Passing Arguments to Methods & Example

    It's generally considered bad form to access fields directly. Instead it is considered good object oriented practice to access the fields only through methods. This allows you to change the implementation of a class without changing its interface. This also allows you to enforce constraints on the values of the fields.

To do this you need to be able to send information into the Car class. This is done by passing arguments. For example, to allow other objects to change the value of the speed field in a Car object, the Car class could provide an accelerate() method. This method does not allow the car to exceed its maximum speed, or to go slower than 0 kph.

 void accelerate(double deltaV) {

     this.speed = this.speed + deltaV;
     if (this.speed > this.maxSpeed) {
       this.speed = this.maxSpeed; 
     }
     if (this.speed <  0.0) {
       this.speed = 0.0; 
     }     
     
  }

void accelerate(double deltaV)

indicates that accelerate() returns no value and takes a single argument, a double which will be referred to as deltaV inside the method.

deltaV is a purely formal argument.

Java passes method arguments by value, not by reference.

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Example

class Car {

  String licensePlate = "";    // e.g. "New York 543 A23"
  double speed        = 0.0;   // in kilometers per hour
  double maxSpeed;    = 120.0; // in kilometers per hour
  
  // accelerate to maximum speed
  // put the pedal to the metal
  void floorIt() {
    this.speed = this.maxSpeed;  
  }
  
  void accelerate(double deltaV) {

     this.speed = this.speed + deltaV;
     if (this.speed > this.maxSpeed) {
       this.speed = this.maxSpeed; 
     }
     if (this.speed <  0.0) {
       this.speed = 0.0; 
     }     
     
  }
  
}

class CarTest4 {

  public static void main(String args[]) {
    
    Car c = new Car();
    
    c.licensePlate = "New York A45 636";
    c.maxSpeed = 123.45;
    
    System.out.println(c.licensePlate + " is moving at " + c.speed + 
     " kilometers per hour.");

    for (int i = 0; i < 15; i++) {
      c.accelerate(10.0);     
      System.out.println(c.licensePlate + " is moving at " + c.speed + 
       " kilometers per hour.");
    }

  }
    
}

utopia% java CarTest4
New York A45 636 is moving at 0.0 kilometers per hour.
New York A45 636 is moving at 10.0 kilometers per hour.
New York A45 636 is moving at 20.0 kilometers per hour.
New York A45 636 is moving at 30.0 kilometers per hour.
New York A45 636 is moving at 40.0 kilometers per hour.
New York A45 636 is moving at 50.0 kilometers per hour.
New York A45 636 is moving at 60.0 kilometers per hour.
New York A45 636 is moving at 70.0 kilometers per hour.
New York A45 636 is moving at 80.0 kilometers per hour.
New York A45 636 is moving at 90.0 kilometers per hour.
New York A45 636 is moving at 100.0 kilometers per hour.
New York A45 636 is moving at 110.0 kilometers per hour.
New York A45 636 is moving at 120.0 kilometers per hour.
New York A45 636 is moving at 123.45 kilometers per hour.
New York A45 636 is moving at 123.45 kilometers per hour.
New York A45 636 is moving at 123.45 kilometers per hour.