Java参数化的构造函数

Question

I'm new to programming and also new to Java. What i'm trying right now is using a parameterized constructor, and then use the created objects with a specific method. This is the code i have:

public class Car {
int fuelcap;
int mpg;

Car(int f, int m) {      //here
    fuelcap = f;         //and here
    mpg = m;             //and here
}

int range() {
    return mpg * fuelcap;
}

public static void main(String[] args) {
    Car sedan = new Car(16, 21);
    Car sportscar = new Car(14, 16);
    System.out.println(sedan.range());
    System.out.println(sportscar.range());

}

}

The problem is, i don't know why the parameters of the constructor Car - 'int f' and 'int m' are different from the fields: 'int fuelcap;' 'int mpg;'. Can't we just create are constructor like this:

Car(int fuelcap, int mpg){
}

and then just pass values to those parameters when creating the objects?

Answer 1

No, you cannot. You must assign the constructor parameters to the class fields in the body of your constructor, or they will not be saved. That's just how Java works. There's nothing "magical" about the constructor parameters.

Also, observe proper style:

Car(int fulecap, int mpg) {     
    this.fuelcap = fuelcap;         
    this.mpg = mpg;            
}

If you have more fields then ctor parameters, that's just fine. Just think of the constructor as a normal method that gets called after the object is created. (That last statement is only sorta-kinda true, but will give you the correct idea for this purpose.)

Answer 2

Yes off course you can, but in that case your constructor should look like as follows:

public Car(int fuelcap, int mpg){
 this.fuelcap = fuelcap;
 this.mpg = mpg;
}

Answer 3

You have the instance variables fuelcap and mpg but if you do the constructor you mention, they will never be assigned to anything.

You could do this to make it more readable,

int fuelcap;
int mpg;

Car(int fuelcap, int mpg) { 
    this.fuelcap = fuelcap; //this is refrencing the instance variable in the class    
    this.mpg = mpg;             
}

int range() {
    return mpg * fuelcap;
}

The constructor just doesn't recognize the way you want because it can cause errors in variables with the same names.

Answer 4

Parameterized constructor is used to provide different values to the distinct objects.

if you want to use

Car(int fuelcap, int mpg){}

you must use this keyword

so the code becomes

Car(int f, int m) {      
this.fuelcap = f;         
this.mpg = m; }

If you dont use this keyword

all objects will have the same values as in the instance variables ie, fuelcap and mpg right now its value is 0

Answer 5

Just a few hints considering good practice:

First you should make your members private ( link ). Moreover, it is good style to make your objets "immutable", meaning, once they are created, they cannot change the state ( link ). You can achieve this by declaring your fields as final .

The Car class would then look like this:

public class Car
{
  private final int fuelcap;

  private final int mpg;


  public Car( final int fuelcap, final int mpg )
  {
    this.fuelcap = fuelcap;
    this.mpg = mpg;
  }


  public int range()
  {
    return this.mpg * this.fuelcap;
  }
}