3.3. Java Basics

One of the most obvious and immediate differences between Objective C (and incidentally C++) and Java, is that Java does not partion classes into "declarations" (header files) and "implementations" (implementation files). All information for any Java class is contained in a single .java file.

Perhaps the best way to illustrate this is to consider the Java equivalent of the previous Objective C Example 3-1.

public class Bug extends SwarmObject 
{
  int xPos, yPos;
  int worldXSize, worldYSize;
  FoodSpace foodSpace;

 public Object setX$Y (int x, int y)
  {
    xPos = x;
    yPos = y;
    return this;
  }
 public Object step()
  {
    // body of step() code
    return this;
  }
 public return_type look(direction_type d)
  {
    return_type returnval;
    // body of look() code
    return returnval;
  }
}

One important distinction to notice is that Java does not have a notion of an id or "generic" data type. All variables must be assigned a type, in the above example the foodSpace instance variable is declared as being of type FoodSpace. This is because Java is a strongly typed language. The compiler checks all types of all variables to ensure all receiver objects respond to the messages that are sent to them by the programmer. Most of the rest of the other differences between the Objective C and Java examples given, lie almost purely in deviations of syntax. Here are a few obvious ones (this list is by no means exhaustive and the reader is encouraged to consult their Java or Objective C reference manual for all the detailed syntax):

• In Objective C, method names and the parameters are interspersed, whilst in Java, the entire method name is given before the parameters.

• In Java self is referred to as this (super retains its meaning and syntax in both languages).