Function to change TextView doesn't work when called from another class

Question

I've written a function to change the text of a TextView. It's in the class of the activity which contains the TextView. However, this function doesn't work when it's called from another class.

The function looks like this:

public class MainActivity extends AppCompatActivity {
    public void changeTest() {
        TextView test = (TextView) findViewById(R.id.textViewTest);
        guessedLetters.setText("test");
    }
}

If I call this function from my MainActivity, it works perfectly fine:

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_gameplay);

        changeTest();
    }
}

But if I call it from a class that extends MainActivity, I get a java.lang.NullPointerException at android.app.Activity.findViewById(Activity.java:1884).

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_gameplay);

        MyClass myClass = new MyClass();
        myClass.myFunction();
    }
}

public class MyClass extends MainActivity {
    public void myFunction() {
        MainActivity mainActivity = new MainActivity();
        mainActivity.changeTest();
    }
}

How could I fix this?

It works when in onCreate you have called setContentView but have you called it by the other way? can you post at least that line on both onCreate methods? — Nanoc
– Nanoc, Commented Nov 19, 2015 at 12:49
@Algorithm_NL I posted some code that you can try to replicate — Jorge Casariego
– Jorge Casariego, Commented Nov 19, 2015 at 13:26
In MyClass you dont need to create object of MainActivity in that method just write this.changeTest(), I think it will change. — Nigam Patro
– Nigam Patro, Commented Nov 19, 2015 at 13:33

Jorge Casariego · Accepted Answer · 2015-11-19 14:02:05Z

1

If you want this result using another activity extending from MainActivity, you can do it in this way:

MainActivity

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //changeTest(this);

       AnotherActivity a = new AnotherActivity();
        a.updateTest(this);


    }

    public void changeTest(Activity activity) {
        TextView test = (TextView)activity.findViewById(R.id.textViewTest);
        test.setText("test");
    }
}

AnotherActivity

public class AnotherActivity extends MainActivity {

   public void updateTest(Activity activity){
       changeTest(activity);
   }
}

edited Nov 19, 2015 at 14:02

answered Nov 19, 2015 at 13:26

Jorge Casariego

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2 Comments

TheLeonKing Over a year ago

Thanks a lot, this did the trick! However, I'm now having the same problem when defining a new Intent (instead of defining a TextView) with Intent intent = new Intent(this, GameplayActivity.class);. Where should I add 'activity' there?

Jorge Casariego Over a year ago

where are you defining this intent? inside the method changeTest()?

Luca Ziegler · Accepted Answer · 2015-11-19 13:55:20Z

1

Might this help you.

MainActivity

public class MainActivity extends AppCompatActivity {

public static TextView tv;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    tv = (TextView) findViewById(R.id.textView);
    OtherClass otherClass = new OtherClass();
    otherClass.setText("TEST");
    }
}

Other Class

public class OtherClass {
    public void setText(String txt){
       MainActivity.tv.setText(txt);
    }
}

answered Nov 19, 2015 at 13:55

Luca Ziegler

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Comments

Nigam Patro · Accepted Answer · 2015-11-19 14:06:15Z

1

Try with this

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_gameplay);

        MyClass myClass = new MyClass();
        myClass.myFunction(this);
    }
}

public class MyClass extends MainActivity {
    public void myFunction(MainActivity mainActivity) {
        mainActivity.changeTest();
    }
}

answered Nov 19, 2015 at 14:06

Nigam Patro

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