JAVA - Variable is always null

Question

I've made this code where the variable DEVICE will change if a file exist or not. So i have made this code but the variable DEVICE is always null

public class MainActivity extends AppCompatActivity{

    String DEVICE;

 @Override
    protected void onCreate(Bundle savedInstanceState) {

        apply = (Button) findViewById(R.id.apply);
        apply.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                    checktypezip(DEVICE);
                    while (DEVICE == null){
                        Log.v("Check","Check non completo");
                    }
            }
        });

    }

    public void checktypezip(String string){
        String percorso = Environment.getExternalStorageDirectory().getPath()+"/BAC/.unzipfile/";

        File normalzip = new File (percorso+"desc.txt");
        File flashzip = new File (percorso+"/system/media/bootanimation.zip");
        File samsung = new File (percorso+"/bootsamsung.qmg");
        File flashsamsung = new File (percorso+"/system/media/bootsamsung.qmg");
        String disp;

        disp=string;
        if (normalzip.exists()){
            disp = "Normal";
            string=disp;
        }
        else if (flashzip.exists()){
            disp = "Flashnormal";
            string=disp;
        }
        else if (samsung.exists()){
            disp = "Samsung";
            string=disp;
        }
        else if (flashsamsung.exists()){
            disp = "Samsungflash";
            string=disp;
        }
        else
        {
            disp = "Unknown";
            string=disp;
        }

    }

}

You are not updating DEVICE in the checktypezip methods because string is not a pointer. string = "foo" will not update DEVICE. PS : this is a nice infinite loop ;) — AxelH
– AxelH, Commented Jan 27, 2017 at 12:16
Add return type to checktypezip or remove param from it and access member variable directly. — Anurag Singh
– Anurag Singh, Commented Jan 27, 2017 at 12:18
Possible duplicate of Is Java "pass-by-reference" or "pass-by-value"? — AxelH
– AxelH, Commented Jan 27, 2017 at 12:18

0xDEADC0DE · Accepted Answer · 2017-01-27 12:59:02Z

5

Java uses 'pass by value'. This means that the value of DEVICE is passed to your function, not the reference. Although you are assigning a value to the parameter string, it will never be assigned to DEVICE.

You must return the value of disp from your function and assign it to DEVICE

Define your function like this

public String checktypezip()

and call it like this

DEVICE = checktypezip();

At the end of checktypezip, you must add return disp

On a side note:

while (DEVICE == null){
    Log.v("Check","Check non completo");
}

This will block your main thread indefinitely and cause an ANR after 5 seconds. I would suggest replacing while with if

edited Jan 27, 2017 at 12:59

answered Jan 27, 2017 at 12:19

0xDEADC0DE

2,4831 gold badge19 silver badges22 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

JAVA - Variable is always null

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related