How do I change a variable from a different class

问题:

How do I change a variable from a different class in Java?

I'm trying to change a variable from another class, and use it back in the first class.

I make a variable in class First, and give it a value of 1. Then I try to change the value of the same variable to 2 in class Second, but it changes back to 1 when I use it in class First.

I'm new to Java and don't know very much yet, so if you could try and keep the answers simple, that would be great :)

---

Class First:

public class First {

    public static void main(String args[]){


    int var = 1; //Variable i'm trying to change from class "Second"

    Second Second = new Second();

    System.out.println(var); //Prints out 1

    Second.test(var); 

    System.out.println(var); // Prints out 1 again, even though I changed it

}
}

---

Class Second:

public class Second {

    void test(int var){
    /*
     * 
     * I try to change var to 2, and it works in this class
     * but when it doesn't change in the class "First"
     * 
     */
    var = 2;
    System.out.println(var); //Prints out 2

}
}

---

What the output looks like:

1
2
1

What i'm trying to get:

1
2
2

---

Ive tried to find answers to this, but all of the answers that I could find didnt make any sense to me, as im very new to Java and programming.

回答1:

The problem is

Second.test(var); 

It's not a bug. It's just not doing what you think its doing.

A primitive (an int is called a primitive...it's not an object) passed to a function may be changed in that function, but in a copy. As soon as the function is done, the original value is the same, because it was never altered to begin with.

What you want is

int test(int var){
   var = 2;
   System.out.println(var); //Prints out 2
   return  var;
}

And then instead of

Second.test(var); 

use

var = Second.test(var);

There is actually no point in the parameter at all. It is equivalent to

var = Second.test();

...

int test(){
   int var = 2;
   System.out.println(var); //Prints out 2
   return  var;
}

I hope this helps. Good luck, welcome to Java, and welcome to stackoverflow!

回答2:

Java is pass-by-value, it means that you are passign a copy of var. What you can do is to make method test() to return a int

int test(int var){
    ...
    var = 2;
    System.out.println(var); //Prints out 2
    return var;
}  

and then assign its result (the value the method test() returns) to var:

var = second.test(var);
System.out.println(var); //Prints out 2

Note:

In

Second Second = new Second();

you shouldn't use the name of a class as a name of a variable. Do this sintead:

Second second = new Second();

回答3:

The var parameter in test method of class second is a copy, not a reference or pointer. So you can not change the value of var in class First.

To achieve your goal, you could declare var as a global variable, like this:

public class First {

    public int var = 1; 

    public static void main(String args[]){

    Second Second = new Second();

    System.out.println(var);

    Second.test(); 

    System.out.println(var);

}
}

public class Second {

    void test(){
    /*
     * 
     * I try to change var to 2, and it works in this class
     * but when it doesn't change in the class "First"
     * 
     */
    First.var = 2;
    }
}

回答4:

Well you've changed it in second class, but you did not return it back to the first class. First class can't possibly know you've changed it in the other class if you don't return it. You can do it this way, hope it helps.

First class:

public class First {

    public static void main(String args[]){


        int var = 1;

        Second Second = new Second();

        System.out.println(var); //Prints out 1

        var = Second.test(var); 

        System.out.println(var); //Prints out 2

    }
}

Second class:

public class Second {

    int test(int var){

        var = 2;

        System.out.println(var); //Prints out 2

        return var;

    }
}