title: Why is my method undefined for this type?
body: <p>I'm not sure why Eclipse is giving me this error, I cannot run the exchange method for any of my desired classes: </p>
  
  <blockquote>
    <p>The method exchange(Currency, double) is undefined for the type Currency</p>
  </blockquote>
  
  <p>Some of the code methods listed have not been fully implemented due to my issues compiling to begin with.</p>
  
  <p>What simple mistake am I making?</p>
  
  <pre><code>public abstract class Currency {
    String currencyName;
    double totalFunds;
  
    //Exchange money between planets
    public abstract double toEarthDollars(double amount);
  
    public abstract double fromEarthDollars(double EarthDollars); 
  
    public static void main(String[] args) {
      //TEST INPUT CODE
      Currency mars = new Mars(100.00);
      Currency neptune = new Neptune(100.00);
      Currency saturn = new Saturn(100.00);
  
      System.out.println("----- Exchanges -----");
  
      mars.exchange(saturn,25.0);
      neptune.exchange(saturn, 10.0);
      saturn.exchange(mars, 122.0);
      saturn.exchange(mars, 121.0);
    }
  } 
  
  
  public interface Exchangeable {
    //rates should be encapsulated and accessed from here
    double EarthDollar = 1;
    double MarsMoney = 1.3;
    double SaturnSilver = 0.87;
    double NeptuneNuggets = 2;
  
    public void exchange(Exchangeable other, double amount);
  }
  
  
  public class Mars extends Currency implements Exchangeable {
    public Mars(double amount) {
      currencyName = "MarsMoney";
      totalFunds = amount;
    }
  
    public double toEarthDollars(double amount) {
      return amount * (Exchangeable.EarthDollar/Exchangeable.MarsMoney);
    }
  
    public double fromEarthDollars(double EarthDollars) {
      return EarthDollars * (Exchangeable.MarsMoney/Exchangeable.EarthDollar);
    }
  
    public void exchange(Exchangeable other, double amount) { 
      System.out.println("Hello");
      //double transfer = this.toEarthDollars(amount);
      //transfer = ((Mars) other).fromEarthDollars(transfer);
    }
  }
  </code></pre>