爪哇，無法找到符號和麻煩從數組列表中提取dataType

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（）給我錯誤。這裏是包類：爪哇，無法找到符號和麻煩從數組列表中提取dataType

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; 
    } 
}

toString方法觸發從主要方法的調用正常，但是，我不能從包類訪問任何其他方法。構造函數在將數據包添加到包含所有數據包的Arraylist時工作。我試圖在displayLightPackages中定義一個數據包，但是我收到了一個不兼容的類型錯誤。在displayLightPackages方法中，當我使用代碼

shipmentList.get(i).isLight()

我在編譯器找不到符號isLight時出現錯誤。我懷疑這個錯誤與arraylist中的dataType有關，但除此之外，我現在處於虧損狀態。

謝謝

來源

2014-01-25 user451711

你ArrayList沒有類型的，所以一切從它的到來是一個Object。相反，Object沒有isLight()方法，因此「無法找到符號」錯誤。

爲了解決這個問題，無論是鍵入ArrayList ...

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

...或轉換爲Packet當您使用get：

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

兩個選項中，鍵入ArrayList是更簡單和更安全，因爲每當你從中獲得價值時，你都不必記得投。

來源

2014-01-25 20:28:18 Makoto

哇，謝謝你。徹底解決了我的問題，我學到了一些東西！再次感謝。 – user451711

變化

private ArrayList shipmentList = new ArrayList();

到

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

編譯器不知道shipmentList持有包。事實上，編譯器知道shipmentList是一個Objects的列表。所以你可以做shipmentList.get(i).toString()，因爲toString()是Object類的方法。但是您不能在object上調用isLight()方法，因爲它不存在於Object上下文中。你必須明確地說，shippingList是ArrayList<Packet>的類型，或者將元素拋到Packet：Packet thisShipment = (Packet) shipmentList.get(i);

來源

2014-01-25 20:30:50 MGorgon

爪哇，無法找到符號和麻煩從數組列表中提取dataType

回答

相關問題