A good answer might be:

Each constructor requires the user to supply different data.

Multiple Constructors

Any of the three constructors can be used to create a Point. It is a matter of convenience which constructor you use. Here is a program that creates some Points: 


import java.awt.*;      // import the class library where Point is defined
class PointEg1
{

  public static void main ( String arg[] )
  {
    Point a, b, c;              // reference variables

    a = new Point();            // create a Point at (0,  0); 
                                // save the reference in "a"
    b = new Point( 12, 45 );    // create a Point at (12, 45); 
                                // save the reference in "b"
    c = new Point( b );         // create a Point containing data equivalent
                                // to the data referenced by "b"
  }
}

For the compiler to be able to use the definition of Point in the class library, an import must be used. The statement import java.awt.* says to use the AWT library that is part of Java. The "*" says that everything defined in the library can be used (although this program only uses the Point class.)

QUESTION 4:

Say that the program has just been loaded into main memory and is just about to start running.
How many reference variables are there?
How many objects are there?