所以我很难理解什么时候使用静态以及什么时候从函数头中排除它

Question

In my introductory Java course, we are required to create a program that gets and sets info on an array of cars based off what our teacher posted on our class website. One of the requirements is to make a function that will print the info of all three cars

 public static void printAllCarsInfo() // function for printing info needed for all three cars { for ( int index = 0 ; index < NUM_CARS ; index++ ) cars[index].printInfo();

The printInfo() function is in another class and looks like:

 public void printInfo() { if ( fuelGallons > 0 ) System.out.println("The " + getModel() + " has driven " + getMilesDriven() + " miles and has " + getFuelGallons() + " gallons left."); else System.out.println("The " + getModel() + " has driven " + getMilesDriven() + " miles and is out of gas.");

Now when I debug it, I get an error message that goes:

 Smith12.java:31: error: cannot find symbol cars[index].printInfo(); ^ symbol: variable cars location: class Smith12 Smith12.java:36: error: cannot find symbol cars[index].drive(miles); ^ symbol: variable cars location: class Smith12 2 errors

(31 & 36 are the line numbers where the error arises) The error comes from the for loop, but even before I switched this to a loop and just had them all listed outright, I was getting the same error. I've tried so many things that I'm just beat at this point. Someone please help me out.

Here's the main code since it isn't too long:

 import java.util.Scanner; public class Smith12 { public static final int NUM_CARS = 3; public static void main(String[] args) { Car[] cars = new Car[NUM_CARS]; cars[0] = new Car( "Toyota Camry", 3400 ); cars[1] = new Car( "Ford F-150", 5000 ); cars[2] = new Car( "Honda Civic", 3000 ); for ( int index = 0 ; index < NUM_CARS ; index++ ) cars[index].drive(10); printAllCarsInfo(); cars[0].setFuelGallons(5); Scanner scanner = new Scanner(System.in); System.out.println("How many miles is your road trip? "); double roadTrip = scanner.nextDouble(); driveAllCars( 10 ); printAllCarsInfo(); } public static void printAllCarsInfo() // function for printing info needed for all three cars { for ( int index = 0 ; index < NUM_CARS ; index++ ) Car[index].printInfo(); } public static void driveAllCars( double miles ) { for ( int index = 0 ; index < NUM_CARS ; index++ ) cars[index].drive(miles); } }

And here is the Car class:

 public class Car { private static String model; private static double mpg; private static double milesDriven; private static double fuelGallons; public Car( String carModel, double weight ) { model = carModel; if ( weight > 4000 ) mpg = 20.0; else mpg = 30.0; milesDriven = 7; fuelGallons = 15; } public String getModel() { return model; } public double getMPG() { return mpg; } public double getMilesDriven() { return milesDriven; } public double getFuelGallons() { return fuelGallons; } public void setFuelGallons( double gallons ) { fuelGallons += gallons; } public void setMilesDriven( double distance ) { milesDriven += distance; } public double getMilesLeft() { return mpg * fuelGallons; } public void drive( double miles ) { if ( miles <= getMilesLeft() ) { milesDriven += miles; fuelGallons -= ( miles / mpg ); } else { milesDriven += getMilesLeft(); fuelGallons = 0; } } public void printInfo() { if ( fuelGallons > 0 ) System.out.println("The " + this.getModel() + " has driven " + this.getMilesDriven() + " miles and has " + this.getFuelGallons() + " gallons left."); else System.out.println("The " + this.getModel() + " has driven " + this.getMilesDriven() + " miles and is out of gas."); } }

I've changed some of the variables to static, though I'm not sure it's correct. I've tried the comments but I'm here asking for help because I'm not very good at programming in general. Thank you all for your help and comments.

Answer 1

In static funcions you can only use other static variables and arguments that were passed to this functions. There is no this context.