setStrokeColor:blue andThickness:2.0]
double length = [rectangle circumference];
[NSArray arrayWithObjects:one, two, three, nil];

Java vs. Objective-C

Java
Rectangle rect = Rectangle(someWidth, someHeight);
println(rect.getHeight());
boolean inside = rect.contains(x,y);

Objective-C
Rectangle *rect = [[Rectangle alloc] initWithWidth:someWidth andHeight:someHeight];
NSLog(@"%f", [rect height]);
BOOL inside = [rect containsPointWithX:x andY:y]

Objective-C Identiy vs. Equality

// test for object identity:
BOOL identical = @"Peter" == @"Peter";     //YES
            identical = @"Peter" == @" Peter ";    //NO

// test for string equality: will return YES
NSString *aName = @"Peter";
NSMutableString *anotherName = [NSMutableString stringWithString:@"P"];

[anotherName appendString:@"eter"];

BOOL same = [aName isEqualToString:anotherName];     //YES
BOOL equal = (aName == anotherName);         //NO

// test for existence
BOOL exists = (object != nil);