JAVA设计简单的飞机行李托运计费系统。假设飞机上个人托运行李的条件是：行李重量在20公斤以下免费托运；20~30公斤超出部分5元/公斤；30~40公斤超出部分10元/公斤；40~50公斤超出部分15元/公斤；50公斤以上不允许个人携带。

在Java中，我们可以创建一个简单的行李托运计费系统，通过定义一个`BaggageCalculator`类来计算费用。以下是基本的设计步骤：

1. **定义包结构**:
- 创建一个名为`flightutils`的包，用于存放所有相关的类。

2. **创建行李类(Luggage)**:

   public class Luggage {
       private int weight;
   
       // 构造函数、getter和setter方法
       public Luggage(int weight) { this.weight = weight; }
       public int getWeight() { return weight; }
       public void setWeight(int weight) { this.weight = weight; }
   }

3. **创建行李费用计算器(BaggageCalculator)**:

   public class BaggageCalculator {
       private static final int[] PRICE_PER_KILO = {0, 5, 10, 15};
   
       // 计算费用的方法
       public double calculateCharge(Luggage luggage) {
           if (luggage.getWeight() <= 0 || luggage.getWeight() > 50) {
               throw new IllegalArgumentException("Invalid luggage weight");
           }
           int category = (int) Math.min(luggage.getWeight(), 50);
           return luggage.getWeight() * PRICE_PER_KILO[category];
       }
   }

4. **测试代码**:

   public static void main(String[] args) {
       Luggage luggage = new Luggage(25); // 正常行李
       System.out.println("Charges for " + luggage.getWeight() + " kg: " + BaggageCalculator.calculateCharge(luggage));
       
       luggage = new Luggage(55); // 超限行李
       try {
           System.out.println("Charges for " + luggage.getWeight() + " kg: " + BaggageCalculator.calculateCharge(luggage));
       } catch (IllegalArgumentException e) {
           System.out.println(e.getMessage());
       }
   }

在这个例子中，`BaggageCalculator`类有一个静态数组表示每个重量段的价格，然后根据输入的行李重量计算出对应的费用。如果输入的重量不在允许范围内，会抛出异常。