Question

Everyone, I am a newbie to android development. Now I have a question that cannot be solved by myself. Anything wrong with the code below(especially the line marked in the code)?

MainActivity.java:

package com.amaker.ch02.app;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends Activity {
    /** Called when the activity is first created. */
    private TextView displayTextView = (TextView)findViewById(R.id.DisplayTextView);  <--Possibly this line

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        displayTextView.setText("change in the code"); 
    }
}

Run, and i got a message in AVD: The application has stopped unexpectedly. Please try again. But if i dont assign displayTextView immediately after declaration, IOW i change the code as follows, then everything goes well.

package com.amaker.ch02.app;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends Activity {
    /** Called when the activity is first created. */
    private TextView displayTextView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        displayTextView = (TextView)findViewById(R.id.DisplayTextView);
        displayTextView.setText("change in the code"); 
    }
}

Why? Any difference with the two codes?

Answer 1

The TextView is not part of the activity’s view hierarchy until after you call setContentView(R.layout.main). When you declare the variable like this:

private TextView displayTextView = (TextView)findViewById(R.id.DisplayTextView);

the view does not yet exist, so displayTextView gets set to null. Then you are getting a NullPointerException when you try to call setText() in onCreate().