java: get private variable from another class, without setting a getter

Question

I am not allowed to alter class A, but i need to get the value of 'a'(set in the constructor). Can anyone tell me how to do that? Every other solution seems to propose a getter in class A. I am not allowed to make changes to class A in any way, no changing to public etc. The magic has to happen in class B.

public class A{
    private int a;
    public A(int x){
        a = x * 10;
    }
}

public class B{
    public B(int x){
        A instA = new A(x);
    }

    public int geta(){
        ???
    } 
}

PS: Posted this question, because every similar question was answered with altering class A, whereas i am not allowed to do so. But since there seems to be only reflection, this question can be closed. Thanks guys and gals.

if member variable is private, the only way to access it outside is via a some method... — Jos, Mar 24 '16 at 15:28
You are not allowed to add a getter to A, but are you allowed to modify A in any way? If you are, you could potentially change `private int a;` to `public int a;`. Also, please state WHY you are not allowed to add a getter. — Kevin Cruijssen, Mar 24 '16 at 15:28
see: http://stackoverflow.com/questions/1196192/how-do-i-read-a-private-field-in-java — Meiko Rachimow, Mar 24 '16 at 15:30
Since the options for this (reflection, bytecode hackery, alternate jvm language that doesn't care about access modifiers) are already covered in http://stackoverflow.com/q/1196192/217324, voting to close. — Nathan Hughes, Mar 24 '16 at 15:51

score 3 · Answer 1 · answered Mar 24 '16 at 15:39

3

That is the whole point of private fields; they are not accessible outside the scope of the enclosing class.

You got the following options:

Change that class to provide a getter
Change your overall design, to either not need that value; or find another way to make it available to both classes
Not recommended: use reflection and its ability to override the "private" protection at run time

answered Mar 24 '16 at 15:39

GhostCat

137,827
25
176
248

Thanks. I understand the point of private fields. I guess the wording of the excercise is wrong. I don't think that reflection is the solution we are supposed to use. I was hoping someone would point me here to a solution i was just unable to see. I think we are not allowed to change the given methods, but add some, otherwise there seems only to be reflection... – Peter Mar 24 '16 at 15:49

Danail Alexiev · Answer 2 · 2016-03-24T15:40:48.950

0

The best decision is to have a getter. It's the safest and cleanest solution when it comes to programming best practices.

It's possible to achieve what you want using reflection - you can find the field by name (what if the name changes in a future version of the class?) and the extract it's value like this:

A a = new A(5);
Field field = A.getField("a");
field.setAccessible(true);
int value = field.get(a);

But bear in mind that this is considered super hacky and it strongly advice against.

edited Mar 24 '16 at 15:40

answered Mar 24 '16 at 15:31

Danail Alexiev

7,624
3
20
28

Partially agree, but one more line should be added before accessing the field: `field.setAccessible(true);` – Aliaxander Mar 24 '16 at 15:39
@Aliaxander true that. – Danail Alexiev Mar 24 '16 at 15:40

score 0 · Answer 3 · answered Mar 24 '16 at 15:48

Here is how you can do it

    public int geta(){
        Field field;
        int x = 0;
        try {
            field = A.class.getDeclaredField("a");
            field.setAccessible(true);
            x = field.getInt(this.a);
        } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
            e.printStackTrace();
        }
    System.out.println(x);
    return x;
}

java: get private variable from another class, without setting a getter

3 Answers3

Linked