在类外更改Java对象

Question

Here's my question, how can I change an object outside of it's class, so that it maintains the changes made in the outside class?

Here's an example of the code:

Main class:

    public class Main {

        public static void main(String[] args)
        {
            Variable var = new Variable(1,2,3);
            Change.changeVar(var);
            System.out.println("" + var.geta() + "" + var.getb() + "" + var.getc());
        }
    }

Variable class:

public class Variable {

private int a;
private int b;
private int c;

public Variable(int a, int b, int c)
{
    this.a = a;
    this.b = b;
    this.c = c;
}

public int geta()
{
    return this.a;
}

public int getb()
{
    return this.b;
}

public int getc()
{
    return this.c;
}

}

Change class:

public class Change {

public static void changeVar(Variable var)
{
    Variable var2 = new Variable(4,5,6);
    var = var2;
}

}

Answer 1

In your example, no. When changeVar() exits, the parameter var is discarded, and the var in your main() method retains its original value. Read up on pass by reference .

Answer 2

public class Variable {

    private int a;
    private int b;
    private int c;

    public Variable(int a, int b, int c)
    {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public int geta()
    {
        return this.a;
    }

    public int getb()
    {
        return this.b;
    }

    public int getc()
    {
        return this.c;
    }

    // depending on your use case, setters might be more appropriate
    // it depends on how you want to control the changing of the vars
    public void update(int a, int b, int c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

}

public class Change {

    public static void changeVar(Variable var)
    {
        var.update(4,5,6);
    }
}

Answer 3

Doing it that way? You can't.

You're passing a reference to the instance. However, inside the function, you use a new reference. Assigning to the new reference does not affect others.

Answer 4

You cannot do it in a way that you described, because in Java variables are passed by values. However you can achieve the desired effect in a different way:

public class Variable {

    private int a;
    private int b;
    private int c;

    public Variable(int a, int b, int c)
    {
        this.a = a;
        this.b = b;
        this.c = c;
    }
    public int geta()
    {
        return this.a;
    }

    public int getb()
    {
        return this.b;
    }

    public int getc()
    {
        return this.c;
    }

    public void seta(int a) { this.a = a; }
    public void setb(int b) { this.a = b; }
    public void setc(int c) { this.a = c; }
}

public class Change {

    public static void changeVar(Variable var)
    {
        var.seta(4);
        var.setb(5);
        var.setc(6);
    }

}

Answer 5

You need to provide setter methods and call them on the original object:

public void seta(int newa) { this.a = newa; }

Then you would say

public static void changeVar(Variable var)
{
    var.seta(4);
    //etc
}

You are merely repointing the local variable reference var to point to your new instance var2 . It has no effect on the value of the original instance passed into the method.

Answer 6

public static void changeVar(Variable var)
{
    Variable var2 = new Variable(4,5,6);
    var = var2;
}

first, u can write some setter methods in Variable class, then you can call these setter methods in the above code, like var.setA(4) ... and so on. enter code here