构造函数打印出 NULL 而不是对象值

Question

I created a new class "Lecturer" which extends another class "Person", i wanted to make 2 constructors for Lecturer and one would accept a name and a stipend (just a constant to say how much pay is), the other just accepts the name and uses the default stipend set in the code. i included appropriate getters and setters. I then wrote a writeOutput method to print an output similar to this

Name: (name) which gets the name and prints it Stipend: (stipend) same process ^

heres what i have so far

Lecturer.java

public class Lecturer extends Person{

private static String name;
static double stipend;

public Lecturer(String name) {
    super(name);

}

public Lecturer(String name, double stipend) {
    super(name);
}

public String getName() {
    return name;
}



public void setName(String name) {
    this.name = name;
}



public double getStipend() {
    return stipend;
}



public void setStipend(double stipend) {
    this.stipend = stipend;
}

public static void writeOutput() {
    System.out.println("Name: " + name);
    System.out.println("Stipend: " + stipend);
    }

 }

Person.java

public class Person {

/** Every Person has a name */
private String name;

/** Person requires a name */
public Person(String n) {
    this.name = n;
}

/** return this Person's name */
public String getName() {
    return this.name;
}

/** Change this Person's name */
public void setName(String nn) {
    this.name = nn;
}

Main file (Inheritance.java)

Lines 41-53

    Lecturer l1 = new Lecturer("Zachary");
    Lecturer l2 = new Lecturer("Wilhelmina", 11017.00);
    l1.writeOutput();
    l2.writeOutput();
    pause();
    l1.setName("Zack");
    l1.setStipend(10800.00);
    l1.writeOutput();
    pause();
    System.out.printf("%s's stipend is $%,4.2f.\n", 
                        l1.getName(), l1.getStipend());        
    System.out.printf("%s's stipend is $%,4.2f.\n", 
                        l2.getName(), l2.getStipend());    

This is the output

Name: null
Stipend: 0.0
Name: null
Stipend: 0.0

press enter...

Name: Zack
Stipend: 10800.0

The 2nd part works as it should but the first one isnt and i tried to change the code but nothing is working properly.

Answer 1

In Lecturer you are declaring another name variable. This variable is separate from the name variable declared in Person . The call to the superclass constructor is setting the name variable in Person , not in Lecturer . But you don't need the second variable; remove it. You can access the name in Person via the getName method you've already declared. This means that you also don't need to re-declare getName and setName in Lecturer , so the Lecturer class can inherit them.

Also, in Lecturer , the two variables you've declared shouldn't be static . Per the above reasoning, name shouldn't even be there, but even if it should be there, it shouldn't be static . The variable stipend should be there, but it shouldn't be static . When you declare a member variable static , then there is only one variable for the entire class, no matter how many instances you create, which doesn't sound like what you want.

Your constructors should initialize stipend .

Answer 2

You have a static variable inside Lecturer which has the same name as the inherited one from Person and your getter is referring to that static one - are you sure you want these static variables? For completeness if you really want to keep the static one and the inherited one with the same name then change your getter to read return this.name; which will return the inherited name instance variable.... But that method can be inherited from Person class...

Answer 3

There are two name fields in your program , one is private static String name; in Lecturer.java and another is private String name; in person.java .

The thing is that you are just calling Lecturer javs's name field but not setting it.

Answer 4

Fixed the project based on rgettman answer.

Lecturer class should look like this:

public class Lecturer extends Person {
    double stipend = 9144;

    public Lecturer(String n) {
        super(n);
    }

    public Lecturer(String n, double stipend) {
        super(n);
        this.stipend = stipend;
    }

    public double getStipend() {
        return stipend;
    }

    public void setStipend(double stipend) {
        this.stipend = stipend;
    }

    public void writeOutput() {
        System.out.println("Name: " + this.getName());
        System.out.println("Stipend: " + getStipend());
    }
}