Java，找不到符号并且无法从数组列表中提取数据类型

Question

    public class Packages 
    {
    private ArrayList shipmentList = new ArrayList();
    private double totalWeight;

    public Packages(String shiplist) throws IOException
    {
        Scanner scanFile = new Scanner(new File(shiplist));
        scanFile.useDelimiter("\n");
        while(scanFile.hasNext() == true){
            String pack = scanFile.next();
            Scanner splitter = new Scanner(pack);
            splitter.useDelimiter(" ");
            int id = splitter.nextInt();
            double weight = splitter.nextDouble();
            String state = splitter.next();
            shipmentList.add(new Packet(id, weight, state));
        }
        this.totalWeight = 0;

    }

    public String toString()
    {String allShipments ="";
    for(int i =0;i<shipmentList.size();i++)
        allShipments += shipmentList.get(i) + "\n";
    return allShipments;
    }
    //needs work. Cannot find method isLight() from class packet, yet toString() fires fine.
   public void displayLightPackages()
   {
    for (int i=0;i<shipmentList.size();i++){
       Packet thisShipment = shipmentList.get(i);
        if(thisShipment.isLight() == true){
            System.out.println("Package: "+ shipmentList.get(i).toString()+" Is light.");
        }else{
            System.out.print("");
        }
    }
}

方法 displayLightPackages() 给我错误。这是数据包类:

public class Packet
{
    // instance variables - replace the example below with your own
    private int idNumber;
    private double weight;
    private String state;

    public Packet(int idNumber, double weight, String state)
    {
        this.idNumber= idNumber;
        this.weight = weight;
        this.state = state;
    }
    public boolean isHeavy()
    {
        return (weight>10);
    }  
    public boolean isLight()
    {
      return (weight<10);
    }  
    public String toString()
    {
       return "ID: "+idNumber+" WEIGHT: "+weight+" STATE: "+state;
    }
}

方法 toString 在从 main 方法调用时可以正常触发，但是，我无法访问 Packet 类中的任何其他方法。构造函数在将数据包添加到包含所有数据包的 Arraylist 时起作用。我尝试用

在 displayLightPackages 中定义一个数据包

Packet thisShipment = shipmentList.get(i);

但是我得到一个不兼容的类型错误。刚用代码的时候

shipmentList.get(i).isLight()

在 displayLightPackages 方法中，我收到编译器无法找到符号 isLight 的错误。我怀疑该错误与数组列表中的数据类型有关，但除此之外，此时我不知所措。

谢谢

Answer 1

您的 ArrayList 没有类型化，因此来自它的所有内容都是一个 Object。相反，Object 没有 isLight() 方法，因此会出现“找不到符号”错误。

要解决此问题，请键入您的 ArrayList...

private ArrayList<Packet> shipmentList = new ArrayList<>();

... 或在使用 get 时转换为 Packet:

((Packet)shipmentList.get(i)).isLight()

在这两个选项中，键入 ArrayList 更简单、更安全，因为您不必每次从中获取值时都记得进行转换。