The operator > is undefined for the argument type(s) boolean, double

问题

I am a new programmer, so sorry if this is really basic. I have looked around for this website for an answer, I could find very similar questions, but none was what I needed.

import java.util.Scanner;

public class sortThreeIntegers 
{
    public static void main (String[] args) 
        {
        Scanner input = new Scanner(System.in);
        Scanner input2 = new Scanner(System.in);
        Scanner input3 = new Scanner(System.in);
        System.out.println("Enter the first number: ");
        System.out.println("Enter the second number: ");
        System.out.println("Enter the third number: ");

        double firstNumber = input.nextDouble();
        double secondNumber = input2.nextDouble();
        double thirdNumber = input3.nextDouble();

        if (firstNumber > secondNumber > thirdNumber == true) 
        {
            System.out.println(firstNumber + ", " + secondNumber + ", " + thirdNumber);
        }
        else if (firstNumber > thirdNumber > secondNumber == true) 
        {
            System.out.println(firstNumber + ", " + thirdNumber + ", " + secondNumber);
        }
    }
}

Note: This is in the middle of me writing the code, I am not finished with this.

Why am I getting the error "The operator > is undefined for the argument type(s) boolean, double"? Again, sorry if this is really simple.

回答1:

if (firstNumber > secondNumber > thirdNumber == true)

This is invalid; you want:

if (firstNumber > secondNumber && secondNumber > thirdNumber)

The same goes for your else if condition.

You cannot chain comparisons in Java like you can in, say, Python. Also, there is never a need for an == true in boolean expressions. After all, x == true will always be x.

The error you received makes sense, since your expression is being evaluated as:

(firstNumber > secondNumber) > thirdNumber

Now (firstNumber > secondNumber) is a boolean, which cannot be compared to a double (thirdNumber).

Finally, it looks like you're just trying to output the numbers in sorted order. Why not just sort them regularly, then output the result? Also: don't use three different scanners! Just use the same one three times.

This is what I mean by sorting the numbers:

double numbers[] = {firstNumber, secondNumber, thirdNumber};
Arrays.sort(numbers);
System.out.println(numbers[2] + ", " + numbers[1] + ", " + numbers[0]);

回答2:

You cannot combine operators such as > as we would do as a shortcut in expressing a mathematical inequality. You must list out such expression explicitly and join the expressions with the && and operator.

if (firstNumber > secondNumber && secondNumber > thirdNumber) {

Also, I took out the unnecessary comparison to true, because the result of the > (and other comparison operators) is already a boolean.

You can change your else if condition similarly.

回答3:

You can't have this : if (firstNumber > secondNumber > thirdNumber == true).

You need to break it down to:

if (firstNumber > secondNumber && secondNumber > thirdNumber) {

来源：https://stackoverflow.com/questions/22305632/the-operator-is-undefined-for-the-argument-types-boolean-double

标签

java

boolean-operations