空超載構造

Question

我有下面的類。

class Student(id: String, name: String) { 

    var id: String? = null 
    var name: String? = null 
    var grade: String? = null 

    constructor(id: String, name: String, grade: String) : this(id,name) { 
     this.grade = grade 
    } 
} 

用途：

var student = Student("AB001","Smith","N/A") 

    prinln(student.id + student.name + student.grade) 

輸出：

nullnullN/A

任何人都可以解釋爲什麼我從默認的構造函數的輸出空？

Answer 1

除了其他答案，還有另外一種方法。初始化直接使用主構造函數的參數屬性：

class Student(id: String, name: String) { 

    var id: String? = id 
    var name: String? = name 
    var grade: String? = null 

    constructor(id: String, name: String, grade: String) : this(id,name) { 
     this.grade = grade 
    } 
} 

注意，因爲id和name總是與非空值初始化，您可以省略?。除此之外，你可以在主構造使用默認值忽略次要構造：

class Student(id: String, name: String, grade: String? = null) { 

    var id: String = id 
    var name: String = name 
    var grade: String? = grade 

} 

但是現在我們只剩下一個構造函數，所以我們可以直接拉動的屬性到構造：

class Student(
     var id: String, 
     var name: String, 
     var grade: String? = null 
) 

因爲類的身體現在是空的，我也省略了大括號。

Answer 2

class Student { 
    var id: String? = null 
    var name: String? = null 
    var grade: String? = null 

    constructor(id: String, name: String) { 
     this.id=id 
     this.name=name 
    } 

    constructor(id: String, name: String, grade: String) : this(id, name) { 
     this.grade = grade 
    } 
} 

或

class Student(var id: String?, var name: String?) { 
    var grade: String? = null 

    constructor(id: String, name: String, grade: String) : this(id, name) { 
     this.grade = grade 
    } 
} 

Answer 3

或者

class Student(id: String, name: String) { 
    var id: String? = null 
    var name: String? = null 
    var grade: String? = null 

    init { 
     this.id = id 
     this.name = name 
    } 

    constructor(id: String, name: String, grade: String) : this(id,name) { 
     this.grade = grade 
    } 
} 

初始化模塊是你如何在主構造，當他們沒有標記爲丘壑或增值經銷商必須做使用參數那裏。

換言之，由於您沒有將主要構造函數中的參數（類名旁邊的括號）標記爲val或vars，因此它們不會自動指定爲屬性。爲了在主構造函數中使用參數，特別是那些沒有被標記爲變量和val的參數，你需要一個init塊。

我不禁想，你真正想要的是這雖然：

class Student (var id: String, var name: String, var grade: String? = null) 

甚至其更改爲丘壑，而不是增值經銷商。