Access modifiers help with the OOP principle of encapsulation. If you create a well-written class anyone will be able to use it without knowing all the internal details of how it works. When you define a method as public any code that uses your class will have access to that method. You only want to expose methods the end user of your class will need. If it's a helper method that's does some internal calculation relevant only inside your class, declare it as private. It will cut down on the methods available to the end user and make your class much easier to use. Here's an example:
Public Class Car {
private int speed;
private float fuelNeeded;
private float afRatio = 14.7;
public void speedUp { speed++; }
public void speedDown { speed--; }
public float getFuelNeeded {
calculateFuelNeeded(speed);
return fuelNeeded;
}
private void calculateFuelNeeded (int speed) {
// Do some complex calculation
fuelNeeded = someValue / afRatio;
}
}
Anyone who creates a Car object will have three methods available, speedUp, speedDown, and getFuelNeeded. calculateFuelNeeded is only used internally by the class, so there's no reason for the user to see it. This way they don't need to know how the amount is calculated, or when the method needs to be called, or what values it sets internally. They just get the value easily with getFuelNeeded();
When declaring variables, it is usually always correct to declare them private, having a public get() and set() method for any that need to be accessed from outside the class. The main reason for this is to prevent an outside user from setting the value something invalid. In our Car class, afRatio needs to be nonzero because we are using it as a divisor. If we declare it as public, a user could easily write this code:
Car car = new Car();
car.afRatio = 0;
someFloat = car.getFuelNeeded();
Of course this would cause our class to throw an ArithmeticException for divison by zero. However, if we did this:
private afRatio = 14.7;
public void setAfRatio(float ratio) {
if(ratio <= 0)
throw new IllegalArgumentException("A/F ratio must be greater than zero");
afRatio = ratio;
}
This allows us to check user given parameters and ensure our variables don't get set to some invalid value.
