public class Spacecraft
{
   // private, hidden fields

   private String name;   // i.e.,  USS Challenger
   private int speed;   // m/s
   private int maxSpeed;   // m/s
   private int fuel;   // L
   private int maxFuel;   // L
   private double position;  // m traveled from 
                             //start of trip (like an odometer)
	
   private final int mpL = 2000;   // fuel mileage in m/L
	
   // a constructor ================================================

   public Spacecraft(String n, int maxS, int maxF)
   {
	   name = n;
	   speed = 100;   // default speed
	   maxSpeed = maxS;
	   fuel = 1000;   // default fuel
	   maxFuel = maxF;
	   position = 0.0;
   }
	
   // get methods ==================================================
	
   public String getName()
   {
	  return name;	
   }	
	
   public int getSpeed()
   {
	   return speed;	
   }
	
   public int getMaxSpeed()
   {
	   return maxSpeed;	
   }
	
   public int getFuel()
   {
	   return fuel;	
   }
	
   public int getMaxFuel()
   {
	   return maxFuel;	
   }
	
   public double getPosition()
   {
	   return position;	
   }
	
   // set methods ==================================================
	
   public boolean setSpeed(int s)
   {	
	   boolean resultFlag = false;
	
	   if (s <= maxSpeed)
	   {
		   speed = s;	
		   resultFlag = true;
	   }
		
	   return resultFlag;
   }
	
   // some other public methods ====================================
	
   public boolean addFuel(int f)
   {
	   boolean resultFlag = false;
	
	   if ((fuel + f) < maxFuel)
	   {
		   fuel += f;	
		   resultFlag = true;
	   }
	   else
	   {
		   fuel = maxFuel;	
	   }
		
	   return resultFlag;
   }
	
   public boolean travel(int t)   // travel time in s
   {
	   boolean resultFlag = false;

	   int requestedTravel = speed * t;   // in m
	   int maxTravel = fuel * mpL;   // in m

	   if (requestedTravel < maxTravel)
	   {
		   position += requestedTravel;
		   fuel -= ((speed * t) / mpL);
		   resultFlag = true;
	    } 
	   else
	    {
		   position += maxTravel;
		   fuel = 0;
	   }
		
	   return resultFlag;
   } 
	
   public String toString()
   {
	   String s1 = " Spacecraft name: " + name + "\n";
	   String s2 = "           speed: " + speed + " m/s \n";
	   String s3 = "   maximum speed: " + maxSpeed + " m/s\n";
	   String s4 = "            fuel: " + fuel + " L \n";
	   String s5 = "    maximum fuel: " + maxFuel + " L\n";
	   String s6 = "current position: " + position +
	   " m traveled so far.";
	   return (s1 + s2 + s3 + s4 + s5 + s6);
   }
}

public class TestSpacecraft
{
   public static void main(String args[])
   {
	   Spacecraft sc1 = new Spacecraft("USS Challenger", 10000, 4000);
		
	   System.out.println(sc1);
		
	   if (sc1.travel(1000))
	   {
		   System.out.println("\nTravel fully successful!\n");
	   }
	   else
	   {
		   System.out.println("\nTravel not fully successful.\n");
	   }
		
	   System.out.println(sc1);
   }	
}

• The final variables cannot be given a new value; they're essentially constants.
• The get and set methods provide read and write access to private fields.
• Most classes implement a toString()method that overrides the simple one inherited from Object (all Java classes are descendants of class Object).
• TestSpacecraft's only method is static; this means that it can be used without creating an instance of the enclosing class; in this case, the Java Virtual Machine invokes main() without creating any instance of class TestSpacecraft.

public class SuperSpacecraft extends Spacecraft
{
   // private, hidden fields
	
   private int photonTorpedoes;   // count
   private int phaserMegaJoules;   // in MJ

   // a constructor ================================================

   public SuperSpacecraft(String n, int maxS, int maxF,
   int torps, int phaserCharge)
   {
	   super(n, maxS, maxF);
		
	   photonTorpedoes = torps;
	   phaserMegaJoules = phaserCharge;	
   }	
	
   // get methods ==================================================
	
   public int getTorpedoes()
   {
	   return photonTorpedoes;
   }
	
   public int getPhaserCharge()
   {
	   return phaserMegaJoules;	
   }

   // some other public methods ====================================

   public void addTorpedoes(int t)
   {
	   photonTorpedoes += t;
   }
	
   public void addCharge(int c)
   {
	   phaserMegaJoules += c;	
   }
	
   public boolean firePhotonTorpedoes(int t)
   {
      // implementation here
	  
	  return true;
   }
	
   public boolean firePhasers(int p)
   {
	   // implementation here	
		
	   return true;
   }
	
   public String toString()
   {
	   String s1 = super.toString();
	   String s2 = "\n     photo torps: " + photonTorpedoes + "\n";
	   String s3 = "   phaser charge: " + phaserMegaJoules + " MJ\n";
	   return (s1 + s2 + s3);
   }
}

And here is the associated test file:

public class TestSuperSpacecraft
{
   public static void main(String args[])
   {   
	   Spacecraft craft[] = new Spacecraft[10];
		
	   for (int index = 0; index < 10; index += 2)
	   {
		   craft[index] = new Spacecraft("USS Enterprise " +
		   index, 5000, 12000);
		   craft[index + 1] = new SuperSpacecraft("USS Voyager " +
		   index, 5000, 12000, 50, 4525);
	   }
		
	   for (int index = 0; index < 10; index ++)
	   {
	   	   System.out.println("Craft " + index + "\n:"
		   + craft[index].toString() + "\n==========\n");
	   }
		
   }	
}

• There's a superclass call in the constructor and in the toString() method.
• The test file uses arrays and introduces polymorphism by using references to Spacecraft objects to refer to both Spacecraft and SuperSpacecraft objects.