Importing data from a main method into another method, and then storing the value before running the program again

Question

I have two classes, one that only has a main(String args[]) method and one with all the rest. I am trying to get the data from the main method into the second class to run it, and then give the information back to the main method. My program is compiling but the numbers should be going down each time I input new number, but they are not. Any help is appreciated!

public class LunarLander
{
  public static void move(double efficiency, double fuelLeft, int maxFuel, int time, double Altitude, double inputFuelRate)
  {
    //formulas to change outputs
    double velocity = time * (efficiency - 1.62); //given formula to caluculate velocity
    double altChng = time * velocity; //creates a variable for atitude chage 

    //exceptions
    if (efficiency > 0 && fuelLeft == 0){ //changes efficiency to 0 when there is no fuel left
      efficiency = 0;
    }
    else{
    }

    //new outputs
    Altitude = Altitude - altChng; //calculates new altitude by subtracting altitude change
    velocity = time * (efficiency - 1.62); //given formula to caluculate velocity
    altChng = time * velocity; //creates a variable for atitude chage 
    double verticalSpeed = velocity; //since the ship would only move and not go back and forth velocity is speed

    efficiency = inputFuelRate / maxFuel; //recalculates efficiency

    double fuelLoss = time * fuelLeft * maxFuel;// new variable to determine how much fuel was burned during time period
    fuelLeft = fuelLeft - fuelLoss; //changes the values for fuel left

  }

  public static boolean crashed(double Altitude, double verticalSpeed)
  {
    if (Altitude == 0 && verticalSpeed <-1){
      return true;
    }
    else{
      return false;
    }
  }

  public String toString(double Altitude, double verticalSpeed, double fuelLeft){
    String output = "";
    output += "Eagle: \n";
    output += "Altitude = " + Altitude + "\n";
    output += "Speed = " + verticalSpeed + "\n";
    output += "Fuel = " + fuelLeft + "\n";

    return output;
  }

}

main method:

import java.util.Scanner;

public class Pilot
{

  public static void main(String args[])
  {
    Scanner kb = new Scanner(System.in);

    LunarLander lander = new LunarLander(); // create a LunarLander object 
    double Altitude = 10.0;

    String Name = "Eagle";

    double fuelLeft = 1000.0;

    int shipWeight = 400;

    int maxThrust = 10000;

    int verticalSpeed = 0;

    double efficiency = 0;

    int maxFuel = 400; //max fuel flow

    System.out.println("Initial data: ");
    System.out.println("Altitude = " + Altitude);
    System.out.println("Speed = " + verticalSpeed);
    System.out.println("Fuel = " + fuelLeft);

    while (lander.crashed(Altitude, verticalSpeed) != true && Altitude > 0)
    {

      System.out.println("Please enter a time in seconds: ");
      int time = kb.nextInt();
      System.out.println("Please enter a fuel rate between 0 and 1");
      double inputFuelRate = kb.nextDouble();
      System.out.println("Input time increment: " + time);
      System.out.println("Input fuel rate: " + inputFuelRate);

      lander.move(efficiency, fuelLeft, maxFuel, time, Altitude, inputFuelRate);


      System.out.println(lander.toString(Altitude, verticalSpeed, fuelLeft));
    }

  }
}

Please read http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value Your LunarLander methods will not modify the variables in the way you're expecting. — user1071777, Jul 29 '14 at 19:38
Also, your design is not good. Consider that each LunarLander instance should have its own variables (altitude, fuel, speed, etc). — user1071777, Jul 29 '14 at 19:45
Although I appreciate your response that is really not helpful. If something is "not good" giving examples is a lot more helpful — user3889083, Jul 29 '14 at 19:52
I'm saying that altitude (and some other variables you're using) belongs to a LunarLander instance. `class LunarLander { double altitude; static int maxThrust = 10000; // etc.}` and in your main, you create a LunarLander like `new LunarLander(10.0, 1000.0, ..);` or create a default constructor that uses default values. — user1071777, Jul 29 '14 at 19:55

score 0 · Answer 1 · answered Jul 29 '14 at 19:41

0

If you redesigned your program so the output of one program was the input of the other you could run it like this using pipes:

prog1.exe > prog2.exe
prog1.exe < prog2.exe
prog1.exe | prog2.exe

Another thing you could do is have a client server solution using networking http://docs.oracle.com/javase/tutorial/networking/index.html

