无参数构造函数的目的是什么? [英] What is the purpose of a no-arg constructor?
问题描述
我了解下面的大多数代码.我只是不了解Employee和Person的无参数构造函数的目的，因为删除它们时会得到相同的结果.我是Java的新手，所以如果这是一个愚蠢的问题，我深表歉意.这是我的一个课程的代码段.任何帮助将不胜感激.
I understand most of the code below. I just don't understand the purpose of the no-arg constructors of Employee and Person since I get the same results when I delete them. I am new to java so I apologize if this is a dumb question. This is a code snippet for one of my classes. Any help will be appreciated.
public class TestEmployee {
public static void main(String[] args) {
// Define some employees
Employee president = new Employee("Lucy", "President", 100000);
System.out.println(president);
Employee cto = new Employee("Vincent", "Chief Tech Officer", 70000);
System.out.println(cto);
}
}
class Employee extends Person{
// ADD YOUR CODE HERE!!!
// Nothing above needs to change.
private String jobTitle="Unknown";
private int salary = 0;
public Employee() {
}
public Employee(String name, String jobTitle, int salary) {
super(name);
this.jobTitle = jobTitle;
this.salary = salary;
}
@Override
public String toString() {
return this.getName() + " is the " + jobTitle + " and makes $" + salary + " a year.\n";
}
// You need data fields, a constructor or two, and a to-string method
}
// IF YOU ALREADY HAVE THE PERSON CLASS IN YOUR WORKSPACE
// YOU CAN DELETE THIS. OTHERWISE YOU'LL GET AN ERROR:
// 'The type Person is already defined.'
class Person {
private String name = "Default Name";
// No-arg Constructor
public Person () {
}
public Person (String name){
this.name = name;
}
// Getter method
public String getName() {
return name;
}
// Setter method
public void setName(String newName) {
if (!newName.equals("")) {
name = newName;
}
else
System.out.println("Can't change name. Empty names aren't allowed!");
}
public String toString() {
return "Name: " + name + "\n";
}
}
推荐答案
1.由于您只是Java的初学者，所以这是一个完全正确的问题(尽管我建议在发布问题之前对此主题进行一些研究).
1.Since you are just a beginner in Java,this is a perfectly valid question(although, i recommend doing some research on the topic before posting questions).
2.现在，提一个问题.
2.Now, coming to the question.
(a)您要引用的no-arg构造函数是对默认构造函数"的显式定义替代.如果程序员未明确定义构造函数，则编译器(javac)会自动将默认构造函数定义为 ClassName(){} .此构造函数在代码中不可见，但是编译完成后，它存在于Java字节码中.
(a)The no-arg constructor you are referring to is a explicitly defined substitute to the "default constructor".If the programmer doesn't explicitly define a constructor,then the compiler(javac) automatically defines a default constructor as ClassName(){}.This constructor is not visible in the code,however after the compilation is done,it is present in the java bytecode.
(b)在您的情况下，有一个显式定义的no-arg构造函数，原因是您已显式定义一个参数化的构造函数 public Employee(字符串名称，字符串jobTitle，内部薪水)公众人物(字符串名称)如果在程序中显式定义了带参数的构造函数，则Java编译器不会插入隐式默认构造函数.因此，如果希望在不传递任何初始值的情况下创建该类的对象，则 Employee President = new Employee(); 此语句将引发错误.<代码>未找到默认构造函数；默认值为0.嵌套异常是java.lang.NoSuchMethodException:.因此，为no-arg构造函数提供了明确的定义来支持此类对象的创建，用户可以先为对象分配堆空间，然后使用设置方法或其他一些机制.
(b)In your case, there is a explicitly defined no-arg constructor.The reason being that you have explicitly defined a parameterised constructor
public Employee(String name, String jobTitle, int salary)
public Person (String name)
If parameterised constructors are defined explicitly in a program the Java compiler doesn't insert the implicit default constructor.So if you wish to make the object of the class without passing any initial values Employee president = new Employee(); This statement would throw an errorNo default constructor found; nested exception is java.lang.NoSuchMethodException:.Thus there are explicit definations for the no-arg constructors to support such object creation,where a user may allocate heap space for a object first,and then initialise it later using the setter methods or some other mechanisms.
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