Accessing a Variable from Another Class

Best way to access variables from another class?

In general there are 2 ways to access a variable from another class:

1. You create an object of that class. Then this object has all the variables from the scope of that class assigned to it. For example:

Test t = new Test();
t.name = "test";

2. You can also create a static variable. Then the variable is assigned to the class not the object of that class. This way you will not need to create an object, but all instances of the class will share the same variable.

//In the scope of the class
static String name; 

-------------------------
//when classing the class
Test.name = "The Name of the Test";

If you do not want to create a new instance of a class every time, and always use the same instance, you can create a singleton object. You write a getter method that gets you the object. It looks like this:

public class Test {
    Test t;

    public static void main(String[] args) {
       t = new Test();
    }
    
    public Test getTest() {
        if (t != null) {
            return t;
        } else {
            t = new Test();
            return t;
        }
    }
}

I see you work with a JFrame. Then you will probably want to make it a singleton. Else you will open a new instance of the JFrame every time you call upon it, which is not recommended.
Does this answer your question?

How do I access variables from another class in Java?

If you simply want to access the value of a property from outside of the class, then you need to create getters for each one of the properties you want to retrieve.

If you want to modify these values from outside of the class, you need to create a setter.

Getters and Setters are simply methods that allow you to access properties of a class outside of it.

If you have the class Customer and want to access and change it's name from outside, you'd create 2 methods,

//declare the property
private String name;

//getter
public String getName(){
   return this.name;
}

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

//You can then instantiate a Customer object anywhere else and have access to those //properties

Customer cust = new Customer();
cust.setName("Mark")
System.out.println("Oh hi " + cust.getName());
//output "Oh hi Mark"

Read more on getters and setters

Also, best practices tip: instance variables should always be declared as private to help encapsulation. If no access modifier is provided for an instance variable in Java, it defaults to the default modifier, which makes the variable accessible for every class within the same package.

Edit:

Your specific error is that you are creating a new Customer object within your customerDetails method, and you're not returning this Customer object. Therefore, as soon as the method has been executed, the Customer object you created inside is destroyed (cleared from memory,Garbage collected), because there is no further reference to it.

You either need to

Method 1: Change the return type of your customerDetails method from void to Customer and add a return a statement at the end, then you would simply need to instantiate a Customer object from your booking class, like so

public Customer customerDetails(){               
    Customer a = new Customer();           
    //your logic to set all of the properties          
    return a;        
}

in your booking class

Customer myCust = new Customer();
myCust = myCust.customerDetails();

I would not prefer this method, because as you see, you're just creating an empty object then reassigning to it. You may alternatively add the static keyword after public so that you can call it without having instantiated an object of the class, like so

booking class

Customer myCust = Customer.customerDetails();

Method 2: remove the Customer a = new Customer() from the customerDetails altogether and simply use this.name = sc.nextLine() to set the name of whatever instance is calling this method.

Then on bookings class, instantiate a new Customer object and call the method.

Customer myCust = new Customer();
myCust.customerDetails();

How Do I Access One Variable From Another Class?

• If the field myNum is declared as public, you can access it by
  any other class by typing the name of the object instance.myNu

• If the filed myNum is declared as public static, you can access it
  from any other class by typing the name of the class.myNum

• If the field myNum is private, you need getters and setters, namely,
  methods to access the file from an instance of the class that
  contains it. Google them to get up to speed on why they're useful and
  why you should use them.

Ex.

//public

ClassOne instance = new ClassOne();

ClassTwo instante2 = new ClassTwo();

instance2.myNewInt = instance.myNum;

//public static

ClassTwo instante2 = new ClassTwo();

instance2.myNewInt = ClassOne.myNum;

//getter

 ClassOne instance = new ClassOne();

 ClassTwo instante2 = new ClassTwo();

 instance2.myNewInt = instance.getMyNum();

 //and inside of ClassOne you'll have

  private int MyNum = 5;

  public getMyNum(){

     return MyNum;  

  }

Note:

If the variable was only declared locally (inside the body of one of ClassOne's methods), you're gonna need to assign it to a filed, so that you can later access it from other classes.

Reading Material:

Getters and Setters

Access modifiers

Accessing a class' member variable from another class

public class FirstClass {
    int x;
    ...... 
    public int getX(){
        return x;
    }
}

There is no maggic. it's by java specification.
Controlling Access to Members of a Class

The four access levels are −

• Visible to the package, the default. No modifiers are needed.
• Visible to the class only (private)
• Visible to the world (public)
• Visible to the package and all subclasses (protected)

your case int x; has default access. All classes in some package have access to it.

public int getX()

you can get x variable everywhere by in your project.

FirstClass fc = new FirstClass();
int value = fc.getX();

You should use exactly this (call method instead of direct x) approach in 90% cases outside of your class and 10% getX() - inside class. There might be cases with testing , some extra logic for getX() -- but it's in common practice , not in your case

Can't access a variable from another class in Java?

You need to say Person.numFriends. This is because numFriends is a static member of the Person class, so you need to use the Person class to reference numFriends.

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