primitive type double error

Question

sorry if my question looks very stupid. I get error on .compareTo() Cannot invoke compareTo(double) on the primitive type double! how can i fix this ? Thank you!

Vehicle class:

public class Vehicle implements IOutput {
private double cost;}

public double getCost(){
        return cost;
    }

Array Class:

public static void sortByVehicleMakeModel(Vehicle[] vehicles) {

    boolean swapped = true;

    for(int y = 0; y < vehicles.length && swapped; y++) {
        swapped=false;
        for(int x = 0; x < vehicles.length - (y+1); x++) {
            if(vehicles[x].getCost().compareTo(vehicles[x + 1].getCost()) > 0){
                swap(vehicles, x, x + 1);
                swapped=true;
            }
        }
    }
}

my other codes works fine:

public static void sortByOwnerName(Vehicle[] vehicles) {
    boolean swapped = true;

    for(int y = 0; y < vehicles.length && swapped; y++) {
        swapped=false;
        for(int x = 0; x < vehicles.length - (y + 1); x++) {
            if(vehicles[x].getOwner().getName().compareTo(vehicles[x + 1].getOwner().getName())> 0) {   
                swap(vehicles, x, x + 1);
                swapped=true;
            }
        }
    }
}

Answer 1

Change the return type of your getCost() method from double to Double and it will all work. Auto boxing will take care of the rest.

Answer 2

if(vehicles[x].getCost().compareTo(vehicles[x + 1].getCost()))

您需要在某处>0 ！

Answer 3

compareTo method is not available on premitive type. Use Wrapper Double as:

     if(Double.valueOf(vehicles[x].getCost())
          .compareTo(Double.valueOf(vehicles[x + 1].getCost()))>0){

Please Note: Double.valueOf(double) returns the Wrapper type Double with value as double .

Please Note: If your objective is to use compareTo then its fine otherwise, you may want to directly compare double values using comparison operators <, >, == as appropriate.

Answer 4

You can only invoke methods on a reference type, double is a primitive type. As the error message suggests vehicles[x].getCost() returns a double .

One thing you can do is manually box your double into Double :

int costComp = Double.valueOf(vehicles[x].getCost()).compareTo(Double.valueOf(vehicles[x + 1].getCost());

if(costComp < 0) {
    //...
} else if(costComp == 0) {
    //...
} else {
    //...
}

Answer 5

This code of yours works fine

if(vehicles[x].getOwner().getName().compareTo(vehicles[x+1].getOwner().getName())> 0)

because vehicles[x+1].getOwner().getName() must be returning an object of String and compareTo method accepts an object as an argument.

This code doesn't work

if(vehicles[x].getCost().compareTo(vehicles[x + 1].getCost()))

because vehicles[x + 1].getCost() must not be returning an object( in your case it must be returning a primitive double ) so there is mismatch in the type and the compiler complains that there is no such compareTo method that accepts double (a primitive)

Answer 6

I changed this to :

public Double getCost() 

instead of

public double getCost(){ 
return cost; 
}