Cup.java

public class Cup implements Replenishable {
  private int capacity;
  private int amount;

  public Cup(int c){
    capacity = c;
    amount = 0;
  }

  public void fill(int foo) {
    if foo + amount > capacity {
      amount = 0;
    } else {
      amount += foo;
    }
  }

  public void empty(int foo) {
    if amount - foo >= 0 {
      amount -= foo;
    }
  }

  public String checkAmount(){
    System.out.println(amount);
  }

}

Person.java

public class Person implements Replenishable {
  private int capacity;
  private int amount;

  public Person(int c){
    capacity = c;
    amount = 0;
  }

  public void fill(int foo) {
    if foo + amount > capacity {
      amount = 0;
      System.out.println("*BLARGH* ... I just threw up.");
    } else {
      amount += foo;
      System.out.println("Yum! That tasted good.")
    }
  }

  public void empty(int foo) {
    if amount - foo >= 0 {
      amount -= foo;
      System.out.println("I just pooped.");
    }
  }

  public String checkAmount(){
    if (amount > 7){
      System.out.println("I feel full.");
    } else if (amount > 5) {
      System.out.println("I feel pretty darn satiated.");
    } else if (amount > 3){
      System.out.println("I feel a bit hungry.");
    } else {
      System.out.println("I am starving.");
    }
  }

}

Replenishable.java

interface Replenishable {

  void fill(int amount);
  void empty(int amount);
  String checkAmount();

}