toString方法打印空变量，无法理解为什么（Java）

Question

I haven't found any help on here so I'm gonna just ask.

public  Ride(String n, int tr, double hr , int nr){
    String name = n;
    int ticketsRequired = tr;
    double heightRequirement = hr;
    int numberOfRiders = nr;
}

public String toString(){

    String info;

    info = this.name + " Requiring " + this.ticketsRequired + 
            " tickets with a height restriction of " +       heightRequirement;

    return info;
}

That is in once class, then i try to call it in a main method in another program within my package via this:

r1 = new Ride("Death run",0, 5.2, 5);
System.out.println( r1.toString());

Which will return this

null Requiring 0 tickets with a height restriction of 0.0

I'm sure that I'm missing something simple but this is driving me insane.

Answer 1

You're shadowing your instance variables in your constructor :

public Ride(String n, int tr, double hr , int nr){
    name = n;
    ticketsRequired = tr;
    heightRequirement = hr;
    numberOfRiders = nr;
}

Using this with a field in the constructor is not required here, but it is a good pratice as they are often shadowed by the constructor's parameters (see using the this keyword ).

Here's a version using the this keyword:

public Ride(String n, int tr, double hr , int nr){
    this.name = n;
    this.ticketsRequired = tr;
    this.heightRequirement = hr;
    this.numberOfRiders = nr;
}

Answer 2

You are treating the variables like if they were instance variables, however you have declared them locally inside the constructor. Therefore, once it gets out of the scope you declared them in (the constructor), the value of the String object will be null and the value of the int and double variables will be 0 (since it's the default value).

Answer 3

I think this is how you want your class definition:

public class Ride
{
  String name;
  int ticketsRequired;
  double heightRequirement;
  int numberOfRiders;

  public  Ride(String n, int tr, double hr , int nr){
    name = n;
    ticketsRequired = tr;
    heightRequirement = hr;
    numberOfRiders = nr;
  }

  public String toString(){

    String info;

    info = this.name + " Requiring " + this.ticketsRequired + 
        " tickets with a height restriction of " + this.heightRequirement;

    return info;
  }
}

EDIT: fixed a minor problem in the code snippet