關於if語句的詢問

Question

public String toString() { 

String details = number + ", " + RACE_DESCRIPTIONS[typeIndex] + "[" + date + "]" + ":"; 
if (!hasFinished) { 
    details += "Race not finished"; 
} else if (hasFinished = true) { 
    details += "\n  1st: " + competitors[0].getName(); 
    details += "\n  2nd: " + competitors[1].getName(); 
    details += "\n  3rd: " + competitors[2].getName(); 
} else if { 
    details += "n/a"; 
} 
return details; 

如果沒有設置位置，則應該返回字符串字面值「n/a」而不是人名。 - 我試圖編寫一個IF語句，如果第一，第二和第三不正確，並且返回n/a。

Answer 1

你永遠不會得到details += "n/a"的原因有兩個執行：

1. 你在if分配hasFinished到true（其他人點）：

   else if (hasFinished = true) 
   

   應該

   else if (hasFinished) 
   

2. 比這更重要的，你的代碼有以下形式：

   if (false) { 
       ... 
   } else { 
       if (true) { 
       } else { 
        //this is false, it was supposed to be hitted before 
       } 
   } 
   

所以，即使你做的第一個定位，背後的邏輯沒有任何意義。您應該在第二個if中使用另一個評估來處理最後的else聲明。例如：

if (!hasFinished) { 
    ... 
} else { 
    //the hasFinished usage here has no sense, being commented 
    //just left it to show this is supported 
    //if (hasFinished && someValue <= someOtherValue) { 
    if (someValue <= someOtherValue) { 
     ... 
    } else { 
     ... 
    } 
} 

Answer 2

=用於賦值，==用於比較。在下面的語句使用==，而不是=：

else if (hasFinished = true) 

這實際上賦予hasFinished值爲true，也是條件將始終評估爲true。將其更改爲：

else if (hasFinished == true) 

做了對比，而不是分配

Answer 3

不要使用=運營商的比較boolean值 - 這是賦值運算符，所以它總是true。它已經boolean，只需使用：

} else if (hasFinished) { 

此外，無論是它的false或者它true，所以沒有必要對第三種情況。您可以刪除

else if { 
    details += "n/a"; 
} 

無論如何，這在語法上是不正確的。

Answer 4

基本上，你需要檢查你是否真的有三個終結者，這就構成了你的else-if子句。

public String toString() 
{ 
    String details = number + ", " + RACE_DESCRIPTIONS[typeIndex] + "[" + date + "]" + ":"; 
    if(!hasFinished) // The race has not yet finished. 
    { 
     details += "Race not finished"; 
    } 
    else if(competitors.length >= 3) // Implies that hasFinished evaluated to true 
    { 
     details += "\n  1st: " + competitors[0].getName(); 
     details += "\n  2nd: " + competitors[1].getName(); 
     details += "\n  3rd: " + competitors[2].getName(); 
    } 
    else // The race has finished, but there are less than three competitors 
    { 
     details += "n/a"; 
    } 
    return details; 
} 

如果你想允許少於三個競爭對手，但仍然顯示第一/二/三，你可以使用一個三態比較，並與其他人取代你的別的，如果條款。

public String toString() 
{ 
    String details = number + ", " + RACE_DESCRIPTIONS[typeIndex] + "[" + date + "]" + ":"; 
    if(!hasFinished) 
    { 
     details += "Race not finished"; 
    } 
    else // Implies that hasFinished evaluated to true 
    { 
     details += "\n  1st: " + (competitors.length >= 1 ? competitors[0].getName() : "n/a"); 
     details += "\n  2nd: " + (competitors.length >= 2 ? competitors[1].getName() : "n/a"); 
     details += "\n  3rd: " + (competitors.length >= 3 ? competitors[2].getName() : "n/a"); 
    } 
    return details; 
}