Java“无法取消引用double”错误-double数据类型上的调用方法

Question

I'm trying to use the following method for conversion between temperature scales in Java. Problem is when I call the method I get a "double cannot be dereferenced" error.

I understand that the reason is that methods cannot be called on primitive data types, and I think the solution is to use a wrapper but I'm not sure what that code would look like (my initial attempts at using Double wrapper all produced errors).

public double celsius(double f) {        
    return (f - 32.0)/1.8; 
}

public double fahrenheit(double c) {        
    return c * 1.8 + 32.0;
}

public void doConversion() {
    double tempC = 0.0;
    double tempF = tempC.fahrenheit; // double cannot be dereferenced error
}

Answer 1

Instead of

double tempF = tempC.fahrenheit;

use

double tempF = fahrenheit(tempC);

Answer 2

Do something like this:

public class temperature{
    public double T; //in C

    public temperature(double T){
        this.T = T;
    }

    public double getC(){
        return T;
    }

    public double getF(){
        return T * 1.8 + 32.0;
    }
}

Answer 3

You already have the methods defined in you class. While doing conversion, you just have to call those methods like below:

public void doConversion() {
    double tempC = 0.0;
    double tempF = fahrenheit(tempC);
}

Hope this helps!

Answer 4

For your code to work, you need to:

public void doConversion() {
    double tempC = 0.0;
    double tempF = fahrenheit(tempC);  // call your function
}

The reason double tempF = tempC.fahrenheit; is a problem is that the . (ie dot) is meant to allow classes and objects to access their member fields and methods.

A variable of type double is a primitive type; it's not a class or an object, so the dot operator cannot be used with them.

The wording in the messaging is because classes and their instances (ie objects) are always references . In other words, if you do something like:

List<DisplayMode> displayModes = new ArrayList<>();

the variable displayModes holds a reference (ie points to) the list, but is not the list itself. You can then do something like displayModes.add(new DisplayMode(...));

Notice you need to use the new operator with classes, but not with primitives.

On the other hand, if you write:

int x = 5;

the variable x is the value itself.

If you really want to use the dot operator, then consider something like:

Temperature temp = new Celsius(0.0);
System.out.println(temp.toFahrenheit().toString());

Assuming that you have an interface called Temperature with at least a method called toFahrenheit() and concrete classes Celsius and Fahrenheit implementing said interface.

public interface Temperature {
    Temperature toFahrenheiht();
}

public class Celsius implements Temperature {
    // ...
}