爪哇 - 對象存在DILEM

Question

我寫了下面，這是在同一個包市級一類國家的一個toString和_cities是一個數組代表我的國家範圍內的城市： ** 編輯： * *

public String toString(){ 

    String allCitiesData = ""; //must be initialized 

    for(int i=0;this._cities[i] != null;i++)//run all over the cities until reach the end(null cell) 
    { //concat new city to the string and adds a new line between 
     allCitiesData = allCitiesData.concat(this._cities[i].toString()+"\n\n"); 
    } 
    return allCitiesData; 
}//toString method 

public String citiesNorthOf(String cityName){ 
    String allCitiesNorthOf = "";// must be initialized 

    for(int i=0; this._cities[i] != null ; i++) 
    { 
     if (this._cities[i].getCityName() == cityName) 
     { 
      Point referenceCityCenter = new Point(this._cities[i].getCityCenter()); 
     } 
    } 

    for(int i=0; this._cities[i] != null ; i++)//we don't need to exclude the comparable city itself because it will give a false 
    { 
     if (this._cities[i].getCityCenter().isAbove(referenceCityCenter)) 
     { 
      allCitiesNorthOf = allCitiesNorthOf.concat(this._cities[i].toString()+"\n\n"); 
     } 
    } 
}//citiesNorthOf method 

但是，當我運行它，它僅顯示在這一行的單個錯誤：

if (this._cities[i].getCityCenter().isAbove(referenceCityCenter)) 

和Eclipse說：「referenceCityCenter不能解析到VARI能夠「..任何建議？

謝謝！

Answer 1

referenceCityCenter超出範圍。把它放在你的if聲明之外，並確保您檢查null事後像如下：

public String citiesNorthOf(String cityName){ 
String allCitiesNorthOf = "";// must be initialized 

Point referenceCityCenter = null; 

for(int i=0; this._cities[i] != null ; i++) 
{ 
    if (this._cities[i].getCityName() == cityName) 
    { 
     referenceCityCenter = new Point(this._cities[i].getCityCenter()); 
    } 
} 

for(int i=0; this._cities[i] != null ; i++)//we don't need to exclude the comparable city itself because it will give a false 
{ 
    if (referenceCityCenter !- null && this._cities[i].getCityCenter().isAbove(referenceCityCenter)) 
    { 
     allCitiesNorthOf = allCitiesNorthOf.concat(this._cities[i].toString()+"\n\n"); 
    } 
} 
} 

Answer 2

您已聲明referenceCityCenter在您的代碼的該行不可見的範圍內。嘗試在方法開始時聲明它（並且如果它是null，那麼當它到達您的驗證.isAbove()！時：P）