android - 空重载构造函数

Question

我有以下类(class)。

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
    }
}

使用:

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

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

输出:

nullnullN/A

谁能解释为什么我从默认构造函数得到输出 null？

Answer 1

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
    }
}