如何从另一个Java类更改变量的值？

Question

I am new to java so help is much appreciated.

I have a few classes. I want to pass the user input from my GUI class to my Basket class where it will update the value stored in the variable. Below is what i currently have but it doesn't seem to be updating the variable.

Class One - GUI Class: I would like to pass the 'seats' to the basket class

Basket b;
b = new Basket();   
String seats = JOptionPane.showInputDialog("Enter the number of seats to book");
try {
    int currentValue = Integer.parseInt(seats);
    int newValue = currentValue;    
    b.setSeatsBooked(newValue);
    //some code emitted
}

Second Class - Basket Class, I would like the 'seats' to be passed into this class and stored in the instance variable.

public class Basket {
    private int seatsBooked;
    public int getSeatsBooked() {
        return seatsBooked;
    }
    public void setSeatsBooked(int seatsBooked) {
        this.seatsBooked = seatsBooked;
    }
}

This is in another class where i see the result as 0:

Basket b;
b = new Basket();
lblMovieSelection = new JLabel("You have booked:" + b.getSeatsBooked());

Answer 1

This is the reason of the wrongly behaviour:

Basket b;
b = new Basket();

lblMovieSelection = new JLabel("You have booked:" + b.getSeatsBooked());

you are creating a new object! so the default value is zero for that

Answer 2

You need to pass the class object for which you have stored the value. If you create a new instance of a class, how can you expect that object will contain that value?

In the following two cases,

Basket b;
b = new Basket();   
String seats = JOptionPane.showInputDialog("Enter the number of seats to book");
try {
    int currentValue = Integer.parseInt(seats);
    int newValue = currentValue;    
    b.setSeatsBooked(newValue);
    //some code emitted
}

and

Basket b;
b = new Basket();
lblMovieSelection = new JLabel("You have booked:" + b.getSeatsBooked());

Are the object b of class Basket same in both blocks? Answer is No. So, you are getting zero value when you call b.getSeatsBooked() from the newly created object b of class Basket in the second block of code.

There are two way to solve this problem.

1. Best solution is to pass the class object (as parameter) to the third class where you are calling the getSeatsBooked() method.

Example : Suppose your another class is X and there is a function called method() . Then you can pass Basket class object as parameter to that function where you can access it to get the updated value. The code may look like as follows.

class X{
    // other code
    public void method(Basket b){
        lblMovieSelection = new JLabel("You have booked:" + b.getSeatsBooked());
    }
}

2. If you want share the variable seatsBooked across all class instances, you can declare the variable as static. (I guess this is not what you are looking for)

Answer 3

Try making the variable static, an example for this would be:

public static int testInt = 0;

The way you would call this in the other class is up to you, there are 2 options that you can use.

//NOT CLASS NAME IS FOR YOUR ACTUAL CLASS NAME.
ClassName varName = new ClassName();
varname.testInt;

//OR YOU CAN USE
ClassName.testInt;