为什么 getName() 返回 null？

Question

EDIT: Thanks for all the answers! I didn't know that the object p in List.java is different from the p in Main.java. I passed it as a parameter and it works fine now. Thank you!

In Main.java:

        System.out.println("Enter your name:");
        String name = scan.next();
        name+=scan.nextLine();

        String words[]=name.split("\\s");  
        String capitalizeWord="";  
        for(String w:words){  
            String first=w.substring(0,1);  
            String afterfirst=w.substring(1);  
            capitalizeWord+=first.toUpperCase()+afterfirst+" ";  
        }

        Person p = new Person(capitalizeWord);

In Person.java

    private String name;
    private int age;

    Person(String newName){
        name=newName;
    }

    Person(int newAge){
        age=newAge;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

In List.java:

public void printInvoice(){
        System.out.println("Enter your age:");
        int age = scan.nextInt();

        Person p = new Person(age);
        System.out.println("Thank you for shopping with us, "+p.getName());
}

The last output is

Thank you for shopping with us, null

I don't know why this is happening. Did I do something wrong? I think the code is correct?

Answer 1

You have two constructors. One that accepts a string, one that accepts a number.

In your latest example you called the one that accepts a number and, since there's nothing initializing the name member, it gets initialized to null as default.

The p in Main.java has nothing to do with the p in List.java (they're two different variables), so initializing the former will have no effect on the latter.

Answer 2

Each time you call a constructor you get a distinct instance of the object. The object created in Main.java with a name is stored in a local variable p that is only existing in the scope of Main.java . In List.java you create a second object with an age, but no name. It's also stored in a variable called p but that one is in the scope of List.java only and has nothing to do with the earlier created object.

It sounds like you want to add the name to the earlier object and not create a new one. For that you should pass the first object as a parameter to the code that adds the age, perhaps like this:

public void addAge(Person p) {
  System.out.println("Enter your age:");
  int age = scan.nextInt();
  p.setAge(age);  // will have to make this method in the Person class
  System.out.println("Thank you for shopping with us, "+p.getName());
}

Calling Person p = new Person(age); gives a brand new Person object that has no name yet.

Answer 3

As I can see in your List.java class, you have instantiated a new Person object with age as a parameter, so for that object, the name would be null , since the two reference variables are pointing to the different Person object

The first reference p to object Person has instantiated the person object using the contructor:-

Person(String name) {
    this.name = name;
}

Second reference p to object Person has instantiated the person object using the constructor:-

Person(int age) {
    this.age = age;
}

You should have used setter to set the property age to an already created Person object

Answer 4

code that create your person -> Person p = new Person(age);

constructor called -> Person(int newAge){ age=newAge; } Person(int newAge){ age=newAge; } This constructor let the name with his default value.. so null