需要重新审视我的 Java 代码

Question

I am hoping the community can provide me with a fresh set of eyes, as i have gone a bit code blind,

I have attached my code from both the classes that I have to connect together for a udemy tutorial.

As I compiled them, the usual errors came up, mis-spelled variables and missing semi colons. It also through up an unexpected type, which was solved by changing a int to a String.

The problem I have is on line 25, the error it gives me is

incompatible types, There was an expression of a certain type required here. You provided an expression of a different type hat is not compatible. (eg you wrote a String where an int was expected.)

But variable being called is declared a string as far as I can see in all instances.

I wrote it in intelliJ and used that to generate the getter/setter methods so these should all be correct, and I just can't for the life of me see where the error is coming from.

I know it will be something simple but just can't see the woods for the trees.

Car class.

public class Car
{
   // instance variables
   private int numOfMilesDone; // a car has-a number of miles drive, "20000"
   private int yearBought; // a car has-a year it was bought "1997"
   private int carValue;  // a car has-a value of what it is worth "300"
   private Model modelName; // a car has-an model of type Model

   /**
    * Constructor for objects of class Car
    */

   public Car(int aNumMiles, int aYearBought, int aValue, Model aModelName)
   {
      this.numOfMilesDone = aNumMiles;
      this.yearBought = aYearBought;
      this.carValue = aValue;
      this.modelName = aModelName;
   }

   public Model getModelName()
   {
      if (this.modelName == null || this.getModelName() == null )
      {
         return ("needs to be checked");
      }

      return modelName;
   }

   /**
    * Getter method for number of miles the car has done
    * @return
    */

   public int getNumOfMilesDone()
   {
      return numOfMilesDone;
   }

   /**
    * Setter method for number of miles the car has done
    * @return
    */

   public void setNumOfMilesDone(int numOfMilesDone)
   {
      this.numOfMilesDone = numOfMilesDone;
   }

   /**
    * Getter method for the year the car was bought
    * @return
    */

   public int getYearBought()
   {
      if (this.yearBought == null)
      {
         return "Needs to be checked";
      }

      return yearBought;
   }

   /**
    * Setter method for the year the car was bought
    * @return
    */

   public void setYearBought(int yearBought)
   {
      this.yearBought = yearBought;
   }

   /**
    * Getter method for the year the cars value in pounds
    * @return
    */

   public int getCarValue()
   {
      return carValue;
   }

   /**
    * Setter method for the year the cars value in pounds
    * @return
    */

   public void setCarValue(int carValue) {
      this.carValue = carValue;
   }

   public boolean isClassic()
   {
      return(Integer.parseInt(this.modelName.getYearOfModel()) < 1969);
   }

   /**
    * returns the a String describing the object
    * @return
    */

   public String toSting()
   {
      return this.getModelName() + " has done " + this.numOfMilesDone + ", it is worth " + this.carValue + ", it is " 
              + this.isClassic() + " it's a classic.";
   }

}

My other class, Model, this compiles no problem.

public class Model
{
    private String modelName; // the model has a model name
    private String yearOfModel; // the year the model was created


    /**
     * constructor method for no model attributes
     */

    public Model()
    {
        this.modelName = null;
        this.yearOfModel = null;
    }


    /**
     * constructor method for known modelName attribute, but no yearOfModel attribute
     * @param bModelName
     */

    public Model(String bModelName)
    {
        this.modelName = bModelName;
        this.yearOfModel = null;
    }

    /**
     * constructor method for known modelName attribute, and known yearOfModel attribute
     * @param bModelName
     * @param yearOfModel
     */

    public Model(String bModelName, String yearOfModel)
    {
        this.modelName = bModelName;
        this.yearOfModel = yearOfModel;
    }

    /**
     * modelName getter method
     * @return
     */

    public String getModelName() {
        return modelName;
    }

    /**
     * modelName setter method
     * @param modelName
     */

    public void setModelName(String modelName) {
        this.modelName = modelName;
    }

    /**
     * yearOfModel setter method
     * @return
     */

    public String getYearOfModel() {
        return yearOfModel;
    }

    /**
     * yearOfModel setter method
     * @param yearOfModel
     */

    public void setYearOfModel(String yearOfModel) {
        this.yearOfModel = yearOfModel;
    }

    /**
     * returns the modelName and yearOfModel variables as comprehensible information.
     * @return
     */

    public String toString()
    {
        return this.modelName + " was launched in " + this.yearOfModel;
    }
}

Thanks in advance for your help.

