Thread: Acess methods

could some one please explain to me in laymen terms why access methods are used. I know how to use methods but, I am confused about why access methods are used. I put an example from my book that uses acess methods. I hope this will help someone explain it to me.

Thanks,
Truck35

//This class is used to calculates fuel efficiency
 
class Vehicle {
private int passengers;   //number of passengers
private int fuelcap;   //fuel capacity in gallons
private int mpg;  // fuel consumption in miles per gallon
 
//This is a constructor for Vehicle.
 
Vehicle (int p, int f, int m) {
passengers = p;
fuelcap = f;
mpg = m;
}
 
//Return the range.
 
int range() {
return mpg * fuelcap;
}
 
//Compute fuel needed for a given distance.
 
double fuelneeded(int miles){
return (double) miles/mpg;
}
 
//These are the access methods  I was talking about.
//The book says they are for instance variables.
 
int getPassengers() {return passengers;}
void setPassengers(int p) {passengers = p;}
int getFuelcap () {return fuelcap;}
void getFuelcap (int f) {fuelcap = f;}
int getMpg() {return mpg;}
void getMpg (int m) { mpg = m;}
 
}

Originally Posted by Truck35

...why access methods are used...

Since the instance variables (passengers, fuelcap, and mpg) are defined as having "private" access, an application program can not set their values directly. An application program can not retrieve their values directly.

In other words, the following would not work. (Why not make a test program and try it?).

/*
  Example of bad code from Zaphod_b.  Compiler error message:
 
 Z.java:11: fuelcap has private access in Vehicle
        System.out.println("Vehicle number 1 has a fuel tank capacity of " + v1.fuelcap + " gallons.");
 
*/
public class Z {
    public static void main(String [] args) {
        Vehicle v1  = new Vehicle(2, 8, 40);
        System.out.println("Vehicle number 1 has a fuel tank capacity of " + v1.fuelcap + " gallons.");
    }
}

This causes a compiler error message as indicated in the opening comment.


So, a "getter" function allows access to the private instance variables:

/*
  Example of code using "getter" access function from Zaphod_b.
*/
public class Z {
    public static void main(String [] args) {
        Vehicle v1  = new Vehicle(2, 8, 40);
        System.out.println("Vehicle number 1 has a fuel tank capacity of " + v1.getFuelcap() + " gallons.");
    }
}

Output:
Vehicle number 1 has a fuel tank capacity of 8 gallons.

Maybe defining the individual variables to have "private" access and, therefore, requiring "getter" and "setter" access methods, seems to be making extra work. I mean, why not just define them with "public" access? The first program would work "just fine," and you wouldn't need a "getter" access method.

Well...

In a little program like this, there might not seem to be much of a downside to having "public" access to the variables, but as programs get bigger and more complicated, there are real advantages to "private" variables. The reasons are covered in every competent introductory discussion of object-oriented programming.

Bottom line: Get into the habit of making the variables "private" and using "getter" and "setter" methods if the application will need to retrieve or set their values. My observation is that bad habits (like making instance variables public to get an improperly-designed program to work "just fine") formed early in one's development are notoriously difficult to break.

By the way: I think you have typographical errors where you copied the method names from the text. I'm pretty sure they should be:

    int getPassengers() {return passengers;}
    void setPassengers(int p) {passengers = p;}
    int getFuelcap () {return fuelcap;}
    void setFuelcap (int f) {fuelcap = f;}
    int getMpg() {return mpg;}
    void setMpg (int m) { mpg = m;}

Cheers!

Z

Thanks for the reply. Your reply was awsome.

Bless,
Truck