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;
        }

    }

}

Answer 1

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