Java notes day 2.


1) Arrays

	An array is an object into which you can put other things.
		you can put in primitives

		you can put in objects


	Declare an array as

	type[] name ;

	And the create the array with

	name = new type[n_of_items];
	

	Access the i'th element as

	name[i]
	
	example2.1  (note overloaded printTheArray method)

	Note items in an array are numbered from 0 to N-1, and this
index number is sometimes called the offset.



	example2.2. Put objects into an array, get them out!


2) Static



Take a close look at a class:



public class Student extends Object {
    static String teacher;  //look, it's a class variable.
    String myName;  //look, it's an instance variable

    public Student (String s){  //look, it's a constructor
	myName = s;
    }


	//look, it's a static (class) method
    public static void setTeacherName(String s){ 
	teacher = s;
    }




1. key terms in creating classes/fields

	static:

	a static field means there is only one of these for each
class. You can set the value of this field even if no instances exist.


	All instances share this thing.
	
	
		

	static methods are ones that the class itself can execute.
	 	must not directly access instance variables

		I (personally) think there is a mistake in java's
design because static methods can be called either by the class name

		Student.setTeacherName("bob");

Or, if you have an instance of the class, you can tell the instance to
do the same thing:

		aParticularStudent.setTeacherName("bob");

As far as I know, other languages don't allow instances to execute
class methods.

	Please also note that since teacher is a public static
variable, we could access it directly as:

		Student.teacherTeacherName;

instead of using the method setTeacherName.  I don't like doing it
that way for information hiding reasons.



3) Inheritance and project design.


	interface--list of methods all subclasses must implement.  Any
class can adopt this interface, which means a promise to implement its
methods.

	abstract class--methods list, similar in nature to interface,
but it cannot be adopted by any class except a subclass.


	inner classes


4) Using packages

	If it is a jar file, add the jar file to the CLASSPATH
	if it is *.class files, add the directory to the CLASSPATH