Unable to swap out double for void as errors follow

I apologize if the title is extremely vague however, i'm not entirely sure how else to word it. Every time I swap out double for void, I get an error in which tells me that my program's exit value is incorrect. It's all on CodeLab as well so everything has to be clear and precise.

My assignment is to:
Write a class named GasTank containing:
An Instance variable named amount of type double, initialized to 0.
A method named addGas that accepts a parameter of type double. The value of the amount instance variable is increased by the value of the parameter.
A method named useGas that accepts a parameter of type double. The value of the amount is decreased by the value of the parameter. However, if the value of amount is decreased below 0, amount is set to 0.
A method named isEmpty that accepts no parameters. isEmpty returns a boolean value: true if the value of amount is less than 0.1, and false otherwise.
A method named getGasLevel that accepts no parameters. getGasLevel returns the value of the amount instance variable.

public class GasTank {
    private double amount=0;

    public double addGas(double addg){
        amount=amount+addg;
        if(amount>0.1){
            amount=0;

        }

    }

    public void useGas(double useg){
        amount=amount-useg;
        if(amount<0){
            amount=0;
        }
    }

    public boolean isEmpty(){
        if(amount<0.1){
            return true;
        }
        return false;
    }

    public double getGasLevel(){
        return amount;
    }
}

Error

Let's just debug your code: We go through it step by step, figure out what is supposed to happen. Doublecheck your work by running the code. Use a debugger to witness what the computer is doing. If you can't be bothered to learn how to use one, well, println statements are a crappy standin and works well enough for beginning programmers:

public double addGas(double addg){
        amount=amount+addg;
        if(amount>0.1){
            amount=0;

        }

    }

So, we take the amount field, add the gas to it as specified..

and then if amount is above 0.1, we set it back down to 0.

So, as the image shows, the checker code adds 78 gallons, and as that's more than 0.1, your code will set amount to 78.0 and then IMMEDIATELY sets it back down to 0.0. I have no idea why you added that line, but computer just does what computer is told. And what you told it to do fails the assignment. the field should have been 78.0 afterwards.

I have absolutely no idea what you're on about with:

> Unable to swap out double for void as errors follow

But the whole 'I reset amount down to 0 if you try to add more than a tenth of a gallon' thing is the only problem with your code.

I fixed it for you. Check the notes in the comments

public class GasTank {
   private double amount=0;
   private double v;
   
   //Constructor
   public GasTank(double a){
      v = a;
   }
      
    //Changed to void, we aren't returning a double val  
   public void addGas(double addg){
       amount=amount+addg;
      
       //changed your if statement
      if(amount>v){
         amount = v;
      }
   }

   public void useGas(double useg){
      amount=amount-useg;
      if(amount<0){
         amount=0;
      }
   }

   public boolean isEmpty(){
      if(amount<0.1){
         return true;
      }
        //added else statement here
      else{
         return false;
      }
   }

   public double getGasLevel(){
      return amount;
   }
}