Answer 1

return "Needs to be checked"当您的方法签名建议是ModelName时，您将返回一个字符串。

Answer 2

You have a problem because you use C like style, when return value is used to detect a problem. In this given case, you cannot use it, because return type and string error message are not the same (eg String.indexOf() returns either position or -1 if not found).

In yous situation it's better to throw NullPointerException with the message.

public Model getModelName() {
    Objects.requireNonNull(modelName, "needs to be checked");
    return modelName;
}

public int getYearBought() {
    Objects.requireNonNull(yearBought, "Needs to be checked");
    return yearBought;
}

---

This is not an answer your question, but I think you have another problem with your code. Below a few comments.

// it's better to check value when set it, but not when get (class instance should always contains correct value, this is plain old dto)
// do hot use useless JavaDoc: make code self documented
class Car {

    private int numOfMilesDone; // a car has-a number of miles drive, "20000"
    private int yearBought; // a car has-a year it was bought "1997"
    private int value;  // a car has-a value of what it is worth "300"
    private final Model model; // a car has-an model of type Model

    // the name of method's parameters and local ones usually the same (use this for local ones)
    public Car(int numOfMilesDone, int yearBought, int value, Model model) {
        setNumOfMilesDone(numOfMilesDone);
        setYearBought(yearBought);
        setValue(value);
        // use null object instead of null
        this.model = model != null ? model : Model.NULL;
    }

    public Model getModel() {
        return model;
    }

    public int getNumOfMilesDone() {
        return numOfMilesDone;
    }

    public void setNumOfMilesDone(int numOfMilesDone) {
        this.numOfMilesDone = Math.max(0, numOfMilesDone);
    }

    public int getYearBought() {
        return yearBought;
    }

    public void setYearBought(int yearBought) {
        this.yearBought = Math.max(0, yearBought);
    }

    public int getValue() {
        return value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    @Override
    public String toString() {
        return model + " has been done " + numOfMilesDone + ", it is worth " + value + ", it is " + model.isClassic() + " it's a classic.";
    }
}

// you retrieve instance from `Car` class. It is better to make `Model` immutable and do not worry about encapsulation
final class Model {

    public static final Model NULL = new Model(null, null);

    // no need to use `Mode` in the name of internal properties
    private final String name;
    // usually this is integer, not a string
    private final int year;

    public Model(String name) {
        this(name, 0);
    }

    public Model(String name, int year) {
        this.name = name;
        this.year = Math.max(0, year);
    }

    public String getName() {
        return name;
    }

    public int getYear() {
        return year;
    }

    // this method belongs to the Model, but not to a Car
    public boolean isClassic() {
        return this != NULL && year < 1969;
    }

    @Override
    public String toString() {
        return name + " was launched in " + year;
    }
}

Answer 3

You need to correct two of the methods getYearBought() and getModelName() . Both of them are returning strings whereas getYearBought() expects an int to be returned and getModelName() expects an object of Model class to be returned

Answer 4

A few issues I can see. Your method getModelName() calls itself this.getModelName()

Also it returns the class Model and modelName is of type Model . But then in you if statement you return a String and not the class Model

if (this.modelName == null || this.getModelName() == null ) {
    return ("needs to be checked"); // This needs to return a Model
}

Answer 5

There are a couple of things wrong that jump out:

You have errors in at least two places:

In getModelName() you have two problems:

1) You intend to return a Model but return a String instead in error cases.

2) You make a recursive call to getModelName() without a terminating condition.

 public Model getModelName()
 {
  if (this.modelName == null || this.getModelName() == null )
  {
     // NOTE: This is a String!
     return ("needs to be checked");
  }

  return modelName;
 }

This could be rewritten like this:

public Model getModelName() {
    return modelName;
}

It would return null when modelName is null, and a non-null Model otherwise.

Also, in getYearBought() , you do the same thing - return a String when you intend to return an int .

public int getYearBought() { if (this.yearBought == null) { // NOTE: This returns String return "Needs to be checked"; }

  return yearBought;

}

Checking that yearBought is null isn't required, this is an int and it can't be null. You can rewrite it like this:

public int getYearBought() {
    return yearBought;
}