子类中的Java构造函数错误

Question

class Person 
{
    String name = "No name";

    public Person(String nm)
    {
        name = nm;
    }
}
class Employee1 extends Person
{
    String empID ="0000";
    public Employee1(String id)
    {                            // Line 1
        empID = id;
    }
}
public class EmployeeTest 
   {
       public static void main(String[] args) {
           Employee1 e = new Employee1("4321");
           Person p = new Person("Hello");
           System.out.println(e.empID);
       }
   }

I get compilation error saying constructor Person in class Person cannot be applied to given types; required String found no arguments constructor Person in class Person cannot be applied to given types; required String found no arguments but I am passing arguments for both the parent and the child class when I create new object in main method. Unable to figure out why compilation error?

Answer 1

You need to properly create the parent class, passing it a name as the Person construtor requires:

public Employee1(String id) {
    super("person name");   // create the parent
    empID = id;
}

Or maybe something like:

public Employee1(String id, String name) {
    super(name);   // create the parent
    empID = id;
}

public static void main(String[] args) {
    Employee1 e = new Employee1("4321", "Hello");
    // ...
}

Answer 2

Because constructor in child class implicitly calls the parameter-less constructor of it's immediate super class only if the default constructor does not explicitly call a superclass constructor so

public Employee1(String id)
{                            // Line 1
    empID = id;
}

will try to call constructor in super class as you have not called it explicitly so you can say your code will be like this

public Employee1(String id)
{
    super();                            // Line 1
    empID = id;
}

but in you parent class there is no "no argument" constructor, so it is giving error like this.

Answer 3

On the constructor of employee you must call first the constructor of person as in super(name); and then initialize the subclass

class Employee1 extends Person
{
    String empID ="0000";
    public Employee1(String id, String name)
    {                            // Line 1
        super(name);
        empID = id;
    }
}

Answer 4

The implicit super constructor Person() is undefined.

So you must explicitly invoke its parent constructor.