answered Jul 29 '14 at 19:41

user3386190

13
4

From what I deciphered from the question, it's one program with 2 classes. – user1071777 Jul 29 '14 at 19:43
Yes it is one program! – user3889083 Jul 29 '14 at 19:45

user2833557 · Accepted Answer · 2014-07-29T20:10:58.517

Your method variable Altitude is local to the method. This means its recreated and reassigned value every time you call the method. In java all parameters to a method are passed by value, which means the change in the parameter inside the the method is not visible outside it, basically it creates a copy of the parameter when invoked. You need to make it an instance variable by defining in the class not inside any method. same goes for the other variables that you want to be changed every time you call the method.

import java.util.Scanner;

public class Pilot
{

  public static void main(String args[])
  {
    Scanner kb = new Scanner(System.in);


    double Altitude = 10.0;

    String Name = "Eagle";

    double fuelLeft = 1000.0;

    int shipWeight = 400;

    int maxThrust = 10000;

    int verticalSpeed = 0;

    double efficiency = 0;

    int maxFuel = 400; //max fuel flow

    LunarLander lander = new LunarLander(Altitude, verticalSpeed, fuelLeft); // create a LunarLander object

    System.out.println("Initial data: ");
    System.out.println("Altitude = " + Altitude);
    System.out.println("Speed = " + verticalSpeed);
    System.out.println("Fuel = " + fuelLeft);

    while (lander.crashed() != true && Altitude > 0)
    {

      System.out.println("Please enter a time in seconds: ");
      int time = kb.nextInt();
      System.out.println("Please enter a fuel rate between 0 and 1");
      double inputFuelRate = kb.nextDouble();
      System.out.println("Input time increment: " + time);
      System.out.println("Input fuel rate: " + inputFuelRate);

      lander.move(efficiency, maxFuel, time, inputFuelRate);


      System.out.println(lander.toString());
    }

  }
}

And the LunarLander class

public class LunarLander {

    double Altitude;
    double verticalSpeed;
    double fuelLeft;

    public LunarLander(double Altitude, double verticalSpeed, double fuelLeft){
        this.Altitude = Altitude;
        this.verticalSpeed = verticalSpeed;
        this.fuelLeft = fuelLeft;
    }

    public void move(double efficiency, int maxFuel,
            int time, double inputFuelRate) {
        // formulas to change outputs
        double velocity = time * (efficiency - 1.62); // given formula to
                                                        // caluculate velocity
        double altChng = time * velocity; // creates a variable for atitude
                                            // chage

        // exceptions
        if (efficiency > 0 && fuelLeft == 0) { // changes efficiency to 0 when
                                                // there is no fuel left
            efficiency = 0;
        } else {
        }

        // new outputs
        Altitude = Altitude - altChng; // calculates new altitude by subtracting
                                        // altitude change
        velocity = time * (efficiency - 1.62); // given formula to caluculate
                                                // velocity
        altChng = time * velocity; // creates a variable for atitude chage
        verticalSpeed = velocity; // since the ship would only move and
                                            // not go back and forth velocity is
                                            // speed

        efficiency = inputFuelRate / maxFuel; // recalculates efficiency

        double fuelLoss = time * fuelLeft * maxFuel;// new variable to determine
                                                    // how much fuel was burned
                                                    // during time period
        fuelLeft = fuelLeft - fuelLoss; // changes the values for fuel left

    }

    public boolean crashed() {
        if (Altitude == 0 && verticalSpeed < -1) {
            return true;
        } else {
            return false;
        }
    }

    public String toString() {
        String output = "";
        output += "Eagle: \n";
        output += "Altitude = " + Altitude + "\n";
        output += "Speed = " + verticalSpeed + "\n";
        output += "Fuel = " + fuelLeft + "\n";

        return output;
    }

}

Thank you so much! How do you recommend fixing? should I just just move the variable declarations outside of the methods — user3889083, Jul 29 '14 at 19:50
NO not exactly. Let me write and give you the code. hold on :P — user2833557, Jul 29 '14 at 19:54
this is very helpful! Using the this. is great and a lot of places are much simpler. Thank you — user3889083, Jul 29 '14 at 20:37

Importing data from a main method into another method, and then storing the value before running the program again

2 Answers2