title: Unable to swap out double for void as errors follow
body: <p>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.</p>
  <p>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.</p>
  <pre><code>public class GasTank {
      private double amount=0;
  
      public double addGas(double addg){
          amount=amount+addg;
          if(amount&gt;0.1){
              amount=0;
  
          }
  
      }
  
      public void useGas(double useg){
          amount=amount-useg;
          if(amount&lt;0){
              amount=0;
          }
      }
  
      public boolean isEmpty(){
          if(amount&lt;0.1){
              return true;
          }
          return false;
      }
  
      public double getGasLevel(){
          return amount;
      }
  }
  </code></pre>
  <p><a href="https://i.stack.imgur.com/zvyLk.png" rel="nofollow noreferrer">Error</a></p>