如何从Java中的不同类访问变量？

Question

I have been having trouble accessing a variable from another class. Here are my variables:

public static byte agressivePoints = 0;
public static byte peacefullPoints = 0;
public static byte meanPoints = 0;
public static byte happyPoints = 0;
public static byte sadPoints = 0;

I put them outside of my Main class. In this class, I am tying to access those variables from the other class to put them in this one.

public class Check_Anwser {
    public static void Check_Answer(int Input) {
        boolean TryAgain = false;

        do {
            switch (Input) {
                case 'a': {
                    ++agressivePoints;
                    break;
                }
                case 'b': {
                    ++sadPoints;
                    break;
                }
                case 'c': {
                    ++meanPoints;
                    break;
                }
                case 'd': {
                    ++peacefullPoints;
                    break;
                }
                case 'e': {
                    ++happyPoints;
                }
                default: {
                    System.out.println("You have entered an invalid anwser. Please try      again");
                }
            }
        } while (TryAgain != true);
    }
}
}

But when I do this, there is an error. For some reason, the variables from the first class aren't shared with the second class.

Answer 1

How do you access variables from different classes in Java?

If the variable (or member) of another class is non-static. You can create an instance(object) of that class and access to its variable, for example:

class Warrior
{
    private int strength;
    public int getStrength(){
        return this.strength;
    }
}

class TestRunner{
    public static void main(String[] args){
        Warrior w = new Warrior();
        w.getStrength();    //Access to strength of warrior via a Warrior object
    }     
}

If the member is public, you will be able to access it directly like:

w.strength;

But when you learn the concept of encapsulation and data protection, you will realize generally we will try to make the instance variables as private.

---

As for static members, they belongs to the class and not individual objects. So it is encouraged you access them via the class name, for example:

class Warrior{
    public static final int SPEED;    //let say speed for all warriors are unified
}

class TestRunner{
    public static void main(String[] args){
        int speed = Warrior.SPEED;    //Access a class variable via the class name
    }     
}

Answer 2

Use the expression Main.aggressivePoints to access these global variables.

I encourage you to keep learning about object-oriented design, so that you can understand why it will be better when you create objects and protect data like this inside them. It's called encapsulation.

Answer 3

To access variables from other class say A to call the variable into your main class use (instance of classA).varaibleName In your case corresponding class name of the variable say class A contains

public static byte agressivePoints = 0;

by creating an instance of the class like

A inst =new A();
++inst.agressivePoints;

Answer 4

The best way to do this is have a Variables class, in which holds all your variables, and inside the Correct_Answer class you could simply call

Static Variables v;

and infront of each of the variables you wish to call just do c.VARIABLENAME;