尝试创建 Singleton 时出现 getInstance() 错误

Question

I know there is lots online and I have been looking for the last two hours trying to fix my code.

I am making a immutable class called Car I have seen lots of videos and read articles online.

Even my notes from college talk about creating a Singleton class when make a class Immutable (my notes are rubbish as usual).

Anyway the code below has everything I have seen in videos and read in articles the only difference is in everything I watched and read is their Objects did not have other variables.

public final class Car{
    private static Car carSingleton = null;
    private final String owner;
    private final String reg;
    private final String make;
    private final int kilometres;
    private final double price;



    private Car() {}    


    public static Car getInstance()() {
        if(carSingleton == null) {
            carSingleton = new Car();
        }   
        return carSingleton;
    }



    public String getOwner(){return owner;}
    public String getReg(){return reg;}
    public String getMake(){return make;}
    public int getKilometres(){return kilometres;}
    public double getPrice(){return price;}



}

I am getting a an error in the getInstance() constructor Car() is undefined.

EDIT: Here is the actual question I was given

Question 1 Given below is a mutable Car class. Re-write it so that it becomes immutable and write a simple test program.

class Car{
    private String owner;
    private String reg;
    private String make;
    private int kilometres;
    private double price;
    public Car(String owner, String reg, String make, int kilometres, double price){
            this.owner = owner; this.reg = reg;this.make = make; 
            this.kilometres = kilometres; this.price = price; 
    }
    public String owner(){return owner;}
    public String reg(){return reg;}
    public String make(){return make;}
    public intkilometres(){return kilometres;}
    public double price(){return price;}
    public void setPrice(double p){price = p;}
    public void setOwner(String ow){owner = ow;}
    public void setKil(int k){kilometres = k;}
    public void setMake(String m){make = m;}
}

Answer 1

In your getInstance() you are calling a constructor which is not defined:

carSingleton = new Car();

Define a default constructor to fix the error:

private Car() {
    // initialize final variables
}

or use some other constructor in getInstance()

Answering the EDIT part:

If a type (class) is immutable it only means you can't change its instances once you create them. Only thing you need to do in your assignment is remove all of the set methods and make all class attributes final .

Like you said, your notes are not helping you at all. Singleton should not be used for this purpose.

Answer 2

To make this class immutable remove the setters. It should not be Singleton.

class Car{
    private String owner;
    private String reg;
    private String make;
    private int kilometres;
    private double price;
    public Car(String owner, String reg, String make, int kilometres, double price){
            this.owner = owner; this.reg = reg;this.make = make; 
            this.kilometres = kilometres; this.price = price; 
    }
    public String getOwner(){return owner;}
    public String getReg(){return reg;}
    public String getMake(){return make;}
    public getKilometres(){return kilometres;}
    public double getPrice(){return price;}
}

Answer 3

You can to try with this updates:

private static Car carSingleton  = new Car();

private Car() { 
   System.out.println(“creating”); 
   if (instance != null) { 
      throw new RuntimeException(“already exist!!! “); 
   } 
}

public static Car getInstance() { 
     return carSingleton; 
}